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Drivetrain Maintenance

  Experience in a Book
Drivetrain Maintenance


The XJ-S was originally fitted with a Borg-Warner Model 12 automatic transmission, but in 1979 this was replaced with the GM400 automatic. In the mid-70's exactly 273 XJ-S's were fitted with 4-speed manual transmissions and the official Jaguar repair manual dedicates an entire section on maintenance, but there is precious little information available on that tranny for inclusion in this book. The XJ-S's fitted with the six-cylinder AJ6 engine were available with a 5-speed, but there is no info herein on that tranny either.


SHIFT CABLE -- ELECTRICAL PROBLEMS: William F. Trimble reports: "The Jag would not start -- nothing when the key was turned to the start position. Shortly thereafter smoke started coming out of the shifter quadrant on the console. I figured that there had to be some major electrical fault that toasted most, if not all, of the wiring in the console.

"We took part of the console apart and immediately found that the shifter cable had melted, freezing the car in park. A check underneath revealed that the lockout switch and wiring looked OK, and that there were no obvious problems with any of the wiring under the car." Eventually, the problem was found: a loose ground cable. "The high current load imposed by the starter could not get through the loose ground cable that links the gearbox to the body. The current chose the next best available route, which was through the shifter cable to the body. Had it been a beefier shifter cable without all sorts of plastic sleeves, it might have worked fine and I'd never known there was a problem, but such was not the case."

"I called Special Interest Car Parts to order the part, but before I said what I wanted, I told the guy what the symptoms were and that I had a loose ground strap. He immediately said that I had fried my shifter cable and that he moved a lot of those parts."


Borg-Warner Model 12


BORG-WARNER/GM400 BOLT PATTERNS: The bolt pattern for the transmission/engine attachment is the same for the BW12 and the pre-1993 GM400, and apparently was the same for the Series III E-type with manual transmission. However, when the GM400 was incorporated, the dowel pin arrangement was changed. The GM400 alignment relies on a pair of 1/2" dowels located the same way as on GM vehicles; the earlier arrangements used a smaller diameter dowel. All of this means that if you replace your early BW12 automatic with the later GM400, the bolt holes will line up but the dowel pins will fit loosely in the holes and won't serve their intended purpose.

It is possible to remove the dowels and bolt a transmission to an engine despite dowel pin conflicts. However, the alignment of the engine/transmission mating may not be as good. If misaligned, the flexplate may flex at each revolution, and will quickly fail.

Scott Horner points out there is also a difference in the end of the crankshaft. The crank that mates to the GM400 has a relatively large pilot diameter in which the protrusion on the front of the torque convertor sits. The crank that mates to the BW12 has a smaller pilot diameter. When faced with this problem, he was able to machine the stub on the front of his GM400 torque convertor to fit the BW12 pilot hole on the crank.


BAND ADJUSTMENT: Michael Neal offers this tip: "If you attempt your own band adjustment on the B/W don't overtighten them. The car will come to a very sudden halt. A quick adjustment: tighten them finger tight and back off the adjuster 3 flats."


WARMING UP THE TRANSMISSION?: According to a tip passed on from the Heart of America Jaguar Club, if you have a Jaguar with a Borg-Warner automatic transmission (pre-1979 XJ-S), you should let the car idle in neutral for 10-20 seconds before driving. This is because a one-way valve that prevents fluid from draining from the torque converter is unreliable, and the torque converter may have drained. The fluid pump does not function in park, so it must be in neutral to do any good.


FLUID: Please see the comments on fluids in the section on Drivetrain Modifications.





GM400 AUTOMATIC TRANSMISSION INTERCHANGABILITY: It is well known that the later XJ-S's are fitted with the GM400 automatic transmission. What is less known is that this transmission (through 1992) has a housing designed specifically to mate with the Jaguar V12. Because the GM400 has an integral bell housing, a GM400 from any other car will not fit.

The innards are interchangeable, though, so even though it's not easy to replace your transmission, it is easy to have it rebuilt. It's not even particularly difficult to remove (as transmissions go) and there are several access panels under the interior carpet to make maintenance easier.

If the tranny is apart, it is recommended that the sprags be replaced with high-performance aftermarket models. The Jag is hard on even the trusty GM400, and has a tendency to tear it up.

As a result of a complete revamp, the 1993-on 6.0 liter V12 has a standard GM engine/transmission bolt pattern. Cars with this engine come with a 4-speed version of the GM400.


GM400 - MINOR DIFFERENCES: John Goodman reports: "I may be wrong here, but I think Jaguar revised the tranny a few times throughout its history. My '88 car shifted much faster with less "slurr" than my friend's '84."

Richard Mansell says, "The Collectors guide does mention that "the GM400 transmission was recalibrated for improved responsiveness" from 88MY."

Goodman adds that the GM400 in the XJR-S has different shift characteristics. "The factory-modified GM400 came as standard on the 6.0L XJR-S from '89 to '93 Part no. SPE 1027. The owners handbook says "JaguarSport modified shift points". In my car ('89) I would consider under normal driving the shifts to still be quite soft, under hard acceleration they are by no means harsh, but seem to be a little quicker. However, what is noticeable is its ability while in drive to kickdown into first at higher road speeds, (up to about 38-40 mph); this is quite useful."

"The bell housings are different because it no longer needs the Marelli speed sensor. The valve body is changed and recalibrated and a different spring in the rear accumulator (whatever that is). The XJR-S manual and all the other related bumph I have collected makes a big point of stating it is unique."

Richard Mansell says, "As well as suspension mods the SportsPack modifies the gearbox change points. This means that the car will change down into first at up to 49mph (standard GM400 - 30mph) and down into second at up to 98 mph (standard GM400 - 85mph). During full throttle kickdown acceleration the car will also hold first up to around 64mph and second until around 105mph."


SOFT SHIFTS: Yes, those gentle shifts are suitable for the Jaguar image of luxury. Unfortunately, they are apparently to blame for the GM400's poor durability record in this application. Per Jim Cantrell: "Old transmission types know this phenomenon well. Take a cadillac with the same transmission as a truck and the cadillac will last half as long. You can tell when you pull the pans on those bux luxury cars - lots of transmission dust in the bottom (clutch wear). Seems that grandma and grandpa would rather have to replace the transmission every now and then than to have their tushes bumped during shifts. I guess that this allows them to have a deeper sleep while careening down the highway."

If durability is more important to an XJ-S owner than the gentle shifts, use of alternative fluids or installation of a "shift kit" may be in order.


GM400 SLIPPING: It is occasionally reported that the GM400 transmission slips; sometimes when cold, in first gear, or under other specific circumstances. Leonard Berk reported that his slipped until he deliberately overfilled the transmission fluid, then it worked fine. Randy Wilson provided this explanation:

"On GM vehicles, a loose/fallen filter is a normal occurrence, and is known for causing weird problems. The most common problem is the transmission briefly finding neutral right after a hard turn. Most THM transmissions, the 400 included, hang the filter low in the pan. The filter is supported by the o-ring on the pickup pipe (pipe from filter to valve body), which snaps into a machined groove at one end and a steel clip at the other end. If the o-ring is bad, or the filter has fallen, the fluid pickup point is effectively raised to the bottom of the valve body, about an inch up from the filter."

Jim Cantrell provides more elaboration: "The o-rings on the filter an at the pump inlet are known to leak in cold weather which results in pump cavitation. Pump cavitation can usually be heard - a whining sound of the pump. On the 400 in the cold (talking 40 degrees F and below), it will usually last about 30-60 seconds. It still moves since it's actually pumping oil. It's just sucking in air from the o-rings and this gives the cavitation.

"Second hypothesis is that once cold, the o-rings leak and air leaks into the oil pickup tube and drains the oil. The result is the pump runs dry until oil starts flowing."

Since the GM400 dipstick has warnings not to overfill, perhaps overfilling should be done only as a diagnostic measure. If it fixes the problem, suspect a dropped filter. Just go ahead and change the filter (and O-ring!) and fluid.

Wilson added that slipping is not good on the transmission, and the sooner it is corrected the less damage will be done.


CHEAP GM ACCUMULATOR: Sandy Gibbs reports, "My tranny (GM400, 1987 XJ-S) went belly up. I took it to the shop for a rebuild. The mechanic showed me a small plastic cuplike looking thing which he said had caused my transmission to burn up. He claimed this piece (called the accumulator) was usually the problem when a GM400 dies. This information was confirmed by the owner of the parts place (which deals exclusively in transmission parts) where I bought all my parts for the rebuild. GM knows about the problem and now supplies an aluminum accumulator which won't break. It seems to me that anyone who plans to have some transmission work done anyway might want to invest in a new accumulator."

Thomas Alberts adds, "You are talking about an accumulator piston. One of the two accumulators has a plastic piston that can fracture with fatigue after high miles. It is a common failure mode. The piston breaks leaving a 1/2" hole that hydraulically "shorts out" parts of the transmission logic and leads to clutch packs burning up. It prevents kickdown and can cause second and (I think) first gear to engage simultaneously forcing one of the clutches to slip in second. It is pretty easy to replace the piston with the transmission in the car. It involves removal of the oil pan and valve body. I wouldn't advise everyone to rush out and do this, but if you are ready to replace filter and fluid it would be worth the extra effort to put in the aluminum piston."


DEXRON II/III FLUIDS: John Horner passed along this info from "an applications specialist at Texaco's lubrication division.": "Since Type A, Type A Suffix A, DEXRON, DEXRON-II and DEXRON-IIE fluids are obsolete, customers with transmissions calling for any of these earlier vintage fluids should use DEXRON-III. DEXRON-III fluids are designed to be back-serviceable for automatic transmissions used in passenger cars and light trucks manufactured since 1949.

"There are a few synthetic DEXRON-III ATFs available but they may be hard to find. They are also more expensive than mineral-based ATFs. Synthetic transmission fluids basically share all the same advantages that synthetic motor oils enjoy over conventional mineral-based oils. These include improved low-temperature fluidity, oxidation stability and reduced volatility.

"Compared to the old Type A fluids (and even newer generation fluids such as DEXRON-II or IIE), a mineral-based DEXRON-III fluid will help prolong the life of a transmission. DEXRON-III fluids retained the low-temperature fluidity of DEXRON-IIE fluids and incorporated major improvements to the frictional stability, high temperature oxidation stability and material compatibility.

"For your information, and as a point of reference, when GM introduced DEXRON-III fluids they increased the drain interval to fill-for-life under normal service and 50,000 miles for severe service."

John T. Horner adds: "Mobil lists a Dexron type synthetic ATF. Check the web site at: 

"I think most of the other synthetic providers also have competing products."


DRAIN & REFILL CAPACITY: The repair manuals seem to agree that a drain and refill of the GM 400 transmission will require 9.6 US quarts or 19-point-something US pints. However, Larry Barnes reports that it only took about 6 US quarts on his car. Reason unknown.


KICKDOWN: Since there seems to be a lot of confusion about this term, I will first endeavor to define it. Kickdown does not refer to the normal downshifting of the transmission due to increased throttle (reduced vacuum) at low speeds. Kickdown refers to the forced downshifting of the transmission by a very definite pressing of the accelerator pedal all the way to the floor, and pushing it hard enough to operate a separate "snap" that the driver can feel when the pedal is moved the last quarter inch or so. It is intended to be a distinct enough feel that the driver can easily choose whether or not he desires kickdown operation, or merely full throttle without kickdown.

In the case of the pre-1992 Jaguar with the GM400 transmission, kickdown is accomplished via a microswitch mounted on the throttle cable attachment at the bellcrank. After the throttle is wide open and the bellcrank is against the stop, further pressing of the pedal can move the bellcrank no more. Instead, the cable housing moves in the opposite direction against a very strong spring, strong enough to ensure that the driver must really want it to move. When the cable housing moves against this spring, a microswitch closes a contact sending current to a solenoid within the GM 400. This solenoid causes the transmission to downshift now. At any reasonable speed, it will downshift to at least 2nd; below 30 mph or so, it'll downshift all the way to 1st, and the engine will scream.

The results should be obvious and dramatic. Unfortunately, all too often this system doesn't work, and the owner doesn't even know it; since the transmission will downshift normally under heavy throttle, they presume that everything is OK. It is easy enough to test, though; with the car shut off, make sure pressing the accelerator hard operates the microswitch on the throttle cable as it should. Make sure you know what it feels like to press the accelerator all the way without kickdown as opposed to with kickdown. Then, take the car for a drive. If there is an eye-popping difference between full acceleration with kickdown and no kickdown, it's working as it should; if you have a hard time telling the difference, it ain't working. If you're not sure you can distinguish the feel of the pedal, simply disconnect a wire from the microswitch on the throttle cable housing and go for a spirited drive; if you can't tell the difference, it ain't working.

If it doesn't work, check fuse #5; there should be power to the kickdown switch on the LG/W wire when the ignition is on.

If that's OK, check that the kickdown switch itself makes good contact. Physical adjustment may be necessary, since problems may be caused by the switch not positioned correctly or the roller arm bent.

Next, check the inhibit switch on the shifter. One of the microswitches (hey, the car is covered with them) within the console and operated by the cams on the shifter is a microswitch that closes only if the shifter is in D. This prevents the operation of the kickdown solenoid in any gear other than D. Note that this same switch also prevents operation of the cruise control in any gear other than D, and it does so by grounding a signal line from the cruise control through the solenoid in the transmission! Problems with this inhibit switch may be mechanical in nature as well; it might not be positioned properly for the cam to operate it reliably.

The next possibility, as Greg Meboe points out, may be that the wire at the transmission isn't connected right. "The kickdown connector on the outside of the transmission case had two separate connection points on it, in the shape of a "T". The vertical connector which "bisected" the horizontal connector was the kickdown one. I had been explicitly told that the horizontal connector was the proper one to connect the kickdown wire to, so that's what I did. When I took apart the tranny, I saw that the horizontal connector went to what looked like some type of sender on the valve body, whereas the vertical connector (which I hadn't used), went to the kickdown solenoid."

For the final test, it helps to be under the car with your ear near the transmission and all quiet in the area. Have an assistant turn on the ignition, put the shifter in D, and operate the kickdown switch, and you should be able to hear the solenoid within the transmission click. If there's no click, make sure you are getting 12V at the wire to the connector; if so, time to drop the pan and replace the solenoid.

Richard Mansell quotes from a Jaguar publication on the changes for the 1992 model year:

The kickdown switch is no longer located in the underbonnet area but is changed to the XJ6 type and is positioned under the accelerator pedal. 


TRANSMISSION MOUNT: The transmission mount in the XJ-S would have made Rube Goldberg proud. The design utilizes a spring to take the weight of the transmission, as opposed to the rubber supports used on most cars. Jaguar also has a vertical post assembly containing a "special washer with rounded edges" within a rubber bushing. This assembly allows a very limited range of motion: some vertical travel to allow the spring to work via the post assembly sliding up and down within the bushing; very little horizontal or axial travel, since the special washer fits snugly within the bushing; and a little tilting (torque reactions) via the special washer behaving as a ball joint within the rubber bushing.

A picture is worth a thousand words here. Exploded views abound, although many seem to be missing a few parts, which doesn't help someone trying to figure out how it goes together. Figure 8 is a section drawing of the assembly, in hopes that it will be of some help.

Note: If you lower the transmission significantly, Thomas E. Alberts suggests you take care that the top of the engine does not damage the heater valve.

The following sections describe several reasons for disassembling the transmission mount, followed by a section on proper assembly.


SPRING: Michael Neal reports that the tranny support spring (CAC 2327) does not hold up well, and recommends it be replaced every 30,000 miles (when the tranny fluid and filter are supposed to be changed). Apparently it sags as it gets older.

To check the old one, the unloaded length of a new one is 3.925".


GOT A COUPLE EXTRA WASHERS? There have been several reports of washers being found between the vertical post and the housing of the transmission. Reportedly, with the mount assembled correctly it was still possible for the forward U-joint on the drive shaft to hit the support plate, causing a "clunk". The washers were added between the post and the transmission housing to hoist the transmission by the thickness of the washers, just enough to eliminate the clunk.


GM400 FLUID AND FILTER REPLACEMENT: Since the GM400 has no drain plug, draining the fluid requires removing the pan. To remove the pan, the forward mounting bracket must be removed. Therefore, changing the fluid and filter in the GM400 transmission requires complete removal of the transmission support. The correct reassembly of this system is not obvious, and non-Jaguar (Aamco?) mechanics will often reassemble it incorrectly, and perhaps even omit some of the parts. Taking this book along to the shop may be helpful; having an experienced Jaguar mechanic service your GM400 transmission may be the best course of action.

Alan Jenks reports that B&M Racing offers a drain plug kit (B&M #80250) which can be fitted while the pan is off. Also, J. C. Whitney offers two replacement oil pans for the GM400 with drain plugs, one "original capacity" and one that's 1º" deeper for "extra capacity". They're both chrome plated, and cost only about twenty bucks. Note that Larry Barnes reports that "...I purchased a J.C. Whitney chrome transmission oil pan (standard size) with a drain plug ("GM TH400 Original capacity"). It did not fit. Bolt pattern was correct, but the pan was not deep enough for the filter. So, if you are thinking of getting the same pan, get the 1.25" deeper than original "extra capacity" oil pan."

If you're more worried about fresh fluid than filter blockage, one of these changes might be a worthwhile investment. If you do your own work it might still be a worthwhile investment even if you pull the pan every time, since you can drain it first and then remove an empty pan rather than a full one.

John Himes adds a note: "If you are looking to fit a new filter or shift kit on your General Motors Turbomatic 400, check the serial number for the date of manufacture (first 2 numbers), or at least the shape of the pan. Pre-88 models are a little different than 88 and newer."

The filter is held in place by the O-ring on the suction tube on one end, and a bolt at the other. This bolt is shouldered so it cannot be tightened down on the filter, so the filter is free to rattle around a little. This is apparently deliberate; there are no parts missing. Randy Wilson says "That bolt is shouldered for a reason, though I can not tell you exactly why. All GM trannys of that general era use a floating filter that sort of lays in the bottom of the pan. At least the 400 has a positive bolt. Others use a cheesy spring clip."


RUBBER BUSHING & SPRING CUP REPLACEMENT: Finding the rubber bushing (CAC3227) or the rubber spring cups (CBC2517) intact appears to be a rare occurrence; they are often missing, damaged or mislocated. Reportedly, the bushing utilized on earlier cars was made of foam rather than solid rubber, and was even less likely to be found intact.

To properly install a new rubber bushing requires removal of the spring support; you cannot simply slip the rubber bushing in from below. With the spring support removed, install the rubber spool into the center boss so that one lip of the spool is on top of the support and one lip is within the recessed opening on the bottom. It should be securely snapped in place in the boss.

If you are disgusted with the lack of durability of the spring cups, you can easily make substitutes by slitting rubber or vinyl tubing lengthwise and slipping it over the end coils.


UNIVERSAL JOINTS: The XJ-S was clearly not designed with ease of servicing the forward drive shaft U-joint in mind. Applying a grease gun to the zerk fitting, as well as removing the drive shaft itself, require the transmission mount to be removed; if you have it out anyway, you may wish to address the drive shaft. However, drive shaft inspection also requires that the main support plate be removed. Jaguar provided two bolts up within the tunnel to make sure this task was as unpleasant as possible.

According to Chad Bolles, the U-joints in the drive shaft are a GM standard, and Spicer 5-153X or Tru Cross 153 will fit. The U-joints in the rear axles are also a standard; Spicer 5-160X or Tru Cross 160 will fit. All of these should be available at your local auto parts store.

There is a philosophy that having a grease fitting on a U-joint does more harm than good. In theory, if the seals in the U-joint are any good, it will never need regreasing; if the seals are bad, regreasing won't help for long. And undisciplined use of a grease gun is the best way to destroy the seals. Even if you're careful with the gun, it's more than likely that the grease injected will all go to one bearing and leave the other three ungreased; it's a path-of-least-resistance kind of thing. If you believe all that, you might as well choose replacement U-joints that have no zerk fittings and save a couple bucks.


TRANSMISSION MOUNT REASSEMBLY: The two rearmost pan bolts should be double-ended bolts with the head in the middle, so that after installing the pan there are studs available for holding the forward bracket in place. If such bolts are missing and unavailable, normal bolts can be used by inserting suitable washers between the bracket and the pan. The washers should total 3/16" thick.

While apart, the spring support (CAC2438) should be cleaned to remove all the crud that has collected in it. The crud often appears to be the dissolved remains of the original rubber parts, but we will endeavor not to draw any conclusions about Jaguar's materials engineers. The crud must be removed to allow the spring and spring cup to seat properly.

The transmission must be jacked all the way up into the tunnel before the spring support is installed. Once the spring support is securely bolted into place, then the jack can be lowered, allowing the transmission to sit on the spring and the center post to protrude through the bottom.

Once the spring support is supporting the tranny, the assembly of the parts on the center post is as follows: the first item to be installed should be a special washer that has rounded edges, C29011; it will need to be crammed into the rubber bushing somewhat. Next is the sleeve, C30157. Third is a part that looks like two washers welded together; it should be installed with the larger opening facing upward. Then the tie plate is installed, followed by the large self-locking nut; there is no washer directly under the nut.

If the car is fitted with the BW automatic, the post assembly is the same except for an additional sleeve that is installed first. So, the order is sleeve-special washer-sleeve-double washer-tie plate-nut.

If the special washer C29011 is missing, a 1/2" splitring lock washer bent flat will serve.

Two small spacers should be installed between the tie plate and the forward bracket -- at least on the GM400 assembly. If they are missing, 3/8" washers totaling 1/4" in thickness may be used.

After assembly, there should be no more than 3/4" between the tie plate and the bottom of the spring support. Check that the transmission moves freely on the mount; since the spring is soft, you should be able to bounce the entire engine/transmission assembly up and down a little with your hands. If it appears to be immovable, something is not right. It also should not clunk when moved.


SPEED SELECTOR CABLE TRUNNION MOUNT: This is the little bracket that holds the shift cable housing to the transmission housing. The bolt that holds the trunnion mount to the tranny housing goes in from above, and cannot be removed from underneath the car. The only way to remove it is to remove the console, shifter mechanism, and a panel underneath to get at the bolt.

It is suggested that if you ever have an opportunity, cut a hole in the panel inside the console and fit it with a rubber plug or cover, so the trunnion mount bolt can be removed by simply lifting the console cover (three screws) and removing the rubber plug.

If you are trying to remove the engine/transmission assembly and must disconnect the speed selector cable, the above trunnion mount problem may appear to be a real holdup. It is not, however. The attachment of the swivel joint to the end of the cable is a removable clamped-on device. Simply loosen the two hexes from each other, and the entire joint slides off the end of the cable. Then the cable can be slid through the trunnion, allowing the removal of the engine/transmission from the car.


ADDING A TRANSMISSION COOLER: In the May 1995 issue of Australian Jaguar Magazine, John Pearson says "Most Jaguar engines run fairly warm...and transmissions prefer cool running, so you may consider incorporating a neatly positioned transmission oil cooler...This is especially recommended on V12 powered cars with the GM T400 ëbox..."



Final Drive


REAR SUSPENSION/DIFFERENTIAL REMOVAL: It's discussed in the section on Steering & Suspension.


DIFFERENTIAL: The XJ-S has apparently been fitted with at least three different differentials ("final drive units" for you Brits). From 1976-1985, the differential was a Salisbury 4HU Powr-Lok that came with either 3.07:1, 3.31:1 (1976-1982), or 2.88:1 (1982-1985) ratios.

From 1985-1987, a 2.88:1 DANA unit was used. This unit can be distinguished in that the bearings on the output shafts are held in place with three bolts; the differentials both before and after these years have five bolts. It also has no drain plug.

When working on a DANA unit, you can get the seals from Jaguar, bearings from a local bearing house, and the clutches, shims, and those silly little clutch retainers from any performance shop that has access to Dana rear end parts. When ordering, they will usually ask for a ring gear diameter and an "axle" spline count.

Jaguar does not want to admit to the Dana as having ever existed. The problem is those "silly little clutch retainers". The Jag, with it's high weight and sticky tires, really hammers on the clutches, which eventually chew through the retainer. By design, the half retainer can't get out into the gears where they would do major damage. But it does get pushed out the end of its access hole, mucking up the carrier bearings.

From 1987 to 1993, there was a differential referred to as the GKN Power Lock with a 2.88:1 ratio.

Reportedly, the official Jaguar repair procedure for final drive units is to replace them as a whole.

All XJ-S differentials are limited slip, and all the systems operate using essentially the same principle (a series of friction discs, with the compressive load increasing through increased drive torque) although the feature has often been given different names.

The gears for the DANA Type 44 limited slip differential will fit in the Jaguar unit. This includes gears from many larger American cars and small trucks. However, there are problems that must be overcome. First, the DANA ring gear will be threaded for smaller bolts than the Jaguar originals, so bushings will have to be fabricated to make the smaller bolts fit snugly in the holes.

Second, the replacement pinion will not mate with the Jaguar input flange, so a Chevy input yoke will have to be used. The Chevy yoke is the type with U-bolts that retain the U-joint bearing caps directly, so the flange on the rear end of the drive shaft will need to be removed. Since the Jaguar driveshaft uses standard Chevy U-joints, they will match up fine.

The salesman at Quality Jaguar reports that while the DANA gears will work, they are considered inferior to the Jaguar parts. Specifically, they tend to whine more, and of course the smaller mounting bolts are not as sturdy.

From 1993 on, the entire rear end of the XJ-S changed. A suspension similar to that on the XJ40 was used, with outboard brakes rather than inboard. Along with the engine being enlarged to 6.0 litre and the fitting of the 4-speed GM 400, a 3.54:1 final drive was used.

Apparently the AJ6-powered XJ-S's, 3.6 or 4.0 litre, all have 3.54:1 final drives.


DIFFERENTIAL BREATHER: John Goodman reports on special parts for the XJR-S: "Looking through the parts list I notice that there is a revised diff cover with additional baffles. <Quote manual> ..."the diff rear cover is redesigned to improve breathing at high road speeds. The altered baffle plates direct oil flow so that turbulence is kept away from the oil breather.""


DIFFERENTIAL OIL CHANGE: Getting the fill plug out is no picnic. It has been suggested that removing the center reinforcing plate under the final drive unit (14 bolts, 6 with nuts) may be worthwhile. Don't worry, the whole car won't fall apart while the plate is out.

Many Jag owners suggest a length of plastic hose and a squeeze bottle for topping up or filling up the final drive unit. The hose should be long enough to feed out the right side wheel well so the oil can flow downhill.

Brian Sherwood points out that if you open the boot, remove the spare tire, and peel back the matting forward of the fuel pump, there is a round metal plug; removal of this plug provides access to the fill plug on the differential. "I just popped it out with a screwdriver, did my lube thing, then pressed the plug back in with some RTV around the edge."

Note that while the diff can be topped up with conventional gear oil, a drain and refill requires an additive for limited-slip units.

According to a salesman at Quality Jaguar, the XJ-S final drive unit uses both natural leather and natural rubber seals. Therefore, synthetic lubricants are not recommended. It is recommended that synthetics be avoided in the special additive as well.


DIFFERENTIAL GASKET: British Auto USA offers a differential cover gasket (part number 3931) that is supposed to be better than the original -- thicker and better material. Of course, the Jag differential is a common Dana design used in lots of cars and trucks, so gaskets should be available at any parts store.


TEFLON: Dennis Hurvitz reports: "While having a quickie lube joint change the oil in my wife's car (ok, i was pressed for time), a guy pulled up in an older Mercedes to thank the manager. Seems the Mercedes guy had a differential whine and the manager recommended replacing the differential fluid with some new mixture containing teflon. The owner made a point of thanking the manager, because the problem (sound) mostly went away!!"

Unfortunately, lubricants containing teflon are not recommended for limited-slip differentials, and all XJ-S's have limited-slip differentials. Still, if you're facing a diff rebuild anyway, perhaps you can add the teflon to quiet the whine and do without the positraction until you get around to having the diff rebuilt.


INPUT FLANGE RETAINING NUT: It's a really big nut, obviously it should be tightened down really tight, right? Wrong. This nut is used to set the preload on the input shaft bearings. Between the bearings is a "crush sleeve", and during assembly this nut is tightened just enough to provide the proper bearing preload while compressing this sleeve. If the nut is overtorqued, the entire final drive assembly must be torn down to install a new crush sleeve.


DIFFERENTIAL OUTPUT BEARING FAILURE: One of the common failure modes of the Jaguar differential unit is the failure of the bearings in the output shafts, possibly because these bearings take lateral loads imposed by the working of the suspension. Whatever, the failure is usually indicated by clunks from the rear when driving or the tire rubbing the wheel well where it formerly had clearance. Checking by grabbing the top of each rear wheel and shaking vigorously in and out clearly indicates a problem. Closer inspection shows that the output shaft is free to move in and out, and the only thing limiting movement is the brake disk banging back and forth within the caliper!

According to Jan Wikström, "it's very likely that all you need to do is replace the bearings and seals. This is not difficult, but you'll need to take the rear subframe off your car. You can do it from underneath, but that probably takes longer because of the difficulty in getting the brake calipers off and back on. Besides, taking out and dismantling the complete rear suspension gives you a great opportunity to check everything."


CLUNK: Julian Mullaney shares an unpleasant experience: "I had a clunk on my '87 XJ-S from the rear end. My clunk was because the diff itself was loose in the mounts. The mounts in this case were 4 cone-shaped bolts which attach the inner lower wishbone mounts to the diff casting. The bolts screw right into the diff, and are wired in place. These bolts had just become a bit loose even though they were still wired together from the factory. The constant rocking with power on/off had worn the fit between bolts and casting. You can easily check for this by jacking the car up high, putting the trans in P, crawl under and look for movement in the joint while violently rotating the rear tires back and forth." This is reportedly a fairly common problem; it should be noted that proper torquing and lockwiring of these bolts is very important. Many owners use Loctite on the threads as well.

Jan Wikström adds, "If the PO has omitted the security wires on the bolts, they come loose - and then the U-shaped shims fall out and the diff is really loose.

"The inner pivot bracket is shimmed to the diff to line up with the pivot mounting holes in the cage; with the bolts tight, the pivot spindles should slide in and out easily. If those inner pivot carriers aren't correctly shimmed, it's jolly well impossible to get the trunnion bolts out without slackening the diff bolts -- which is very hard to do because the trunnion bolts block access to the bolt heads. You need a very thin open-ended spanner."

"My second XJ had pivot spindles that couldn't be moved; somebody had botched the assembly and just forced the whole thing together without shims. I had the devil's own time finding a thin enough spanner to loosen the mounting bolts enough to slide the spindles out."

"To get this right requires an extra step in the assembly procedure: you measure the space between brackets and diff (bracket mounting bolts finger-tight, bottom plate out) and get spacers to that thickness. Now, there's not a hell of a lot of space between the suspension arms and the bracket-to-diff bolts; unless you have some extremely thin spanners (and thin fingers to get the locking wire on), you now need to pull the inner pivot spindles. Put the spindles back in without the suspension arms and tighten the bejasus out of those bolts (the torque is in the book); now try the inner pivot spindles. If they don't slide out easily, you need to tinker with those shims. Apply lock wires and assemble the inner pivots.

"Now bolt on the bottom plate; you'll find that the complete assembly is rigid.

"Another thing I noted on that car was that the final drive had been flopping around to such an extent that the rear sub-frame was fractured on top, where the four top final drive mounting bolts are (invisible without taking it out). There's a (1/4 in? 5mm?) steel plate inside the double-walled top of the sub-frame (as I found when I welded fatigue cracks in my first one); the retaining bolt heads have an underside taper matching a taper in this plate. With your diff slopping back and forth, there is good reason to suspect fatigue cracks in the top."



On to the Brake System


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