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Lucas Ignition (to 1989)

  Experience in a Book
Lucas Ignition (to 1989)


IGNITION SYSTEM DESIGN: An ignition coil requires a certain amount of time to build up enough energy to produce a spark. The faster an engine is turning, the less time there is between sparks, so the output of an ignition coil starts to drop off. It is also apparent that the more cylinders there are, the less time there is between sparks, and the output of the ignition coil drops off even faster.

Another lesson in physics is that the higher the compression, the more resistance there is for electricity to jump a spark gap, so higher voltage is required.

The Jaguar V12 H.E. has 12 cylinders, turns at 6500 RPM, and has 11.5:1 compression, making it one of the biggest challenges for an ignition system in production automobiles. To cope with this, Jaguar has incorporated some sophisticated ignition technology. Also, Jaguar uses a spark plug gap of only .025" to make it easier for the electricity to jump the gap.


TROUBLESHOOTING: Jan Wikström sends this procedure: "This is the Gum Tree Garage test for a V12 ignition when it cranks but refuses to start:

1. Pull the lead from coil to distributor out of the distributor and jam it under a fuel pipe so there's a gap of about 1 mm (.04in) between the brass and the nearest engine part.

2. Crank the engine. Do you get fat, blue sparks in the gap? Then the ignition is OK and you need to troubleshoot the injection system. If not, continue:

3. Pull the spade connector off the coil terminal marked (+). Turn the ignition on, then test the power with a spare light bulb between the connector and engine bare metal (this is better than a voltmeter, because it will reveal a poor contact with insufficient current carrying capacity). Is the lamp bright? Then continue; if there's no voltage, check the ignition switch and all its wiring including the ballast resistor.

4. Touch the connector quickly several times to the terminal. Do you get a spark now? Then the pickup coil in the distributor or its wiring, or (less likely) the amplifier, may be faulty. If there's no spark, turn the ignition off and continue:

5. Get a test lead with alligator clips. Clip one end to bare metal on the engine, pull the tubular connector off the (-) terminal of the coil and clip the other end to that terminal.

6. Repeat test 4. If you get a spark now, the amplifier or its wiring is faulty. If you get no spark, the coil is dead."


IGNITION TIMING: The proper advance setting is indicated on a decal in the engine compartment. If it differs from the manual, believe the decal.

The next thing to confirm is that the woodruff keys that align the front pulley with the crankshaft are in good condition. They are a known problem, and clearly if the pulley is allowed to reposition itself on the crank, use of the timing marks will be worthless.

On the Jaguar V12, the timing indicator itself is adjustable. If there is any chance it has been tampered with (the oil pan and sandwich plate have been removed), then the position of the indicator must be calibrated before checking the timing.

The official method for setting this indicator is to do it when the right side (A bank) head is off. A dial position indicator can be set up to determine when the 1A or 6A piston is at TDC. If a position indicator that will fit through a spark plug hole is available, this same method can be used with the head in place by removing the spark plug from either cylinder 1A or 6A. Once TDC is determined, loosen the two sandwich plate bolts that hold the timing indicator plate in place, and slide the plate on its slotted holes until 0 lines up with the mark on the pulley.

There is an alternate method to set the timing indicator that doesn't require the position indicator or removing the head. All that is required is a device that will obstruct the motion of the piston near the top of its stroke. Such a device can be made from an old spark plug by breaking the ceramic out of it and installing a bolt through the middle. Ideally, the length of the bolt into the combustion chamber should be just enough for the piston to hit it only a few degrees from TDC. If you make this device strong enough, it might also come in handy for removing the crankshaft pulley someday although for that purpose it would be better if it hit the piston farther away from TDC.

Turn the engine a ways past TDC, and then screw this obstructing device into the spark plug hole of either 1A or 6A cylinder. Then turn the engine backwards until the piston hits the device and you can't go any farther. Note the reading from the timing marks. Then turn the engine forward through one complete revolution until the piston hits the device again, and note the reading of the timing marks. The two readings should be exactly the same amount before and after TDC. If they are different, loosen the sandwich plate bolts holding the indicator plate and move it an amount corresponding to one half the difference between the two readings.

The ignition timing on the V12 is checked with the engine held at 3000 RPM. Although a pain, this method insures the timing is accurate at operating speed rather than at idle, where timing is less critical. However, proper timing by this method requires that the tachometer be at least reasonably accurate. It's not very critical, since it would have to be in error by about 300 RPM to cause an error of 1 in the timing. Nevertheless, if one ever finds his car hooked up to one of those fancy, super-accurate computerized engine analysis machines, it is suggested that the location of the tachometer needle be noted when the engine is running at a real 3000 RPM. That point can then be held whenever the timing is checked in the future.

When checking the ignition timing on the V12, the vacuum line to the distributor vacuum advance must be disconnected and plugged. The vacuum advance capsule is at the bottom rear of the distributor, making it quite difficult to get to this line (the larger hoses connecting to the distributor cap itself are for the distributor ventilation system, and do not affect timing). Do yourself a favor and cut the vacuum line somewhere convenient, and reconnect it with a small piece of tubing. From then on, all you have to do is disconnect it at the break and plug it when checking the timing.

Since the battery is in the trunk, connecting the power leads of the timing light requires ingenuity. The ground lead can be connected anywhere on the car. The positive lead must go to 12V, which exists at any solid brown wire. There are two terminals on the firewall adjacent to the valve covers, and a terminal on the back of the alternator -- all difficult to get at. Another solution is to use a pair of jumper cables from the battery in the trunk. Peter Smith: "I connect my timing light to the 12V power source at the headlights fuse box."

Since disconnecting the plug wire from cylinder 1A is difficult, disconnect the wire from the distributor cap instead; make a small jumper from an old ignition wire to use to connect the timing light. Or, just buy an inductive timing light. If it's more convenient, you can also check the timing using the signal from cylinder 6A.

Since you must crawl under the front of the car while an assistant holds the engine at 3000 RPM to read the timing, it is suggested you put an extra long ignition lead on your timing light so you can route it around the fender rather than feeding it through the engine compartment. Interference with moving parts at 3000 RPM would be memorable. Cheap ignition wire with metal conductor works well for making a long timing light ignition lead, and is available by the foot.

The Jaguar V12 design eliminates most wear items that would normally cause ignition timing to vary, such as ignition points or sloppy camshaft drive systems. The only remaining reason for the timing to change would be wear in the timing chain itself, and the Jaguar timing chain normally wears so slowly as to be insignificant. If the timing in your car is way off, it is highly recommended you determine why rather than simply readjust it. For example: if your centrifugal advance unit seizes in the idle position, then when the car is revved to 3000 RPM the timing will be retarded by 15 degrees or more. If you merely adjust the timing rather than correcting the problem, then the timing will be 15 dfegrees too far advanced at idle!

Timing is normally adjusted using a long screwdriver to turn a small eccentric cam on the side of the distributor underneath the cruise control actuator. If your eccentric timing adjustment won't go far enough, the distributor base must be repositioned. Remove the distributor cap and insert a long allen wrench to loosen the three mounting bolts at the very bottom of the unit. Rotate the entire distributor housing in the direction needed, then retighten. Please remember that this much adjustment should never be necessary, and causes should be investigated.


HOLDING A HIGH RPM: Michael Minglin sends a tip: "Every time I went to check the timing, alternator, etc. the book says to set the rpm to such and such. Reaching down to that auxilary air valve when the engine is hot is not my idea of fun. What I did was to weld a nut on top of a short bolt. Threading a longer bolt into this nut gave me a "T" with a long top and short leg. The short bolt goes into the RHD throttle cable bracket (not used for anything on my '84 XJ-S), Then I use the long bolt to adjust the bell crank to the rpm I want. To make it even easier, I forced a short piece of vacuum hose onto the long bolt, at the head. This makes it easy to adjust the bolt with your fingers." This plan will work on either LHD or RHD cars, simply using whichever cable bracket is unused.


CENTRIFUGAL AND VACUUM ADVANCE DATA: When checking distributor advance versus tables, make sure you are looking at the H.E. or pre-H.E. tables, as appropriate. The H.E. system develops less advance in the centrifugal mechanism and more advance in the vacuum mechanism than the pre-H.E..

The data on the pre-H.E. is inconsistent. Page 05-1 of the XJ-S Repair Operation Manual, ©1975, lists data for the centrifugal and vacuum advance mechanisms and Section 86.35.29/4 of the same book lists data for bench-checking the distributor, but the centrifugal advance data doesn't agree. Values listed in distributor degrees and distributor RPM should be exactly half of the engine readings, since the distributor turns at crankshaft speed. Page 05-1 of the Supplement to the Repair Operation Manual, ©1982, lists the same values as page 05-1 of the repair manual itself, but carefully-taken measurements of the distributors from John Nuttall's 1977 and 1979 cars indicates that the actual advance curves match the values indicated in Section 86.35.29/4.

Note that all of these charts are referring to how much the advances change, not to absolute values read from the scale at the front pulley. For example, if it says the centrifugal advance mechanism should provide no advance at idle, that does not mean you can set the timing at 0 at idle. It means that the advance mechanism has not yet begun to move, so the timing at idle should be the same as the static timing. And, if it says the centrifugal mechanism provides 18 to 22 advance at 4000 RPM, it doesn't mean to set the timing at 18 to 22; it means the timing should be 18 to 22 more advanced than it is at idle.

Regarding the charts for pre-H.E. advance, Mike Morrin says, "I suspect the service manual only has data for one of the distributor variants fitted (probably C44663). The parts book shows that there were 5 different models of distributor fitted to pre-HE XJ-Ss, and I think that the only difference was the advance curves.

C.43735 Australia (this is the one with the peculiar vacuum retard system).
C.44663 California to car 2W54183
C.43735 California from car 2W54184
C.44663 CDN/USA to engine 8S5461 (ie the 4460th engine what year??)

All other countries to engine 8S5202

C.46173 CDN/USA from engine 8S5462

All other countries from engine 8S5203

DAC1609 California from engine 8S11161 and Australia from 8S11800
DAC1380 All other countries from 8S11262

"Note that my parts book does not cover the [pre-H.E.] Digital-P cars, so there are probably 2 more I have not counted.

"On reflection, I am surprised that there is not a clearly different distributor model (advance curve) for 8:1 and 9:1 compression engines. As far as I can see, the distributor seems to change according to degree of emission control, not compression. For example: Australia (9:1) and California (8:1) get the DAC1609, all other countries presumably including the UK (9:1) and the rest of the USA (8:1) get the DAC1380."

For our information, Morrin also provides part numbers for the distributors on the Series III E-Type:

C33148 up to engine 7S4663 (emission control) or 7S4879 (no emission control)
C37443 from above.

"The carburetted distributors of course have no FI trigger assembly."


VACUUM ADVANCE MODULE: On most cars, it is a simple matter to determine if the vacuum advance module is intact: connect a hose and, using your mouth, suck and watch the mechanism move. While in advanced position, put your tongue over the end of the hose, and test to see if it holds vacuum and stays in position.

On most cars, but not on the XJ-S! On the H.E., there is a vacuum regulator in the line to the vacuum advance module, and such regulators cannot be depended on to work properly with no flow. Therefore, the vacuum advance module on the XJ-S has a deliberate bleed hole, so the module will not hold a vacuum even when in perfect condition.

According to Michael Neal, "The vacuum retention ability of the vacuum unit is dependent on the type of unit. The early units have a limiting adjuster on the end of the module. The common replacement unit has no adjuster and will bleed down slowly. The early units hold vacuum. The adjuster seems to simply limit the travel of the diaphragm."

Roger Bywater says, "Of course the high temperature situation also gives the vacuum capsule a hard time and they usually need replacing every couple of years or so but a lot of so-called mechanics seem to miss that one."

Note that a vacuum advance module that isn't leaking may still need replacement. Val Danilov says, "My diaphragm was fried rock hard, I broke it trying to test the rod movement (CRRRACK!), so I think it wasn't leaking."


VACUUM ADVANCE MODULE REPAIR: The following procedure for rebuilding a vacuum advance module is credited to John Napoli and Val Danilov.

Before removing the vacuum advance module from the engine, you might want to take note of the best position for the vacuum line connection when it is reassembled. And after removing the vacuum advance module but prior to taking it apart, it is suggested that you first carefully measure the distance which the actuator rod extends out of the unit. Also, push the rod back into the unit (fully retracted position) and measure that as well.

Raw material is a generic replacement vacuum advance from a Chevy V8 -- dirt cheap at any auto parts store. Take the Chevy unit apart by prying open the case; Napoli suggests it may be easier to open the case by grinding the case all around its periphery to weaken the metal, but be careful not to get it too hot and damage the diaphragm. What you want to get ahold of is the diaphragm/rod assembly.

Open up the stock Jag vacuum advance by prying around the crimped-on case. In this case, you'd like to reuse everything except the diaphragm/flat link assembly, so try to do as little damage as possible to the casing; if you'll read ahead to understand how you'll be reassembling it, you may decide to simply cut or grind the lip off the edge to avoid mangling the dome itself. Even though the diaphragm/flat link won't be reused, keep it on hand for taking measurements.

Cut the rod from the Chevy diaphragm/rod assembly to the same length as the Jag original, then set it down on an anvil or something and pound a flat spot at the end with a hammer. No problem -- it's a Chevy part, remember?

Reassemble the vacuum advance unit using the new diaphragm/shaft assembly and all the old Jag parts. If the casing wasn't damaged when disassembled, you may be able to simply clamp the unit togeher and gently bend/hammer the dome shut. This works but the cannister will become work-hardened so you will never be able to do this repair this way again.

If the edges were too mangled (or you cut them off), drill the body and dome for eight fasteners; Napoli suggests 3/4" long 3-48 fillister head screws and nuts, but notes that the exact sizes aren't critical. Sandwich the body, diaphragm (poke a hole through for the screws with a needle), spring, and dome, and bolt it all together, being careful that the flattened section of the rod is aligned properly. This method has the distinct advantage that it permits repeated repairs later -- important, since the heat in the valley of the V12 will eventually bake the Chevy diaphragm as assuredly as it did the original.

Check that the unit responds to vacuum and pulls the shaft in.

Measure carefully the extended length of the rod, mark, and drill a hole in the flattened area the same size as the hole on the original part. Check the retracted length as well, and if there is an adjustment screw on the dome adjust it to provide the same total travel (if your extended length was off a little bit, make the retracted length off by the same amount; the variance can then be corrected by setting the timing normally). If you don't have an adjustment screw, don't worry about it, the total travel will probably be close enough.

Reinstall on the distributor. Be sure to check the timing; the timing procedure requires that the vacuum be disconnected, but the module still has an effect -- you might not have gotten that extended length as perfect as you thought you did.

Note that the original diaphragm may have had a tiny orifice built into it to provide a deliberate leak. This will usually be evident as a tiny hole through the metal disks on either side of the rubber diaphragm itself. The vacuum advance plumbing includes a vacuum regulator and such regulators don't work reliably when there is no flow, so the orifice is in there to provide a little flow. It appears that this orifice may not be necessary, since some genuine Jaguar replacement vacuum advance modules reportedly don't have it. If having an orifice proves to be necessary, it can be added anywhere in that branch of the vacuum advance plumbing; you might wanna make the hole in the dome rather than in the diaphragm itself. It might make more hiss, but you can seal it later if you decide to with a small piece of aluminum tape. Or you may be able to fit a separate device with an orifice with a tee connection into the vacuum line. You could, for example, take a piece of brass tubing, drill a tiny hole in the side, and connect it in the line to the vacuum advance module.

If, for some reason, you are unable to reuse other parts of the original Jaguar vacuum advance unit, you may be able to use the corresponding parts from the Chevy unit. Note, however, that doing so may result in differences in the advance curve and/or limits. Use of a different spring will change the advance rate. Use of a different dome with a different depth, and therefore a different place where the spring seats, will also affect the advance rate. And use of a different dome may also affect the retracted length, allowing the diaphragm to travel too far or preventing it from travelling far enough.

If the Chevy dome happens to include a limit adjusting screw, you may be able to adjust the full-retract position after assembly to make sure it is the same as the original was. If there is no adjusting screw, you might actually be able to add one, being careful to seal it when done to avoid vacuum leaks. If the Chevy dome is shorter and compresses the spring too much, you may want to add a spacer ring between the diaphragm and the dome to space the dome back. If the Chevy dome is taller and compresses the spring less, you might want to insert something within the dome for the spring to sit on.

Napoli adds, "I have seen in this month's Jeg's catalog that Accel manufactures a replacement Chevy vacuum advance that is adjustable. The ad copy states that the unit is adjustable for total advance and rate. It comes with instructions. The unit looks just like the generic replacement I used, so the same repair approach should work." This fancy aftermarket Chevy part is still cheaper than the stock Jaguar vacuum advance module by a long shot.

If you tore things apart and then read the suggestions to take measurements first, Napoli sends some reference measurements from a junk unit he had laying around: "For reference, the measurements were taken with 1/16" of the adjusting screw exposed. That is, from the face of the adjusting nut to the top of the unmolested screw is 1/16". Also, this is a stock '88 V12 advance (non-Marelli) from my donor engine and the unit does not operate (won't build vacuum). With the unit relaxed, the length from the end of the housing to the center of the hole in the actuating rod is exactly 2 inches. Full compression of the rod reduces the length by 5/16" (i.e., 5/16" maximum travel)."


VACUUM ADVANCE MODULE -- PRE-H.E.: Mike O'Neill noticed that the vacuum advance module from an MGB will fit the pre-H.E. distributor. However, John Nuttall found that the MGB advance unit -- as well as some units for Triumphs -- look similar and will fit, but provide differing amounts of advance at different vacuum levels. So, if your engine is largely stock and you want to keep it that way, you probably should seek a Jaguar advance unit. On the other hand, if your engine is not stock and you need to provide nonstandard amounts of vacuum advance, this provides some places to look. Also, these units might provide materials for rebuilding your old vacuum advance module in much the same way Napoli used a Chevy part in the procedure described above.


VACUUM RETARD: Ian Macfarlane, regarding his 1985 XJ-S: "In Australia the distributor has a retard connection on the vacuum module which is supposed to operate for about 15 min. after starting via a solenoid valve. But the wire leading to the solenoid valve had been cut (as had the wire to the solenoid air switch). Thus, even though the distributor timing was correct with the vacuum tubes disconnected, the engine ran at about 6 deg retarded under normal conditions.

"Although this would not exactly apply to other emission systems it is another option for those with an overheating problem - if the vacuum system was not working properly it is feasible that overheating could occur due to retardation."


CENTRIFUGAL ADVANCE - SEIZING: At this point in this book, I used to report how the pre-Marelli Jaguar V12 centrifugal advance was highly prone to seizing. However, with more and more readers reporting back, I can say with relative certainty: Unless your distributor has been overhauled, it is seized right now. Period. This has proven to be a 100% failure rate, every time, every car, there simply is no doubt anymore. And a seized advance unit results in low power, overheating and major engine damage if not corrected. If you are not absolutely positive that your distributor has been overhauled already, it is imperative that you schedule an overhaul as soon as possible -- and drive the car very gently or not at all until it's done.

Folks, this is probably the single most important item in this book. Centrifugal advance seizure may be the cause behind most of the problems the Jaguar V12 has -- overheating, dropped valve seats, etc. -- and the reason behind most owner dissatisfaction, low resale value, Chevy engine swaps, you name it. Is there any way I can convince you, the XJ-S owner, to take action now? If it'll make you feel better, give me a call, I'll tell you in person: overhaul the distributor NOW.

Sad to say, reports are that this failure mode -- notorious as it is -- is not widely known among Jaguar dealers and mechanics. Despite the frequency of charging customers $10K for a new engine, it has never occurred to many of these shops that the engine failures were preventable. Some people have reported that their Jaguar dealer overhauled or replaced several unrelated components before finally figuring out that the centrifugal advance was seized. Others never figured it out. Perhaps their fancy diagnostic computers don't blink "centrifugal advance seized". Perhaps they just like collecting those fees for new engines. Whatever, keep in mind that just because you have had your car "checked out" by the dealer doesn't mean you don't have the problem. It's not even a certainty that the advance is OK if you specifically asked them to check it; some of these mechanics don't even know how to check it. For those who have mechanical abilities, it is highly recommended that you check your centrifugal advance yourself, or better yet simply go ahead and overhaul it. If you aren't the type to get your hands dirty, it probably would be money well spent to have an independent shop confirm the first shop's conclusions.

John Napoli confirms this: "I know or deal with about a half-dozen repair shops. Some are run by friends (no chicanery ever) and the others, including my local Jag dealer, are pure business relationships. None of them ever heard anything about the chronic Jag problem of seized centrifugal weights. Not even the Jag dealer, although maybe they do know and don't tell. I'll go further and say that no Jag owner I've encountered has ever heard of it, either. You get these strange looks when you bring it up ("If I don't know, then it can't hurt me."). I find it hard to believe, but it looks like a lot of the information regarding the idiosyncrasies of these cars never reaches the people who can use it most. Perhaps this is part of the reason why so many Jag owners have bad experiences with repair shops."

The usual indications of a seized advance mechanism are an XJ-S that lacks power at higher RPM and a nasty tendency to overheat. In some cases, when the throttle is backed off suddenly there is a brief power surge before decelerating. If your car doesn't have the performance at higher RPM that it should, check the advance mechanism immediately; a distributor seizure is easily fixed, but warped blocks, dropped valve seats and burned pistons are much more expensive and are the inevitable outcome of continued operation.

To confirm for yourself that your advance unit is seized, remove the distributor cap. Attempt to turn the rotor counterclockwise. If operating properly, you can easily turn it about 11 (H.E.) or about 18 (non-H.E.) against a spring. Note that this thing should feel really loose, and when you twist it and let go it should snap back with a metallic clink; if at all sticky or gummy-feeling when turned, time to overhaul. The forces that move this thing are balanced against each other, so a very slight difference is supposed to make it move; even a slight amount of drag or stickiness is enough to keep it from moving as it should. It may be movable by hand but not as far as it should or not as freely as it should. Of course, if totally seized, you cannot turn it at all except for backlash in the drivetrain.

Another method of checking for a seized distributor is to compare the timing (with the vacuum line to the distributor disconnected and plugged) at 2000 RPM with the timing at idle (under 900 RPM). The two readings should differ by at least 12 degrees (H.E.) or at least 22 degrees (non-H.E.). If they are the same or very close, the centrifugal advance is seized.


OK, SO IT'S SEIZED -- NOW WHAT? To prevent/correct a seized centrifugal advance mechanism, it's necessary to overhaul the distributor, clean out all of the original lubricant, clean up the parts, and reassemble with a reliable lubricant. Roger Bywater says, "Most of them probably run for years without the cap being lifted so by the time anyone does get round to it the spindle is likely to be running dry and getting built up with debris. Adding lube at that stage may well loosen the debris and aggravate the situation so really it needs complete stripping to ensure that it will not jam at some future time." Bywater, having worked for Jaguar, can be expected to blame the seizures on poor maintenance -- and perhaps he's correct. I personally suspect the original lubricant used at assembly was lousy, based on what the stuff looked like when I got my distributor apart! It really doesn't matter what the cause; either way, the distributor has to come apart.

Of course, you don't have time for overhauling. You will probably want to try the easy fix: remove the distributor cap and rotor, and put some penetrating oil down the center and see if you can work it loose. If you are successful, it is recommended that you check its operation regularly -- or better yet, just go ahead and overhaul it when you get the chance. The results of operation with a seized centrifugal advance are simply too costly to ignore.

This is one place where getting a new part will probably not help. There is little wear in the distributor, and an overhauled and properly lubricated used part can be expected to last the life of the car. But a new distributor may have the same problem as the original distributor did. It is recommended that even a brand new in-the-box distributor be overhauled prior to installation (to get that lousy lubricant outta there).

The good news: the reported incidence of centrifugal advances seizing after they have been overhauled is nil. This may be because the original grease is the problem, and once it's removed the problem goes away. It may simply be because once the overhaul has been done the owner understands what's going on and lubricates it regularly. Whatever, once the distributor has been overhauled, there is apparently no reason to expect to ever have to do it again.

Randy K. Wilson points out that the problems with seized centrifugal advances on pre-H.E. engines may actually be different in nature. "From what I've seen the gummy advance problem is unique to the V12 H.E. distributor. Earlier distributors also had a problem with seizing, but these galled from a total lack of lubrication." Of course, this could mean that the H.E. only gets hot enough inside the distributor to turn the lubricant to varnish while the pre-H.E. got hot enough to burn the lubricant away entirely. Or, it could mean that Jaguar added grease to the assembly in response to the dry seizures, and the grease caused its own seizures.


DISTRIBUTOR REMOVAL: If the centrifugal advance is seized, or if you wish to prevent seizure in the future, the distributor should be removed and disassembled. It's not difficult, but it will help to read the following sections before tackling the job.

Before removal, care must be taken to ensure the distributor can be reinstalled with the gears meshed correctly so the rotor faces the same direction as before. If the crankshaft is not to be disturbed, this is very simple: note the position of the rotor prior to removal, so you can be sure it is in the same position when reinstalled. It's a gear mesh and one tooth off would be obvious, so just note the position well enough you can make sure you are on the same tooth when reassembled.

Note: if you have the older Lucas Opus ignition system, you are advised to position the engine at TDC on cylinder 1A, which is where the timing marks on the crank pulley line up and the rotor is pointing at the front left headlight. If the rotor is pointing at the right side door, you need to turn the engine around one full turn. The reason that this is important is that the Opus has a wheel within the distributor that has three slotted holes that line up with the distributor mounting bolts in this position. Even if you can get it off without moving to TDC (by busting up the plastic wheel, for example) you're still gonna be in a pickle putting the rebuilt distributor back on without tearing it back apart or doing some of the fiddling mentioned below.

If you turn the crankshaft while the distributor is off, you face additional problems. The distributor turns once every time the crankshaft turns twice, and therefore the distributor must not only be lined up facing 1A while the engine is at 10 Before Top Dead Center (BTDC) but it must be on the correct stroke. If not, the spark plugs will fire at the end of the exhaust stroke instead of the compression stroke, and the motor will not run.

To get the setting right, cylinder 1A must be on the compression stroke when the distributor is installed pointing to 1A on the cap. If you know in advance you may be turning the engine, it is easy enough to avoid problems. While the distributor cap is off and the rotor is pointing at 1A and the crankshaft is at 10 BTDC, remove the oil filler cap. Reach beyond the front edge of the chain and mark the camshaft gear itself (the camshaft gear turns at the same rate as the distributor). You can use a dab of paint, or stake it, or whatever will give you a clear indication.

You might also want to mark the position of the base of the distributor, since it mounts on slotted holes. Ideally, the base should be mounted so the correct timing is near the center of the range of the eccentric adjuster, but to begin with it will be easier to put it back where it came from. Peter Smith: "I find that using typist's white out to paint across components can be helpful. It can be applied over slightly greasy surfaces and if precise alignment is necessary I can draw on it with a biro."

You can insert an allen wrench through the openings in the distributor and undo three screws and remove the distributor as a unit. However, for the purposes of doing the marking mentioned above, it may be preferable to proceed with the disassembly described below with the distributor still on the engine until you've removed enough parts to be able to mark the bottom of the housing on the inside.


DISTRIBUTOR DISASSEMBLY: Remove the rotor arm. Right off the bat, this is likely to be somewhat difficult, since it seems to jam. All you can do is twist, rock, and pull, and hope you get lucky and don't break it. Or just have a spare on hand. If the rotor carrier shaft seems to want to come upward with it (the retainer is broken), then you might want to try to hold it down by inserting a screwdriver through an opening in the anti-flash shield or some such. Pulling upward on the rotor carrier shaft may distort the centrifugal advance springs far below.

Remove the EFI trigger board or anti-flash shield (whichever). The biggest problem with removing the anti-flash shield will be dropping the tiny screws and washers, but the trigger board causes more trouble. It is mounted with four tiny plastic screws, and Jan Wikström reports that the screws get stuck to the metal; "On my own experience of three distributors, you can expect to get two out intact. If you're very careful." He also reports that he has used metal screws in a pinch with no apparent problems. When installing the plastic screws, maybe it would be a good idea to use anti-seize compound, even though it looks silly on tiny plastic screws.

Next, remove whatever is used to trigger the ignition; on 1982-89 cars, you need to remove a C-clip, then slide off a wavy washer, then pry out a U-shaped pin. Finally, you need to slide off the iron star-shaped inductor rotor, being careful not to damage it or deform the points. Even more importantly, don't just pull upward on a sticky star rotor, because you may stretch the springs in the centrifugal advance mechanism underneath. Here's a method that seems to work: Lift gently on the star rotor a little bit and insert the claw of a claw hammer under the star rotor. Rest the head of the hammer on the edge of the distributor housing, but do not pry. While holding the star rotor in its slightly elevated position with the claw hammer, tap downward on the center of the rotor carrier shaft with a plastic-surfaced hammer. Once the end of the rotor carrier shaft is flush with the surface of the star rotor, if it still won't simply slide off you can position a small socket with an OD slightly smaller than the rotor carrier shaft on top and continue tapping.

On the pre-1982 cars, a plastic disk with 12 ferrite inserts was used; according to Jan Wikström, "Its hub is about an inch long (estimated; I've never seen an intact one) and all beneath the disk. It's a tight fit on the spindle and in its keyway. And the disk, which is all you have to apply force to, is about 3/16 in thick near the hub. On an old car like mine, the only way to get the disk out would be by breaking it up."

Fortunately, John Nuttall found a better way: "I discovered a technique for removing the circular plastic timing rotor which, according to Jan W in your book, often breaks under extraction. The idea is to undo the three screws which hold the movable part of the distributor body to the base. This can be done with the rotor in place. These screws have springs on them to allow the body to rotate when the vernier is turned. It is then possible to push upwards on the rotor with the body with the force being applied uniformly very close to the central shaft of the rotor - much better than pulling at the edge."

Unfortunately, Wikström counters: "It's better than fingers, although it still bears on the thin part of the disk outside the hub. But sure, it's worth trying before you break the disk up. One hitch: if you don't have the distributor with the little screw inside the top of the shaft, lifting the micro housing (charming name, isn't it?) will pull the centrifugal advance mechanism apart without shifting the disk." And stretch your advance springs beyond recognition. If you can't rely on the retainer to hold the rotor carrier shaft still, you must press down on the center shaft with your thumbs while pulling up on the housing to get the disk off.

Whichever distributor you have, you must remove the three screws with springs and lift off the movable portion of the base to gain access to the centrifugal mechanism below. You could leave the pickup assembly in place, but on the 1982-89 it's in the way for getting to one of the screws with springs; since it's only two screws to remove the pickup assembly, that's the easy solution. Of course, it's a good idea to inspect the vacuum advance mechanism as well as the pickup module while you're there.

Disconnect the springs from the centrifugal weights, noting how they are installed. Yes, the two springs are different, but it doesn't matter which one goes on which weight.

Remove the felt from the top of the rotor carrier (if there is any) and observe the retainer underneath. This is a possible cause of trouble. The early cars had a screw there, but on later cars there's a nylon clip instead. The nylon clip is usually brittle and cracked, sometimes allowing the rotor carrier to rise on the distributor shaft and possibly causing interference damage down in the advance mechanism. If you don't have a screw, the procedure for removing the rotor carrier is to yank, which usually results in breaking the nylon clip.

Normally the rotor carrier shaft would slide off the distributor shaft at this point, but if it is seized some Liquid Wrench or other measures may be called for. Please try not to bend the distributor shaft -- it is remarkably thin within the rotor carrier. Once apart, clean all the crud off the bearing surfaces. Decide for yourself whether Bywater or I am right, whether it seized due to lack of lubrication (dry) or a lousy lubricant (gummed or varnished).


DISTRIBUTOR SERVICE KIT: A distributor service kit, part number DZB105, is available; it is often called an "anti-flash shield kit" since the anti-flash shield is the most obvious part in the bag. The kit contains parts usable on all pre-Marelli distributors.

The mail order catalogs may not list what parts are supplied in this kit, so the list is provided here:

Clear plastic anti-flash shield (used on 1982-89 only)
Screws and washers for anti-flash shield (4 ea)
Ignition pickup mounting screws (2 ea - used on 1982-89 only)
Distributor cap gasket
U-pin for reluctor positioning (used on 1982-89 only)
Reluctor retaining clip and wavy spring washer
Replacement carbon contact for distributor cap, with spring
O-ring for distributor-to-engine joint
Nylon rotor retaining clip (see note under Distributor Disassembly above)

Note that it may not be at all necessary to obtain this kit to perform a distributor overhaul. The existing anti-flash shield, screws, cap gasket, U-pin, retaining clip, wavy washer, and carbon contact may all be reusable. The O-ring can easily be purchased locally, and you'd be well advised to find a Viton one. And the rotor retaining clip, while not likely to survive disassembly, can be replaced with a small washer and tiny O-ring -- see CENTRIFUGAL ADVANCE REASSEMBLY below.


DISTRIBUTOR SHAFT SEAL: The Jaguar V12 distributor has had two common problems, centrifugal advance seizing and cracking distributor caps. Both of these problems have been attributed to crankcase vapors entering the distributor between the shaft and the housing. Supposedly the vapors condense into the type of varnish found inside engines, seizing the advance mechanism. Vapors collecting within the cap are ignited by the arcing of the distributor and cause an explosion which cracks the cap.

Both causes are in question. Hard deposits that seize the advance may very well be the original lubricant exposed to too much heat. And Jaguar's reported reason for venting the later distributor caps was to deal with high temperatures that caused the caps to crack, although clearly the vent system could also remove collecting vapors. It is noteworthy that the later, vented distributors are still prone to advance mechanism seizing, and the still later Marelli distributors are vented even though they have no advance mechanisms to seize.

There is a seal between the shaft and the distributor housing, but it gets hard and brittle, probably due to the heat in this area. If the distributor is being worked on, it might be a good idea to replace this seal. Note that removing the distributor drive gear retaining pin requires some grinding, and reinstalling the pin requires a tack weld to hold it in place. There's a bearing in there, too, you might wanna renew it while you're there.


THINGY: Highly technical term referring to a specially-shaped plastic thrust washer at the bottom of the rotor carrier shaft, just below the centrifugal advance weights. It has ears on it that appear to provide an inner stop for the centrifugal weights, apparently to reduce noise but effectively limiting how retarded the timing moves at idle. It's also a British non-metallic part, so it may be cracked or broken.

If you're lucky enough to have an intact thingy, you need to take care to reinstall it properly during reassembly. If you just look at it and line up the notches with the "wings" on the cam plate on the bottom of the rotor carrier shaft, there's a better than even chance you will have installed it wrong. There is a pair of pins on the thingy that hold it in position relative to the cam plate, and the correct position of these pins is shown in Figure 6. Note that for clarity the thingy itself is not shown, only its pins.

If your thingy is toast, bad news: this part is apparently unobtainable. It's not included in the DZB105 kit. One frustrated owner reported: "I phoned Lucas Aftermarket Operations, Parts and Service. Apparently, they have routine service parts which are readily available at most dealers, other service parts which they can provide part numbers for and which are often available, and then there are non-service parts which of course the thingy and the seal fall under. The parts did not show up on his drawings although he did have DZB105. He could see the shaft bearings in the drawing, but no seal and no plastic thingy. I asked what happens to parts that are not on their drawings anymore. I was advised that once it is not profitable to sell replacement parts, they are removed from the aftermarket listing and so, to Lucas Aftermarket the part numbers are no longer available. He told me that the part numbers could only be obtained from the factory drawings once they were declared non-service parts. He also advised that they factory would not talk to me about this because that is what Lucas Aftermarket is for. So, Lucas Aftermarket is apparently the only information source Lucas has for part numbers and they do not have these parts listed anymore."

You're screwed. The only option -- and it's a perfectly workable one -- is to install a generic 5/16" ID flat thrust washer in place of this thingy. Val Danilov adds "I wouldnt use any washer there larger than 0.5in OD, as it may interfere with the weights." The thrust washer also needs to be at least .060" thick, although if thinner you could merely stack two in there; the ideal thickness is around .078", but you could probably install anything up to about .120" without any problem. The loss of the inner stops for the centrifugal weights will evidently allow the timing to retard a fraction of a degree more at idle than before (timing is set at 3000 RPM, well out of the influence of this thingy, so timing anywhere except idle will be unaffected). There also might be some noise at idle with the weights hitting the metal shaft instead of the plastic stops, although how you would ever hear it above the general din of the engine idling is beyond me.

Plastic, brass or bronze will work nicely; steel might gall against the other steel parts, and aluminum or copper may be too soft and wear quickly. Danilov doesn't like the idea of plastic, since this is the type of application where choosing the wrong plastic will result in a part that gets hard and cracks with age; he found a porous bronze thrust washer at an Ace hardware store with the 1/2" OD and was able to drill out the ID to the correct size. This sintered bronze bearing material is ideal, since the entire purpose of the material is for use as a bearing between steel parts. Another idea would be to buy a 5/16" ID - 1/2" OD bronze sleeve bushing and slice a suitable ring off one end of it (if you find a place that sells them, you might want to pick up a couple more such bushings; see the section on throttle linkage bushing replacement).


CENTRIFUGAL ADVANCE REASSEMBLY: Do not grease the sleeve bearing area between the rotor carrier shaft and the distributor shaft before reassembly; this bearing should be lubricated with synthetic engine oil only. Find a light, high-temp grease (such as distributor grease) for the other moving parts of the centrifugal advance assembly, such as the pivots for the weights, the ends of the springs, and all the way around the cam plate on the bottom of the rotor carrier shaft.

Finding a replacement nylon clip apparently requires buying the entire service kit (see above), which you might want to consider anyway. However, considering the clip's failure tendencies, you may decide not to use it. It has been found that an excellent method to retain the rotor carrier is to insert a small metal washer that fits within the carrier but around the top of the shaft, followed by a tiny, chubby O-ring. Snap the O-ring into the groove on the top of the shaft so that it retains the washer in place, which in turn retains the carrier. David Johnson says, "I found a #61 O-Ring at Home Depot was a perfect fit in the groove. It has the following dimensions: 9/32" x 5/32" x 1/16". The O-Ring has a slightly smaller diameter than the top of the shaft. When you get it on it seems to stretch until it is a perfect fit for the groove." It might even be a good idea to try to find a Viton O-ring in this size, although since sealing isn't the issue and you'd be willing to buy a new O-ring whenever you take the distributor apart perhaps you don't care if it gets hard and brittle. The washer needs to have an OD less than 13/32" (10.3mm) but larger than 5/16" (7.9mm), and the ID needs to be at least 3/16" (4.8mm). If these parts are installed properly the carrier should have a slight amount of up and down play, but if excessive it might be a good idea to install two washers or more; you don't want the rotor carrier shaft to be able to slide up far enough to disengage the pins on the thingy down below.

Another suggestion: Roger Bywater says, "Pulling the spindle apart invariably destroys the plastic retainer at the top but we find a small push on trim lock washer works just as well."

If there is no felt plug in the top of the rotor carrier shaft, install one -- or just jam a cotton ball in there. Something needs to hold some oil.


DISTRIBUTOR OVERHAUL -- PAY SOMEONE ELSE VERSION: Jeff Elmore points out that British Car Service will overhaul your distributor for you. "I called them and they said that they do the whole seals, bushings and new springs. They said that they have an assortment of springs and they keep trying until they get the right advance curve."


DISTRIBUTOR INSTALLATION: Since you read this book first and marked the parts correctly, installation is a snap. If the crankshaft has not been disturbed, reinstall the distributor with the rotor pointing in the same direction it was before removal (note: correcting a seized advance mechanism may make the rotor point slightly differently, but it's real close. You'll know if you miss by one tooth on the drive gear). If the crankshaft was turned, remove the oil filler cap, turn the engine until 10 BTDC and the mark you made on the sprocket is showing, and install the distributor with the rotor pointing towards the 1A contact on the cap.

If you didn't plan ahead and the engine has been turned, it is not so easy. If you just take a blind shot at this you only have a 50/50 chance of getting it right. There is a mark on the jackshaft, but the jackshaft turns at the same rate as the crankshaft, twice the rate of the distributor, so that mark is of no help.

To ensure correct positioning, the 1A spark plug can be removed and the engine turned over with a thumb over the hole; the compression stroke can easily be determined. However, getting the 1A plug out and a thumb over the hole is difficult on the H.E. (it's much easier on the pre-H.E.); alternatively you can check for compression on cylinder 6A (right rear cylinder). This cylinder fires exactly one complete crankshaft rotation before and after 1A. Therefore, you can find the top of the compression stroke on 6A and then turn the crankshaft through one complete turn. Or, you can leave it there and install the distributor such that the rotor points to the 6A contact instead, exactly opposite the 1A contact.

To place the distributor housing in the right place on the slotted holes so the vernier adjustment has a usable range, merely align it with the marks you made before you took it out. You didn't mark it? Tsk, tsk. Read on.

Michael Neal claims, after working on these cars daily, that the optimum location of the distributor body on the three slotted holes is always the same. The correct position "is with the distributor body set at 3.5 degrees retarded at the adjuster. The centerline of the rotor will be 75% past the No. 1 line on the inner shield. The slots for the holddown allen bolts will be 2/3 past the allen heads. The tooth on the pickup will be roughly 5 degrees past the center of the pickup point. Note that the distributor turns anti-clockwise. This setting will give you near perfect timing almost every time with adjustments both ways. Other settings tend to give too much retard or advance with no adjustment."


PREVENTING THE DAMN THING FROM SEIZING AGAIN: After you've overhauled the distributor and fixed the seized advance, the rotor carrier/distributor shaft interface must be lubricated regularly; the Lubrication Chart in the Repair Operation Manual says two or three drops of clean engine oil in the felt under the rotor every 6,000 miles (every oil change - no, Quickie Lube is not likely to be doing this!). This is not easy, as it requires removing the distributor cap, which in turn requires removing the cruise control unit and all the ignition wires. This lubrication procedure is obviously very important; it is recommended that owners do it themselves, or make very sure their mechanic isn't skipping this part of the maintenance. Jan Wikström suggests installing a new felt pad, then taking it in for service. If the pad is dry when checked afterward, cease doing business with that shop. Perhaps good advice, but has a significant probability of eliminating every shop in the area from consideration!

Note that many XJ-S distributors don't seem to have any felt. Considering the known problems here, it'd probably be a good idea to put some in. Or, just a wad of cotton.

Since the seizing appears to be caused by the lubricant getting baked, normal "clean engine oil" might not be the best idea. I recommend that a synthetic engine oil be used, since it won't tend to varnish or sludge and withstands higher temperatures without breaking down. Soak the felt or cotton thoroughly every time you're in there, and make sure you're in there often enough. It might not need to be as often as every 6,000 miles -- especially if you're using synthetic motor oil -- but it's better to be safe than sorry here, so make sure you do it often until you personally have confidence that you can go longer between oilings without the thing drying out or gumming up.

It is suggested that spray lubricants of the flammable variety be avoided within the distributor. When running, there are sparks between the rotor and the electrodes in the cap; vapors will be ignited upon starting, ruining your day.


DISTRIBUTOR CAP REPLACEMENT: Randy Wilson has a low opinion of the aftermarket products, and recommends buying only the genuine Lucas parts: "The last time I checked, there was only one aftermarket cap available. I ordered in a lot of them... and every single one was defective in one way or another. Missing center buttons, missing vent tubes, pre-cracked, molded oval... Flimsy pieces. They're about 1/3 the price of the Lucas pieces... and not worth it." Note that the author has also purchased one of these aftermarket items, and in fact had trouble with the vent tubes simply falling off. Rendering it usable required careful application of JB Weld.


DISTRIBUTOR CAP VENTING: The 82-89 distributor cap has two fittings for a positive ventilation system. Air is drawn through a small filter, through the distributor, and into the engine side of the air filter housing. The filter is normally found laying against the left wing at the crossbrace attachment, and sometimes concerns owners because it appears to have been disconnected from somewhere.

Reportedly this system was incorporated because the earlier distributor caps were cracking due to excessive heat, although some suspect removal of flammable vapors was the real reason -- see the DISTRIBUTOR SHAFT SEAL comments above.

The connection to the air filter housing is not actually to the housing directly, but rather to a hose between the housing and the supplemental air valve. This is therefore a special molded hose with provision for a tiny fitting. The fitting itself is a plastic elbow, and is often found to be broken. It is not to difficult to find a suitable replacement, though, and you might even find a brass one. Or, you can forget the whole thing and install a generic piece of hose between the air filter housing and the supplemental air valve, and install a small fitting right onto the inner side of the air fliter housing for connecting the hose from the distributor.

The filter itself is no more complicated than it looks. Any suitable fuel filter will serve as a replacement.

If you would like to help this system out, reroute the intake filter to somewhere in front of the radiator. This will draw cooler air, and will also draw more air since this is a high pressure area. The cooler air should also help the electronic ignition pickup last longer.

It's possible that this ventilation scheme was retrofitted to earlier cars; SICP lists a "ventilated" cap for these systems. It may also be possible to drill the older cap and fashion suitable fittings, one in the side for an inlet and one out the top for an outlet. A few pieces of hose, a fuel filter, and a connection to the engine side of an air filter housing and it's done.


Lucas Opus Mark 2 (pre-1982)


LUCAS "OPUS" MARK 2 IGNITION -- DESCRIPTION: This is a brief description of how the pre-1982 ignition system works, based on the more detailed description in SAE paper 720163 on the development of the engine. It is not applicable to the 1982-on systems.

The pickup within the distributor consists of an E-shaped transformer with one input coil and two output coils. The input coil is on the center leg of the E and is fed a 600KHz input signal from the amplifier. The two output coils are on the outer legs of the E and are wired in series so that their outputs cancel -- provided the two sides of the transformer are equal.

When one of the ferrite inserts built into the plastic rotor in the distributor aligns itself with one side of this transformer, it magnetically completes the circle on one side of the E. Since the output coil on this side is now more closely coupled with the input coil than the other output coil, its output is greater -- and the two no longer cancel each other. A transistor in the amp is toggled by the resultant output signal, triggering a spark.


OPUS IGNITION AMPLIFIER RELOCATION: If you have a 1982 or older XJ-S, the original ignition amplifier is a finned aluminum block mounted down within the galley between the cam covers. It gets cooked, primarily after the engine is shut off and all that heat from the block rises. Dick Russell says that intermittent failures characterized by the tach reading zero even when the engine is still turning are a sure sign the amp has failed.

Jaguar makes a kit to relocate this amplifier to the plate across the top of the radiator so it stays cooler. The kit includes a new amplifier and a new pickup for inside the distributor, and costs over $300.

It is possible Jaguar includes the amplifier and pickup simply because the only time their mechanics get a call to relocate the unit is after the original unit has fried. On the other hand, perhaps the reason the unit was originally located in such a sorry place was because the wiring needed to be as short as possible, and the replacement amp and pickup have updated circuitry necessary for the longer wires. According to Jan Wikström, the plugs and wires used on the replacement kit are different than the original, indicating you're supposed to replace the amp and pickup together. However, he simply spliced wires and used his old pickup, and it worked fine. He notes, however, that the wire color codes changed; it is necessary to open the amp and verify where the wires go to ensure they are connected correctly.

Clearly, crosstalk between the 600 KHz input signal and the output wire back to the amp would be detrimental to operation, and such crosstalk could be easily caused by the wires merely being located too close to each other for too great a distance. Also, the output wire picking up any other signals -- such as interference from the ignition wires -- would be ungood.

Reportedly, some owners have relocated their original amps by simply lengthening the wires, and have been successful. Others have not been successful. At least one owner reports that the official Jaguar relocation kit caused the wires to pick up so much interference from the spark plug wires that the car wouldn't run, and he couldn't get the system to work until he shortened the wires back to the length of the original.

Russell recommends that ribbon wire with five or more conductors be used to relocate the amp. By using every other conductor, the unused conductors in between provide adequate spacing between the active conductors to prevent crosstalk and interference.

Russell also recommends relocating the amp to the firewall rather than the top of the radiator. In his car, the amp was affected by water and crud thrown up from cars in front of him. Fortunately, he was able to repair it by resoldering some connections inside.

Perhaps another solution is to leave the amp in the valley and attempt to keep it cool there. Fashioning a heat shield from sheet aluminum for underneath it will help. Also, see the suggestion about cutting a hole in the A/C compressor mounting plate.

British Auto/USA claims to have "reinvented" this amp using modern electronics. The amp they offer, part number JLM368/R, looks exactly like the original and is supposedly durable enough that it may be located within the V; apparently some concours judges (yes, these cars are getting old enough to be considered classics) will subtract points if the amp isn't in its original location.

If you have a 1982-89 car with a black plastic amplifier mounted on top of the left intake manifold, you have no problem. Although close to the engine, this is actually a comparatively cool location and there is no significant history of heat-related failure.


IGNITION AMPLIFIER -- REPAIR: Referring to the Opus finned aluminum ignition amp, Jan Wikström says: "As for the amplifier itself, there is precious little pottery involved. I opened up the suspect one (four small hex-head screws underneath) and found that while there's a silicon blob at the cable entry and a silicon slurp for a seal around the lid, the inside is empty and there is complete access to PC board and power transistor. No need to $pend up on a new one if it goes belly up; this unit is eminently repairable."


PICKUP WIRING: Wikström warns: "One problem I've had on both cars is an intermittent break in one of the three ignition trigger wires coming out the front of the distributor, right in the moulded grommet. This seems inherent to the design, as the big, heavy three-pin connector flops around on loose wires and should cause metal fatigue as the wire bends back and forth. I have replaced the wires on the XJC (solder joints inside the distributor) and applied spade connectors instead of the three-pin job. I am about to apply the same cure to the XJ12L, which suffers from the same problem."


PICKUP CLEARANCE: Mike Morrin says "My car suffered for years from an internittantly rough idle. An oscilloscope connected to the ignition showed that at idle, one particular cylinder only sparked about half the time. It turned out that due to wear or some other marginal condition, the clearance between the pickup transformer and the rotating disk in the distributor needed reducing from 0.55mm to 0.50mm (both figures within specified range)."


TACHOMETER WIRING: Wikström reports: "the circuit diagram in the workshop manual is wrong in one particular: it shows the rev counter take-off from the top terminal of the ballast resistor unit; this doesn't work, as that terminal is at direct B+ (I tested). The rev counter is wired to the neg terminal of the coil on both my cars (with a resistor in series for whatever purpose)." Although Wikström's car is an XJ12C, this correction applies to the diagram in the XJ-S repair manual as well; there is a power wire and a ground wire, and the third wire (WS/U) is shown connected to a line from terminal 5 on the ignition switch to the ballast resistor. This would do no good at all, just supplying a constant 12V with no RPM signal.

On cars with the later ignition system, the tach wire -- still WS/U -- is connected to the ignition amplifier, and is shown correctly in the supplement to the repair manual.


OPUS IGNITION SYSTEM REPLACEMENT: Derek Hibbs reports: "My ign amp died as a result of convected heat last year. A reliable replacement unit was not available so the workshop installed a Crane Cams unit. The unit itself is located on the RHS air cleaner and it came with replacement pieces for the distributor because it uses an optical pickup as against the original magnetic pickup."

Bernie Embden came to the same conclusion. "A better solution is to use an aftermarket brand, which has wires long enough to locate the amplifier in a more friendly environment. I installed an Allison optical unit several years ago, mounting the amplifier inside the passenger compartment behind the glove box. This has worked flawlessly, and is significantly cheaper than the original Jaguar unit."

Welsh Enterprises offers two different Lumenition ignition systems for the Jaguar V12.

Note: Lucas designed the plastic wheel within the Opus distributor with three slotted holes that enable the mechanic to insert an Allen wrench, loosen the three screws at the very bottom, and remove the distributor as a unit. On the Lucas CEI (below) this isn't even an issue, since the star wheel used isn't large enough in diameter to impede access to the screws. However, if you install some sort of aftermarket system, you might want to consider whether or not you will be able to get that Allen wrench past or through whatever is used to trigger the pickup. Of course, it might not be your highest priority; you could always simply disassemble the distributor in place far enough to remove the wheel to gain access to the screws.



 Lucas Constant Energy ignition (1982-89)


BENEFITS: Roger Bywater describes the reasoning Jaguar adopted the Constant Energy Ignition system: "The 12.5:1 compression of the H.E. was too much for OPUS to fire and in any case it was getting to be a bit primitive by that time as it had no means of current control other than via the ballast resistor. The Constant Energy system has a "timed turn on" feature which maximises the coil charging time at high speeds whilst at lower speeds it limits current to a predetermined level. This makes possible the use of more powerful coils with lower primary resistance. At the time of the H.E. introduction very low resistance coils were not readily available so the unique dual coil arrangement was adopted to achieve the necessary spark power for high speeds.


TROUBLESHOOTING: Randy Wilson sends this procedure: "With the engine cranking, check for:

  • Power to the ballast source
  • Power to the coil + post
  • Ground switching on coil - post (if so, you should have spark)
  • Ground to amplifier case

"If everything passes except the ground switching on the coil, then it's in the amplifier or its wiring.

"A scope put on the pickup leads should give a modified sine wave pattern typical of magnetic induction sensor. At crank speed, I think it's around 1.5V peak-to-peak. No scope pattern is a bad pickup."

"I've never personally seen the pickup go bad, but have run into cracked wires in the pickup harness. Quite often jiggling the harness will cause the problem to "correct" itself; sometimes for many years. Everything working fine after doing the scope check is bad pickup wires."


IGNITION AMPLIFIER -- REPAIR: The Lucas Constant Energy ignition amplifier is a black plastic box mounted on top of the left intake manifold. This unit is clearly labeled "Lucas" and "Made in UK". The mail-order catalogs call for a part number DAB106, and want serious $$$ for it.

If you unbolt this unit from the intake manifold, turn it over, remove four tiny screws and remove the cover, you will see four components inside. The most predominant component is a GM High Energy Ignition (HEI) module. This unit is so common that you can find it on a bubble card hanging from a hook in any department store with an automotive section -- for around $20.

The other components include a fairly standard condenser and a zener diode. It is probable that this amplifier can be repaired by simply replacing the GM HEI module for considerably less than the cost of replacing the whole unit.

Richard Mansell found the following warning referring to his ignition amp:


Apparently, berylia is an ingredient used in a type of ceramic used to mount electrical parts. This ceramic conducts heat very well, helping keep the part cool. Unfortunately, berylia is really as dangerous as indicated, possibly worse. If you choose to replace the GM HEI ignition module within your amp, just unbolt it and bolt in the new one; don't go grinding or chipping away at things or sawing anything open, the dust created can kill you.


ELECTRONIC IGNITION PICKUP: The electronic pickup used in the Lucas Constant Energy distributor involves a magnet that is mounted with two screws. Be careful tightening these screws; the ceramic magnet is much more brittle than metal items, and can easily crack. If already cracked (notably around one of the screw holes), do not be concerned, it will not affect operation. However, be sure not to leave any loose or small parts inside that could come loose and move around within the distributor. It is better to discard small bits of the magnet that have broken away.


CONNECTORS: Mark Whitnell reports, "For the last several months the only way I could get it started was to use starting fluid. I checked out the spark since that was an area recommended by Kirby. The spark was very small and I suspect that the small spark could not ignite the gas but could ignite the ether (lower vapor pressure).

"I checked the distributor pick-up coil for the proper resistance according to Haynes (2.2K to 4.8K). I checked at the connector attaching to the ig amp first...very high megohms. Then I pulled the connector apart at the distributor. Checked the continuity of the wiring to the ig amp...checked ok. Then measured the resistance at the connector to the distributor...high resistance. Looking at the two prong connector...visually it looked fine. I went ahead an cut off the connector and measured resistance of the two wires to the distributor pick-up. Measured resistance... 3000 ohms within the range described in Haynes. Replaced the connector. Voila, starting problem solved.. good spark, etc."


COILS: The Lucas Constant Energy ignition uses two conventional ignition coils wired in parallel. The high-tension lead of the secondary coil is sealed off, and only the lead from the main coil is connected to the distributor. Between firings, energy is built up in both coils. When the 12V supply is broken ("the points open" in the lingo of the pre-electronic age), the energy stored in the secondary coil cannot escape through the high tension lead because it is sealed off, so the energy comes back through the 12V leads instead. The primary coil then not only has to release the energy it has stored itself, but also the energy coming back from the secondary coil. These two energies add to produce a powerful output at the high tension lead on the primary coil.

The secondary coil, located in front of the radiator, is not a spare or a backup; it is designed into the system for producing a good spark. If either coil goes bad, the performance will suffer. The secondary coil is not special, however, and can be replaced with a conventional coil provided the high tension connection is covered so that it cannot arc to ground.

Since mid-1989, XJ-S's have gone to a Marelli ignition system that also uses two coils. However, these two coils are totally separate; each one fires only six cylinders.

According to Alan Jenks, "Jaguar now recommends replacing both coils with a single "solid" (not oil filled) coil (#DAC 6093) that fits in place of the main coil. The aux coil and wiring is removed." Roger Bywater says, "The best coil to use on Lucas HE V12s is DAC 6093 (Ducelier coil - 0.62 ohms primary); only one needed and works well in place of earlier twin coil set up. If you can find another coil with primary winding resistance of 0.5 - 0.6 ohms maybe it is worth a try. If the resistance is any more than that it will not be able to build up enough coil energy to fire a spark at the higher end of the rev range when the coil "on time" is very short (about 1.4 milliseconds at 6000 revs). It might also struggle around the peak torque point. Although the V12 constant energy ignition module is fairly tolerant (it runs OK with the blanked second coil removed - albeit with a loss of spark energy) I have encountered some that behave very oddly if the coil is not the correct load match. The DAC 6093 might be a bit expensive but it does the job..."

He adds that the original parallel coil arrangement was "conceived when coils of sufficiently low resistance were not available."

John Goodman defends the earlier parallel coil setup: "The only difference (between the two coils) is the HT socket is sealed on the aux coil, and is no longer stocked by Lucas. The primary coil is still available from a Lucas distributor and is cheaper than the universal coil Jaguar sells to replace the two V12 coils. I am experiencing coil/ignition related problems on my car and have tried a few of these Ducelier coils (original equip on the XJR-S). I am convinced it is the coil causing intermittent starting and cutting out problems, thinking of resorting to the early twin Lucas coil system which never gave any trouble on previous cars."

Jim Isbell: "By the way, if you measure the primary, be careful because when you remove the meter the secondary sparks and it can really make you jump; also, when the field collapses you can feel the bite on the primary as well."


On to the Marelli Ignition


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