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Marelli Ignition

  Experience in a Book
Marelli Ignition


MARELLI IGNITION: In this age of electronic wizardry, many modern cars have done away with the mechanical centrifugal and vacuum advance mechanisms and do the whole job with microprocessors. In some cars, the distributor is done away with altogether, and separate ignition coils are used instead.

Since Jaguar incorporated the Marelli ignition system in 1989, this is essentially the type of system used. There is no centrifugal or vacuum advance mechanism; the timing is all done electronically. However, rather than using lots of separate coils, the distributor was kept, using two coils and directing the spark conventionally. This distributor is clearly lacking many of the internal parts of the earlier models, since it no longer handles the timing functions. The rotor is actually two rotors in one, and the cap is two six-cylinder caps in one, so each coil is operated as though it is running a six-cylinder engine. The computer that handles the timing is located near the passenger's feet, and has a vacuum line to it.

The source:

Magneti Marelli S.p.A.
Viale Aldo Borletti, 61/63
20011 Corbetta (Milan) - Italy
+39-2/972001 FAX: +39-2/97200355

As of this writing, this WWW site doesn't have anything on the XJ-S ignition system; only general info on the company.


CATALYTIC CONVERTORS -- MELTDOWN/FIRE (ONE-BANK FAILURE): Greg Maddison says, "My car has Marelli ignition and several mechanics have told me how faults in the system cause the car to run rich and ruin the cats. If you suspect a spark coil is weak or you might have a bad rotor, change them now is the advice I've been getting. Dealer told me they junked two cars last month because of fire from overheated cats."

While any misfire is a cause for concern with catalytic converters, the Marelli ignition system provides a unique and especially threatening possibility: running on six cylinders. While the various Lucas ignition systems were single 12-cylinder ignition systems, the Marelli is actually two separate 6-cylinder ignition systems. Hence, it is possible -- probable, even -- that sooner or later one of these ignition systems will fail while the other continues working. Since each half of the ignition system operates one bank of the engine, one entire half of the engine, along with its dedicated catalytic convertors and oxygen sensor, may see no spark all of a sudden while the other bank continues to run normally.

The V12 can be accurately described as two six-cylinder engines; each bank has the same firing order and the same natural balance as an inline six-cylinder engine (note: conversely, a V8 is not two four-cylinder engines). Hence, the V12 is actually driveable on six cylinders; rather than a "misfire", it runs quite smoothly, and drivers who have not read this section may decide it's not serious and press on. According to LaRue Boyce, symptoms to look for include: "Loss of power, more gas smell but no noise, just the no power feeling. Oh, the loss of vacuum also causes the transmission not to want to shift. You will know when it happens, there is no power, just enough to get you to 45mph on flat ground." Julian Mullaney says, "When my car started running on 6 it was very noticeable. I thought that the tranny was slipping at first because I had to give it so much throttle to get it moving. What was interesting was that it happened all of a sudden. I would expect a bit of intermittent running on 12, 6, 12, 6, etc. before finally shutting down one bank permanently."

Considering that people have been known to drive with seized centrifugal advances, inop vacuum advances, kickdown switches that don't work, one of the two intake butterflies disconnected, or various other problems, perhaps it shouldn't come as a surprise that some people would continue to drive with the engine only running on half its cylinders. There are lots of people driving around every day with their dashboards lit up like Christmas trees, and many drivers seem to feel that such things as idiot lights or drastic reductions in power output are cause to visit a mechanic within a month or two, whenever they get some time or money to spare. Unfortunately, the Marelli-equipped Jag V12 appears well-designed to remove such drivers from the gene pool!

Running a non-cat V12 with no spark on one bank fills the inop side of the exhaust system with fuel -- a disaster looking for a place to happen. Since the XJ-S exhaust system loops up and over the rear suspension, it should be able to hold several gallons of fuel before it starts pouring out the tailpipe! Since there is no spark in that bank at all, the owner might just get away with it -- but he'd better hope his ignition system doesn't mysteriously start working again! The same goes for a cat-equipped car that had an ignition failure when started from cold; the cats won't work when cold, so the fuel in the exhaust system on that side may never be ignited.

If the car is equipped with catalytic convertors and they were hot before the failure occurred, he will not be lucky; the cats will burn the fuel as it arrives, not only keeping them at operating temperature but getting considerably hotter in a big hurry. What's worse, if the car continues to be driven, more throttle will be applied to get any speed out of it, so even more fuel and air will be dumped into these cats. The inevitable result will be a cat meltdown and fire within seconds if the car is not shut off immediately. You don't want to be driving on six cylinders, not even for a minute.


AFTERMARKET DISTRIBUTOR ROTOR PROBLEMS: Unfortunately, a one-bank ignition failure turns out to be a common occurrence on Marelli-equipped V12 Jags. Although anything that kills one side of the Marelli ignition system can cause the car to run on six cylinders and threaten a cat fire, there is one possible cause that clearly outweighs all others: arcing through the distributor rotor to the shaft. This has been blamed on the aftermarket rotor since it has been known to arc through within 5000 miles of installation, while the genuine Marelli rotor apparently causes fewer problems.

The Jaguar-issue Marelli cap and rotor are inordinately expensive, even for Jaguar parts. This provides a powerful incentive for owners and aftermarket shops to look to cheaper aftermarket products -- Bosch, Beck-Arnley, World Parts, Lucas, etc. Unfortunately, the aftermarket items are apparently all made by the same company (InterMotor) and are highly susceptible to defects or failures that result in the engine running on six cylinders. This characteristically results in a fire or meltdown in the right side cats only, since it is always the "A" bank ignition system affected.

Boyce, who was lucky enough to suffer only a coupla thousand dollars worth of damage (cats melted down, cooked several items nearby), had problems with both Bosch and Beck-Arnley parts: "The aftermarket rotor had etching from the spark and you could see where the spark had jumped all over the rotor and cap. I am convinced that the aftermarkets are junk and both failed and caused problems. I will bite the bullet and put in the Jag cap and rotor when I next tune it up."

Randy Wilson: "The most common failure, the one that kills the A bank, is the center post of the rotor burns through, allowing a ground path from the rotor contact straight to the distributor shaft." Michael Aines had one such fault, but he also had a center coil feed on the cap burned away; perhaps one caused the other.

Boyce sent a set of defective aftermarket parts to this author for analysis. The biggest and most obvious problem was that the A bank conductor on the rotor, which contacts the center electrode, had burned right through the plastic rotor and was arcing to the distributor shaft, exactly as Wilson had described. The plastic of the aftermarket rotor is .150" thick at this point, which one would think would be more than enough to insulate against arcing over of this kind. It makes me wonder if the type of plastic itself isn't ideal, or whether there may have been flaws or voids in the molding. There are voids in it now, but it's only too likely those are secondary -- the area around the arcing is burned to a crisp. I don't see any obvious voids anywhere else in the part, but of course we could be talking about microscopic voids and I'm only using a 10x magnifier to inspect it.


AFTERMARKET DISTRIBUTOR CAP PROBLEMS: The problem with the aftermarket cap inspected by this author was less obvious: the spring behind the carbon brush on the center electrode wasn't long enough. If this contact is pushed all the way up into the cap, it barely comes back down to flush with the surrounding plastic. Since it is clear at what level the rotor was riding from the wear on the electrodes, I can establish that a carbon brush held at this position will not contact the rotor. Instead, the ignition pulses will be arcing from the carbon to the rotor, perhaps aggravating the already weak rotor.

Furthermore, upon pulling this spring out, I found that it hadn't been installed all the way -- perhaps because it is too short. As a result, it had been arcing inside between the top end of the spring and the aluminum cup inside the cap rather than being in direct contact with it. Of course, the spring was getting all charred and probably wouldn't last long in this condition.

Offhand, it's not known for sure if this spring problem actually contributes to the one-bank ignition failures. Mike Morrin says, "From my knowledge of high voltage dielectric behaviour in ignition systems, I would hazard a guess that the short spring may well cause the failure of the rotor and cap. I can easily imagine the effect being something like an accelerated life test for the rotor, causing micro-arcing, heating, cavitation and failure."


DISTINGUISHING MARELLI CAPS AND ROTORS: Since it is apparently of some importance to know whether you are buying the genuine Jaguar/Marelli caps and rotors or the aftermarket substitutes, some guidelines on how to tell the difference are in order. Randy Wilson provides the most basic indication: the genuine items will say "Marelli" right on them.

Regarding the rotor itself, the clearly-defective aftermarket one this author cut up is off-white and says "MADE IN ITALY" and "F 258 6E" on it. The metal parts are brass. Greg Maddison says, "I have a genuine Marelli rotor, it's orange and has the Marelli logo clearly embossed on the top of it. I think the contacts are brass."

Regarding the caps, Boyce describes the Bosch and Beck-Arnley items he had trouble with: "The aftermarket caps were identical (maybe just different boxes) and much lighter than the original. They appear to have aluminum as terminals while the Jag cap has copper (?). If you compare the OEM and the aftermarket side by side, there is no comparison!" This author inspected one of these aftermarket caps as well; it's black, and other than the plug numbers and the "A" and "B" coil connection labels, it doesn't say anything at all on it.

Just because the part says "Made in Italy" doesn't mean it's made by Marelli. Randy Wilson points out that "The cheap-junk cap and rotor is made by InterMotor, also an Italian firm. Note the weight of the new cap. If it is as you would expect for a piece of plastic that size, it's the imposter. The OEM cap will feel (weight-wise) more like cast iron."


GENUINE MARELLI ROTOR FAILURES: Julian Mullaney reports: "I recently had a very close call with my '91 V12. It quit firing on the A bank and the cat got red hot. Today I diagnosed the cause as a fault to ground through the distributor shaft via the rotor. This is precisely the way that Kirby diagnosed the fault with an aftermarket cap and rotor. There was a black pitted hole going from the center contact down throught the bottom shorting on the top of the dist. shaft (less than 50K miles on car).

"So why am I telling you? Because this was not an aftermarket rotor or cap. It was the original Marelli cap and rotor. Both have the Marelli name molded on them. So this means that these failures occur the same way with OEM equipment."

So, it appears that the aftermarket parts may cause problems sooner, but eventually the genuine Marelli items will too.


AVOIDING THE PROBLEM: There are already rumors that Marelli has ceased making these caps and rotors; even if false, it's likely they will cease making them sooner or later -- the V12 is no longer in production. And as we have learned, even the genuine Marelli parts may cause trouble. So, unless and until Jaguar issues a recall and installs redesigned caps and rotors (would be easy to do; keep your fingers crossed!) it will be necessary to make do with the parts currently available. Note that, as of this writing, the following ideas have not been tested, so if you put them to use be very sure to keep an eye on things and be certain to pull over immediately if symptoms of six-cylinder operation appear.


AVOIDING THE PROBLEM -- POTTING THE ROTOR: Perhaps the most obvious approach is to "pot" the hole in the bottom of the rotor where it sits on the shaft, in hopes of increasing the amount of dielectric material the arc would have to break through to get to ground. When the rotor is installed, there is a convoluted air space surrounding the top of the shaft within it, and the arc seems to break out of the plastic and run along the surface until it gets to the shaft. filling this space with a dielectric material should make it harder for electricity to break through, hopefully making the rotor perfectly reliable. Of course, I would only suggest trying this at all with a brand new rotor, rather than trying to refurbish one that has already burned through and is all charred and dirty.

The simplest idea is probably to set the rotor down on a table and pour enough potting material in to fill the bottom 1/8" of this hole. Care should be taken to make sure it doesn't get in the way of the shaft, possibly by carefully cutting or drilling some of the potting material back out after it hardens. The hole is shaped like an asterisk (*) and it's gotta be an airtight potting job to do any good, so either a very fluid potting substance must be used or a very careful job must be done filling all the little corners. Mike Morrin suggests, "If you need a 'runny' epoxy, use HV cable jointing epoxy (can't remember the brand name), but you need a mould, or it goes everywhere." No mold needed for this job, just set the rotor on the table hole side up. "The job we used it on also required it to run into a mould and be bubble free, so we mixed it, poured it in, and then heated it to about 100 degrees C to reduce the viscosity."

Pouring potting in the hole will still leave a lot of air space, of course, and Julian Mullaney points out that you might accidentally cause an interference with the fit on the shaft. "Yes, it is very difficult to tell. The plastic ears would flex down when you tighten the screws and you'd never know it." This may result in a stress failure of the plastic, and at speed this could in turn cause all sorts of damage. To avoid both problems, the second idea was proposed: make a dummy plug to simulate the shape of the shaft, and use it to completely fill the space while leaving a formed hole to fit the shaft.

A dummy plug can be made from a metric bolt or cap nut with a suitably-shaped 10mm hex and a flat washer. 6mm cap nuts with 10mm hexes are commonly used on bicycles and come in a wide variety of configurations, so a visit to a bike shop should provide a selection from which one could be found that closely resembles the shape of the stub on the shaft. Obviously, having a shape that makes a hole a little too big on one side or the other is no problem, but you don't want the hole any smaller; either make sure the plug is properly shaped, or make sure you cut any interferences out before installing. Spray a little release agent (Armor All, Pam, whatever) on the plug before using to make sure it comes out easily. Once the plug is created, it can easily be saved for use on replacement rotors in the future. Perhaps local Jaguar clubs could provide such a device, or even pot rotors for their members.

When preparing the plug, you might choose to forget the washer entirely and simply carefully insert the plug into the rotor by hand as far as you think it needs to go; you could scratch a mark on the side to indicate the proper depth. This would allow free access to the six chambers surrounding the center hole so you could see what's going on, air could come out, whatever. You could also opt to use a flat washer that's only slightly bigger OD than 10mm, thereby leaving most of these passages uncovered.

Mullaney suggests an alternate way to form a plug: "Take old rotor and pack it full of plastic molding compound. Press onto the dist shaft. Remove (now you have the negative). Pour potting compound into the rotor with the molding compound still in there. Remove potting compound (this is the replica of the shaft). Spray replica with Armor All (release compound). Install replica of shaft in new rotor. Pour final insulating potting compound into the void between new rotor and replica. Remove replica after potting compound is cured. Install the new "potted" rotor. Preserve replica in glass case for other jag-lovers to use."

Mullaney still ain't happy. "If we pot the rotor, it becomes essentially solid instead of a webbed plastic molded part. I'm concerned that our potting could expand when hot, and cause the rotor to undergo stress during operation which could cause it to crack over time. It's made of pretty strong yet brittle plastic. After all, the reason they don't just mold the thing solid is because it is less strong that way (thermoset vs. thermoplastic resins). If the rotor breaks then it would shatter the distributor cap ($$) and generally wreak havoc in the distributor."

Of course, the expansion issue might be addressed by selecting a flexible potting material. Andrew Corkan says, "I use urethane and silicone potting compounds for various products I manufacture. Generally your local supply house will carry this stuff in small quantities. The 90 durometer hardness stuff is the most common and cheapest, just a tad harder than your average rollerblade wheels. Try that stuff, it should be the combination of features you want. Also, it is best to pull a vacuum on the stuff to get the air out."

Mullaney is also concerned about adhesion, and recommends an RTV compound instead of potting compound. "Believe it or not, we happen to do a lot of testing of potting compounds where I work. We don't do anything with dielectric testing, but we analyze bonding of potting compounds to plastic surfaces extensively. It's very difficult to get a rigid potting compound to stick to a plastic surface because of thermal expansion effects. They tend to disbond with time and temperature cycling. Soft, compliant compounds are much better stickers. Hence the idea of RTV silicone. Silicones are designed to bond well to smooth surfaces such as ceramic, glass and smooth plastics."

Of course, the potting won't do any good if RTV isn't any good as a dielectric. So Mullaney "consulted with one of our resident Ph.D. material scientists to find the best dielectric barrier material for the interior of the Marelli distributor rotor. He (Trey Simendinger) prefers a silcone RTV material because it bonds well to the substrate, and will not delaminate due to temperature fluctuations. He recommended a GE brand silicone RTV standard window caulking material from hardware store. Get a clear one because this will have no filler in it (better dielectric properties). Typical dielectric breakdown is 400 v /.001 inch for silicones versus ~200 v/.001 inch for air. He says to wash the part with soap and water first to ensure a good bond of RTV to the plastic rotor material. He did not recommend potting compounds because they consist largely of filler materials like calcium which reduce the dielectric breakdown voltage and they do not bond well." A clear material has another advantage: you can see any air bubbles.

One idea may be Permatex Flowable Silicone Windshield Glass Seal. It's clear, and it flows -- unlike most sealants (it's about the consistency of honey). The flowing is good because it will help get down into the nooks and crannies inside this rotor.

Mullaney: "How about an alternative and even easier solution: Why not just put a nice big squirt of RTV silicone in the new rotor just before installing it? Of course you would need to take care that air bubbles were not trapped in the cavity prior to installation. I think an excess amount would encourage air voids to be expelled (of course clean up excess ooze). It would form nicely around the complex rotor shaft, and cure in place. Most importantly, it would remain compliant, therefore not exerting any stress on the rotor. I would wipe some oil on the distributor post first so the RTV will not stick to the metal, thus allowing easier removal of the rotor at a later date."

Of course, a combination of ideas may work; the nooks and crannies may be filled using a liquid with the plug, and then a small amount of a thicker RTV could be added to the finished rotor prior to installation as well.

The squirt-and-jam method may be easy, but you will need to wait for the RTV to fully set up before closing up the distributor and driving the car. Hence, the use of a dummy plug has the advantage that you can have the rotor prepared in advance, ready to install and run.


AVOIDING THE PROBLEM -- SHORTENING THE SHAFT: Since it is really the flange and two screws that hold the rotor in place, it might be a workable plan to cut part of the top of the shaft off. Enough should be left to center the rotor before the screws are put in, but it shouldn't need 1/4" to do that. If the 1/4" were cut down to 1/8" it should still work right, and that makes 1/8" farther the arc has to jump. And it allows more potting material to be stuffed into the hole; since the potting material has a particular dielectric rating per thousandths of an inch, increasing the number of thousandths the arc must break through will increase the number of volts it takes to short out. Of course, the dummy plug will need to be redesigned to match.


AVOIDING THE PROBLEM -- CLOSING THE PLUG GAPS: The collapse of the magnetic field in the coil causes a rapidly rising voltage. When the voltage is sufficient to jump the gap at the plug (and the gap in the distributor as well), it does so, and the voltage never gets any higher. This is why it's bad to have the thing firing without a spark plug connected; the voltage continues to rise and has noplace to go, so it may reach high enough levels to damage electronics (the Lucas CEI has a zener diode to protect it from such abuse). If the spark plug gaps are reduced, the spark should occur while the voltage is still at a lower value and the dielectric strength of the rotor doesn't need to be as high to prevent arcing. The V12's with the Lucas CEI ran just fine with .025" gaps, so the Marelli-equipped cars should as well.


AVOIDING THE PROBLEM -- STRETCHING THE SPRING: I can see no inherent problems with the aftermarket cap; the electrodes are aluminum, but I've had no problems with aluminum electrodes in caps for other cars. Offhand, I would suggest that the aftermarket caps would be OK to use with one proviso: prior to installation, the carbon brush and spring should be pulled out of the cap, separated, and the spring stretched. You will find that twisting the spring counterclockwise makes it easier to remove and reinstall; a counterclockwise twist pulls the coils a little smaller.

After some fiddling with the one I had to play with, I suggest the following procedure: Once the spring is out, you will note that it has a large diameter coil in the middle (to hold the spring in the cap), widely-spaced coils tapering gradually smaller in diameter toward the carbon brush, and a smaller constant-diameter set of coils stacked on top of each other (no compressibility at all) toward the cap electrode. I suggest turning this spring upside down and using the constant-diameter coils to push the carbon brush. It is a simple matter to stretch the coils out so they provide some springage. It is a little trickier but still not too difficult to compress the diameter of a couple coils at the end so they will grip down on the carbon brush. Gripping the carbon brush is only necessary to get the cap on without the brush falling down into the works.

Once you have that spring stretched out to a total length of about 1-1/2", install it tapered-coil-end first into the cap, making sure the upper end of the spring is firmly positioned against the electrode at the bottom of the hole. Again, twisting counterclockwise will help, possibly holding the end of the spring in a pair of hemostats or small needlenose pliers while twisting and pushing. Then the carbon brush should be pushed back into the center of the spring. To get the carbon brush to snap in, it helps to turn it clockwise; a clockwise twist makes the spring coils larger. When done, the carbon should protrude at least 1/4" from the plastic each time it is pushed back and released. If you have a VOM, check the ohmage between the carbon itself (without compressing it back into the cap) and the electrode up on top. It should be well under 1 ohm, but it won't be zero because carbon isn't really that good a conductor. If it's an open circuit (infinite ohms), you didn't get that spring seated against the electrode; pull it apart and try again.


ELIMINATING THE PROBLEM: Bill Freeman points out that one sure way of avoiding the Marelli ignition system problems would be to toss it and install an aftermarket ignition system that doesn't rely on the Marelli rotor and cap. Any of the systems that use six double-ended coils would do, but note that some aftermarket systems use only one or two coils along with the stock cap and rotor and therefore won't help.

Another idea might be to retrofit the earlier Lucas system into the later cars. It would be kind of a shame to replace the high-tech electronically-controlled Marelli with the comparatively archaic Lucas with its mechanical advance controls, but it would definitely eliminate the possibility of catalytic convertor fires from that particular cause. Note that the plugs would have to be gapped to the earlier specs (0.025", 0.64mm) and that the centrifugal advance mechanism should be attended to prior to installation.


ELIMINATING THE PROBLEM #2: Hey, the problem isn't the entire Marelli system, it's just the cap and rotor. One positive fix would be to eliminate the suspect cap and rotor altogether by replacing the Marelli distributor with two generic 6-cylinder distributors; connect each coil to one distributor cap and connect the plug leads accordingly, and you'll be good to go. Since the timing is actually handled by the crank and flywheel pickups, coordination of two separate distributors is not a problem; they merely need to be close enough for the spark to jump to the correct terminal. One other plus: if you choose the right 6-cylinder distributors, caps and rotors will be a lot cheaper than the Marelli parts -- even buying two of each!

Really silly idea, right? Well, before you dismiss it out of hand, note that Mike Morrin points out that the Jaguar V12 was designed for dual distributors! If you pull off the valley cover, you will find an extra bore for the jackshaft. The valley cover itself has a boss for a second distributor underneath the bellcrank pedestal, although it's not drilled. So maybe it's not such a silly idea after all.

Not easy, though. If you make this modification, you're gonna face two challenges: 1) coming up with a longer jackshaft with two distributor drive gears on it; and 2) devising a throttle linkage when you have a distributor where the bellcrank pedestal once stood.


DETECTING IGNITION FAILURES: As soon as the Marelli failure mode was discussed and understood on the online xj-s discussion list, several members proposed methods to detect such a failure and thereby avoid disaster. Andrew Corkan promptly designed a dirt-cheap circuit for detecting whether or not there are sparks going to each bank and providing a warning indication if they are not -- see Figure 7

Corkan describes his creation: "The circuit detects a spark in either bank by means of an induction pickup placed on a plug wire, similar to the ones used by tune-up timing lights. When a spark pulse is detected it shorts out an RC circuit that would otherwise cause an LED to light up."

"You have to make your own inductive pickup using a so-called RF choke (really just a two-part inductor core) and magnet wire. Open the core and remove half of it from the plastic holder. Wrap 50 loops of magnet wire around the half of the core still in the plastic holder. You do not want the magnet wire exposed in the engine so solder regular wire to it and pot that half of the pickup sensor, magnet wire and solder joint in RTV. The wires you solder on should be long enough to reach into the car where you will place the circuit. Replace the rest of the choke when you install it around the plug wire. The choke will fit around the plug wire, no need to remove the plug wire.

"There are two options for hooking up indicator lights (see dotted lines in circuit schematic). The first option is to use LED's. Just wire them as D2 and D4 and bring the wires into the interior. Kirbert suggested a second option of using the 'trailer' bulb in the dash as an indicator light. For this option you can wire up D5 and D6 to the base of Q3, which will in turn power up a 12 volt lamp." Note that in this latter case, a spark failure in either bank will light the same light; arguably, the driver doesn't really care which bank just failed, as long as the light tells him a bank just failed (with the Marelli rotor failures, it's always the A bank that fails). The owner may choose to use the trailer warning light, the seat belt warning light, or any other 12V light he wishes, and he may want to relabel the indicator accordingly, color it red, etc.

"All references in the schematic to "+12V" are to a switched (accessory) power source. When you hook things up you should see the lights come on when you move the key to 'accessories' but they should turn off as soon as you start the engine." Hence, no need to provide a separate bulb check circuit.

"All the parts for this circuit are available from Radio Shack (but our overseas friends should have no trouble finding these parts). In addition to what is listed here you will need wire to run from the sensors to the box you put the stuff in, and wires to run the LED's or the signal light into the dash (if you use that option). You will also want to get a small circuit board or breadboard and a little plastic box to put the circuit into; a 3 in. x 4 in. box is just fine."


Part #


Radio Shack #





R3, R5


R4, R6


C1, C2

470uF, 16v


Q1, Q2

IRF510, N-Channel MOSFET



TIP31 NPN Transistor (option B)


D1, D3, D5, D6

Germanium Diodes


D2, D4

LED any kind will do (option A)

L1, L2

RF choke (two part inductor cores)



Lamp on the dash (option B)

Magnet wire

any kind will do



any kind will do

Box and board

any kind will do

"It is easy to build, all you need is a soldering iron. It will monitor both banks and turn on a light if there is a Marelli boo-boo."

Different colored D2 and D4 LED's may be used for the indicator lights to indicate which bank, or red could be used for both to indicate trouble. As shown, the circuit monitors one plug wire (chosen at random) on each bank. One plug wire on each bank would be enough to detect the common Marelli failure (really, you might as well save your money on circuit components and just build one circuit to monitor the A bank), although you could get carried away and build 12 circuits instead of the 2 shown and install a complete set of 12 LED's if you wanna really be sure you detect any possible problem.

Corkan goes into more detail for those interested: "The circuit uses two identical channels for each bank. Pulses from the inductive pickups trigger the MOSFET transistors (Q1, Q2), which are sensitive to voltage. They are pretty standard N-channel MOSFETS that will trigger at about 3.0 - 3.5 volts. To make sure they work reliably with the faint pulses coming from the sensors, R1 and R2 provide an offset voltage of about 2.5 V. (If you can not locate the right MOSFET you may have to change the values of R1 and R2, they should be selected so they bias the sensor pickups to 1.0 V less that the MOSFET needs to trigger.)

The diodes (D1, D3) are very important. The diodes keep the voltage pulses from going back down the sensor wire, forcing it to drain more slowly through R3 or R5. Germanium diodes have a small voltage drop, unlike standard diodes. If you do not use Germanium then you will need to increase the bias voltage on the sensors by about 0.5 V to make up for the additional voltage loss.

Each time the MOSFET's fire they short out the RC circuit formed by R4 and C1 (also R6 and C2). This RC circuit takes about two seconds to charge, and when it does voltage then flows into D2, lighting it up and warning the driver (or flowing through D5 to the NPN transistor which lights the lamp, etc...). When a pulse hits Q1 (or Q2) it drains C1 (or C2) preventing the indicator circuit from ever getting enough voltage to light. As long as there is a steady supply of pulses the RC circuit never charges and the light never lights."

The polarity of the pickups on the plug wires makes no difference, since the pickup will put out a voltage pulse in one direction when the spark starts and a very similar pulse in the opposite direction when the spark stops. Corkan: "...and thus the reason for the first diode. In fact the circuit is sensitive enough that polarity will not make a difference, even though you can see a small difference in the signal on an oscilloscope."

An alternative is to detect the overheating of the cat itself. This has the clear advantage of indicating a problem no matter what the cause.


AUTOMATIC SHUTDOWN: Once you have some sort of failure detection system that will tell you that the Marelli has quit firing one bank, there is still the question of what to do with the indication. You could provide an idiot light, but anyone that drives despite a sudden lack of horsepower may likewise drive despite a warning light; some XJ-S owners allow others to drive their cars and don't want to have to lecture them on the importance of a particular idiot light each time. Another possibility is to have the sensor shut down the car; the power to the entire ignition system could be shut off. If Corkan's ignition monitoring circuit can light one of the dash indicators, it could probably be used to operate a relay to shut off power to the ignition -- but you'd have to install the relay so that it doesn't cut off the ignition when the ignition switch is in the "start" position or you'll never get the car started.


LIMP-HOME MODE: Automatically shutting off the engine in the event of six-cylinder operation protects the car but aggravates the driver, especially if it happens in traffic or in a bad neighborhood; false failure indications would suddenly become very distressing. A very popular idea was to shut off fuel to the bank that has lost spark, thereby making it safe to drive the car on six cylinders. It's not known if this is practical; simply disconnecting the injectors might cause an EFI ECU failure, for example. Just to be safe, maybe it would be a better idea to switch the injector power from the injectors to a resistor pack, so the ECU doesn't see any difference in load. That same relay idea might work for this, using Corkan's circuit with two separate Q3's (you can omit diodes D5 and D6 if each subcircuit controls its own Q3) to control separate fuel injector shutoff relays for each bank. Again, provision will need to be made to get the car started.

The idea of shutting off fuel to one bank and continuing to run does have one important benefit: it is likely to provide a better cooldown of the overheated parts. Several owners, having already been made aware of the Marelli problems, nevertheless reported that the right side cats were cherry red hot by the time they figured out what was happening, shut it off, and got the hood open. When the engine is simply shut off under such conditions, the residual heat within the cat may nevertheless fry nearby rubber parts, sensors, etc. If the fuel is shut off and the engine continues to run, cooling air continues to flow through the engine compartment, and air continues to flow through the cylinders of the inop bank and out through the exhaust system. A running cooldown may be better on the engine and ancillaries than a sudden shutdown. Perhaps even if a sensor provided only a warning light indication, a switch could be provided to manually shut off fuel to the inop bank to allow the car to continue moving.


CATALYTIC CONVERTORS -- MELTDOWN/FIRE (SENSOR-CAUSED): Greg Maddison lamented a large loss to his wallet: "The front two had melted into a solid mass and the debris ruined the back two." Obviously, this symptom differs from the typical Marelli six-cylinder operation described above, since that will toast the cats on one side only.

"It seems there is yet another reason the Marelli system can cause a misfire that brings the cats up to kiln temperatures. This would be problems with the TDC sensor/magnets. There are 3 equally-spaced bar magnets around the crank damper and a sensor that "looks" at the magnets and sends a pulse directly to the Marelli computer. There is a specific air gap between the magnets and sensor that must be maintained for the system to work properly.

The Jag dealer replaced the damper/magnet assembly on my car saying the rubber had split. This caused the air gap to be too wide between one of the magnets and the sensor. They first tried to grind down the other two magnets and move the sensor closer (sounds like the damper deformed and became eccentric), only lasted 600miles so I had to have damper replaced. The dealer's mechanic explained to me that each magnet produces a timing pulse for 4 cylinders. If one of the pulses are missing, 4 cylinders don't get spark and send unburned fuel through the cats. The mechanic said that a different damper is fitted to cars with the 100-amp alternator (like mine) and fails more often than the older cars." Note: the 100-amp alternator requires a grooved belt instead of a V-belt, and the alternator belt is driven by the damper itself rather than the bolt-on pulley, so the 100-amp alternator does require a different damper. Perhaps it fails more often because of the increased torque load of the alternator being applied to the rubber.

"After removing the crank pulley they found the woodruff key sheared and cone split, pulley had rotated 6 degrees on the shaft. Mechanic says the Marelli computer can very the timing up to +/- 28 degrees. So, at this point I don't know if they misdiagnosed the sheared key problem as a damper problem (they are claiming both are toast) all along. I can certainly understand how having the pulley rotate on the shaft can introduce timing errors on a Marelli car."


CATALYTIC CONVERTORS -- MELTDOWN (SECONDARY DAMAGE): Greg Maddison says, "Also check the engine speed sensor located on the flywheel housing. It is a $40 part located very near one of the catalysts. Mine melted when the catalyst overheated. After that the speed sensor became temperature sensitive, car ran well when it was cold. After it warmed up the car wouldn't develop any power and couldn't go over 40 MPH."


INTERMITTENT FAILURES: Greg Maddison suffered from electrical connections: "Occasionally when warm/hot the engine just stops. It will usually restart after a few tries or if left for a couple of minutes. It turned out to be the crankshaft sensor connector. I could jiggle the crank sensor and car would start again. If you have the Marelli digital ignition system you will have a crankshaft sensor and a flywheel sensor; engine will not run if the signal is interrupted from either. Look for a two-wire connector on the left front top of the engine; wires should be in a hard plastic wire protector running down the front of the engine to the back side of the crankshaft pulley. I found the contacts in this connector to be green and nasty. Cleaning with spray-on relay and contact cleaner cured the problem. Run the engine and wiggle this connector and see if it stops. Also check the flywheel sensor connector, it's on the rear top left of the engine, looks like the crank sensor connector and hard to get to."

John J. Lynch had a similar case: "The problem went something like this:

1) Car started fine
2) Ran normally for 15 minutes
3) Died completely due to spark failure
4) Engine cooled down for 1-4 hours
5) Repeat step 1

"Turns out the Engine Speed Sensor was the real culprit. The specs say that the sensor should read about 700 ohms but when I measured it after the engine died I noticed it was only reading about 260 ohms. Since I had previously tested the sensor with an O-Scope and saw that it was producing the square wave I was expecting, I thought everything was fine. I was wrong. For some reason when the engine warms up the sensors resistance is dropping below 700 ohms causing the voltages that are delivered to the ignition computer to be way out of spec. Consequently the computer was shutting down the ignition amps. For kicks I decided to put a 500 ohm resistor in series with the sensor to validate my new conclusion and sure enough the car ran fine. As soon as I removed the resistor, the car died again. Problem solved!!!"


TIMING: You don't really wanna adjust the timing on this thing, do you? Randy Wilson describes the XJ40, which is similar: "Spark timing is not easily adjustable. This car has a crank triggered digital ignition system. Timing changes would require moving the pickup and/or modifying the ECU."

On the other hand, there are two different timing maps built into the ignition ECU. Michael Neal says: "There is a nifty little jumper on the harness at the back side of the left intake manifold. Pull this sucker and your timing retards, perfect for those trips to Mexico. Usually the connector is red."

Mike Wilson quotes from a Jaguar manual: "Map Link Selector- This link permits the selection of either of (2) ignition maps contained in memory. If the link is in place the ignition map suitable for 95 Octane fuel is selected, and if the link is removed the ignition map suitable for 91 Octane fuel is selected."

Mike Wilson continues, "To find this "link" look behind the lefthand intake manifold. This link (barrel shaped and with two wires coming out of it. One is yellow, the other black) is usually wire tied to the Diagnostic Socket."


DISTRIBUTOR POSITION ADJUSTMENT: Randy Wilson, still talking about the AJ6 engine: "Jaguar has a special jig for doing this, but it's not really needed. The whole purpose is to make sure the rotor is pointing to a wire lug no matter what the current computer-chosen timing is. On normal dizzies, the centrifugal advance moves the timing up... but it also advances the rotor by the same amount, as they are attached. No such luxury with the crankfire system.

"First mark the dizzy body directly in line with the #1 and #6 plug wires (note: 1A and 6A on the V12). Then remove the cap. Bring the engine up to TDC, noting which way the rotor turns. When you reach TDC, the rotor should be pointing at one of the two marks. It doesn't matter which one (why we marked both). Now turn the dizzy body until the mark is just past the trailing edge of the rotor contact. Just past it, not more than a degree or two. In this position, the rotor contact will line up with the wire lug in cap in the timing range of about 5 to 40 degrees before TDC; the operative timing range of the engine.

"If the adjustment is off very far, the results will be subtle. The most likely thing is the rotor and cap will tend to burn a little uneven...biased against the direction of misadjustment. The cap and rotor will "wear out" a little faster. If the misadjustment is extreme... and I'm not sure it's possible on the AJ6, there will be a misfire at the extreme end of the timing curve as the spark has to jump a pretty large gap inside.

"Next time, mark the position before you remove the distributor. This is one of those rare cases where "close" is good enough."

Regarding the Marelli on the XJ-S: "Yep, ëtis the same ideas and principles. I've never actually had to pull a Marelli distributor, so I can not comment on the exact position. You would mark 1A and 6A... and set things so the rotor sweep-by happened in the 10 to 30+ before range."


1992-ON: Richard Mansell quotes from a Jaguar Publication on the changes for the 1992 model year:

 The Marelli digital ignition ECU has been software upgraded.


CAP VENTING: The Marelli distributor is vented in the same manner as the Lucas distributor, except that the vent connections are on the base rather than on the cap. Since there are no centrifugal or vacuum advance mechanisms, there are fewer reasons that Jaguar would have decided to provide this system. Either the cap needs cooling to prevent cracking, or explosions can be the result of fumes building up.


On to the XJR-S Ignition


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