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8 - Drivetrain ( )

8.1 - Transmission Warning Note ( )

The California BAR ( Bureau of Automotive Repair ) has an alert out warning about the possibility of transmission failures during the Californian smog test routine. This notice applies specifically to the ZF 4HP series trans. and I believe some of the later Jags may be so equipped. Among other things, it states -

CAUTION - With the transmission in PARK or NEUTRAL - DO NOT EXCEED 2,000 RPM


For service operations that require the engine be at normal operating temp, use the following procedure.

1) Drive the vehicle to bring it to operating temp

2) Shift to PARK and switch OFF the ignition

3) Wait at least 30 seconds before restarting the engine

4) Restart the engine in PARK, and do not move the gear selector through the forward or reverse gears with the engine running

5) perform the service procedure, observing the cautions above

6) Following the service procedure, switch OFF the engine

7) Wait at least 30 seconds before restarting the engine. Do not increase engine speed above idle before selecting a forward or reverse gear

According to Randy Wilson, a Jag mech. extraordinaire in Virginia Beach: "Yes. This is a common known fault with the ZF4hp22. the cause is a single seal... well inside the tranny, will wear a little bit and allow a pressure bleed into the forward clutch. This keeps the forward clutch partially engaged when it's not supposed to be. So, yes, you can toast the forward clutch via slippage by revving long and hard in neutral.

The same conditions exist while in reverse... so backing up fast can do the same thing. The final condition can be no forward movement due to a worn out forward clutch, or *only* forward movement (even in neutral) due to a forward clutch pack that's welded together solid."

8.14 - Speedometer ( )

If you have an erratic speedo check the speed sensor that is mounted on the left side axle(U.S. car). On the 1990, there is a ring with cut outs bolted to the axle flange on the differential. Above that is the sensor that is supposed to pick up and count the cut outs. It does this magnetically....If the sensor gets dirty or damaged, or the connections get cruddy, it could cause the readings to drop. This is only valid if the odometer is off also. If the odometer is correct, but the speedo is off, then it would be in the speedometer/ computer.

8.15 - Differential ( )

8.15.1 - Changing Bearings without removing Diff. ( )

Unbolt the halfshaft and reposition for clearance. Undo the four or five fixing bolts and pull the output shaft out of the diff.

Small loss of oil is possible.

Don't lose the very thin shims behind the shaft. You will need a new collar, seal and bearing. You have to drill the collar and then split it or drive it off with an air chisel, then press off the old bearing and tap out the seal from the housing.

Install new seal, install housing on shaft, press on new bearing and collar at the same time.

Reinstall in diff.

8.15.2 - Changing the Differential Oil ( )

Drain the gear oil out the plug on the bottom of the differential. Use a socket that fits the square plug snugly.

Then to get the new oil in, take the spare tyre out, remove the carpet liner on the back of the boot.

You can then see 2 metal plugs in the boot. Pry out the one that lines up with the differential.

Use a half inch drive extension to remove the plug in the diff.

WARM UP THE GEAR OIL in hot water and use a tube on the end of the squeeze bottles the oil comes in to fill the diff until the oil pours all over the garage floor, scream, and jam the plug back in while getting the oil all over the trunk.

Put some silicone on the boot plugs and put them back in.

8.15.3 - Replacing the Differential ( Brett Gadzinsky,  December 5, 2001 )

Before jumping in and assuming your differential is reaching the end of its life, there are other components which could increase noise and there could be ways to try to solve the problem, at least to bring it down to an acceptable level.

The output bearings, sub frame mounts, and maybe thicker lube can make a worn differential quiet sometimes.... From inspecting some worn and noisy units, it seems only the output bearings wear, and maybe the pinion bearings slightly. The actual gears don't wear out much if you keep lube in the thing, and after all, a rebuilt differential will have used gears in it as well. The ring gear bearings are huge and don't seem to wear much at all.

It seems that 80% of the differentials just need output bearings, thicker lube, and new sub frame bushings.

If you do need to replace your differential, Coventry West is a good way to go when ordering a rebuilt differential.

There are a few other parts around the differential which could be changed at the same time if they show signs of wear but it is hard to say what's needed without inspecting all the bits : differential, drive shaft, axles, etc. No point in going through all this work without tending to other things, so, depending on the state of your car....

To replace the differential, you need a clean garage, the bigger the better, a bigish floor jack (two helps), high jack stands, and a good selection of metric and standard tools. Please check the appropriate manuals before starting !

* Jack the rear of the car up and remove the rear wheels.
* Jack the back of the car up as high as you can, and place.
* Jack stands under the jacking points and chock the front wheels.
* From the wheel well, cut all cable ties to the rear sub frame, and the brakes/abs systems.
* Remove the brake calipers and tie then up high with wire.
* Remove the lower shock mounting bolts.
* Remove the nuts securing the axles to the differential. I found a standard size slightly loose fitting 6 point deep socket got on the nuts better around the U joints, and used a 2 foot extension, and a ratchet with a pipe on it to break the nuts free while working in the wheel well, not under the car. Use the emergency brake to hold the axles from rotating.
* Remove the rear hub pivot bolt, and remove the axle/disk/hub assembly and set it aside.
* Now is a good time to grease the U joints, and clean and grease the pivot bearings.
* Remove all the fuel pump stuff, clamp hoses so fuel does not run out (this is the worst part)
* Be sure all the cable ties on the suction hose are runs along the front of the sub frame.
* Remove the speedometer sensor from the differential.
* Remove the exhaust mounts above the differential (10mm). A small 1/4 inch drive works well here.
* Slacken the emergency brake cable at the adjustment under the car.
* Remove the center pin in the Y of the emergency brake cable, then split the cables that run to each wheel...needle nose vice grips work well here.
* Remove the center section of the exhaust if you suspect the drive shaft U joints, and it makes the job easier, although you can do it without removing it. New hardened bolts and all metal lock nuts are available at Sears hardware.
* Note the exact location of the drive shaft center bearing. Don't count on marking the bolts, it mounts on a sliding plate, and the entire mount has a lot of slop in the mounting points.
* Remove the center bearing mounting bolts, and look for a shim under the bearing.
* Remove the center bearing crossmember.
* Remove the jerid bolts to differential flange, not drive shaft to jerid bolts !
* Place a jack under the differential and take up the weight with it.
* Remove the lower shock mounting bolts.

Now, the only thing holding the sub frame in is the rear dog bones, and the big front mounts. Make sure this is when you lower the assembly, nothing hangs up or gets broken.

* Remove the rear mounts.
* Remove the front sub frame mounts..the plates to the body and the large center bolt.

The differential is now held up only by the jack. Lower it SLOWLY as it wants to tilt and fall off the jack, and get hung up on something.

If you had the car up high enough, you can roll the jack and assembly out from under the car.

Next, you need to disassemble the sub frame, which consists of hundreds of bolts, washers, bushings and spacers. (this thing is really complex!)

If you mark parts, differential cage for front and back, and note the order parts come off, including washers and so on, its not hard, well it is hard, the parts are heavy and hard to deal with, glad my garage has a thick rug on the floor!

The entire rear suspension comes apart, the sub frame has the big lower part, a plate that bolts on the front of it, with another
plate around the differential input shaft, bolts on the bottom of the big part that go into the differential, bolts and nuts everywhere.

With things apart, inspect the rear A arm bushings and pivot shaft, and the washers/spacers/bearings on the A arm pivot points. Also note how the rubber covered big washers go on the back differential mounts. Remove the axles from the differential, all the bolts, and the diff should be separate now.

At this point, I would strongly recommend replacing the rear sub frame mounts (big front ones).

I did it myself, it is a bit of work without the correct power tools... Paint any parts that need it, clean everything up spotless, undercoat the area above the differential, wash all the nuts and bolts and washers in solvent, one at a time as to avoid mix-up. As things come apart, I assemble them as much as possible, and lay them on the floor in the locations they cam out of.

Assembly is the reverse of the above. Grease all moving parts well. Since you cleaned the bolts and bits up, they should go back in easy...all parts should assemble easy, except for the weight of things. Needing force is a very bad sign, back up and think/inspect. The cage around the differential is offset, so you need to know how the bits point (front to back).

Check the drive shaft u joints, center bearing condition, jerid, and if you have U joint problems, I recommend a good used drive shaft off a newer car. I had no luck balancing mine after replacing the U joints, and the insert in the differential end that centers the shaft while the Jurid is fit wears out and the drive shaft to differential alignment becomes impossible to get right.

If things look ok, disturb it as little as possible !

When the job is done, you know everything is right back there, all clean and shiny...and quiet ! The first time I did it, the job took two days. The last time I did it, it took under 6 hours start to finish.

8.15.4 - Rebuilding the Differential ( Martin Violette,  February 22, 2002 )

Before I bought my í91 XJ40 a couple of years and 60,000 miles ago I was lucky enough to find Jag-Lovers and the XJ40 buyerís checklist, on which as I checked over the car I duly noted a faint whine in the differential. Well, after some procrastination (ha ha) and a whole lot of noise from the rear end, I finally got around to dealing with the problem. Here are some notes on what I did to rebuild the differential (Ď91MY GKN, non-Power-Lok). Please note, although this is a bit of a how-to, Iím not a pro, and Jag gives virtually no guidance on this procedure, so try this at your own risk. And I am assuming that you arenít a novice, so I havenít gone into detail on some standard procedures.

Removing and replacing the diff is covered in the on-line book (very well, thanks to Brett Gazdzinsky), in Haynes, and of course in the shop manual, so I am not going to describe that procedure here. But there is no info in Haynes and very little in the shop manual about the diff because Jaguarís official policy is to replace, not rebuild.

But I thought that since I had to remove it anyway, why not have a crack at rebuilding it. The shop manual gives info on tearing down and reassembling the Power-Lok version to replace bad clutch plates, and the info there is critical, so it should be referred to. After the diff is removed it should be securely anchored to a heavy work bench, the two lower wishbone studs removed (30mm box end required, and these are installed with 200ft lbs/270Nm of torque and Loctite 273 so they are difficult to remove. The shop manual states these are to be replaced but it seems you can only get them as part of the Power-Lok clutch kit, so I didnít. I am guessing that the kit has an updated version of the studs, but I donít know for sure.) Then remove the rest of the cover bolts and the cover. From now on everything should be scrupulously clean.

While the diff is bolted to the bench (use the two bottom case to subframe bolts) remove the pinion nut, washer, and drive flange. If you havenít already removed the output shafts remove them now, and keep their shims marked as to which go on which side with which bearing carrier.

At this point the case must be expanded to allow removal of the crownwheel and output bearings. There is a Jaguar special tool, but two pipe-type furniture clamps and two pieces of angle-iron served me well. Use a dial indicator as shown in the manual to be sure you arenít overdoing the case stretch. When you are ready to remove the crownwheel assembly be prepared to capture the bearing cups, which will otherwise fall off. Then remove the pinion gear shaft, and drive out the inner and outer pinion bearing cups. Be sure to keep any shims with notes, illustrations or whatever you like to use to put them back in the right places. (N.B. If you are only replacing bearings, and not shafts or gears, you will reuse the old shims- there is no need to re-shim the assembly or check teeth mesh and backlash, as none of this changes.) Mark the output flange to case and pinion shaft, and the crownwheel position relative to the case for reassembly.

The output bearings will need to be removed by an automotive machine shop, unless you have a large hydraulic press. Be sure to instruct them to keep the shims with the appropriate side of the crownwheel- I marked one side of mine with paint. Take the output shaft assemblies with you and have them taken apart as well, and take your new bearings to have them pressed on the shafts. Total cost for this in my case was less than $50.

Examine the old bearings for edification, even though it should go without saying that you will replace them all. In my case, the starboard output bearing was the most damaged, the inner pinion the next worse, but all were shot. Interesting wear patterns, but thatís another story. All gear teeth looked like they were brand new. A note on bearings: you can buy rebuild kits from the usual sources, or for about half the cost you can buy bearings and seals from a bearing house, which is what I did. But one warning: both the pinion shaft bearings are special. Mine were SKF, and the codes after the size and duty numbers indicated (when I finally found them on the SKF Web site) that they were produced especially for application as differential pinion shaft bearings. The other bearings are pretty standard fare. I paid about $150 for all, including seals.

While all that is going on you will have cleaned up the diff case, and all the subframe parts, so you are ready to put the diff back together. The first step is to properly install the pinion shaft and output flange assembly, which is the most difficult part of the operation. The pinion shaft nut has to be tightened to bring the bearing into a preload state, and either too little or too much preload will cause premature bearing failure. This bearing preload is achieved by tightening the nut on the pinion shaft to crush a steel sleeve until the desired preload is reached, so that the assembly is as rigid as if it were solid. This means it is a tighten and measure, tighten and measure operation. And itís one-way - the nut canít be loosened, if there is too much preload you have to start over with a new crush sleeve (I bought two in case, although I didnít need the spare). Factory assembly uses some very sophisticated computer controlled air driven machinery to do this which just canít be replicated in a shop. It is worth doing some research before attempting this job, and in particular I recommend visiting the Timken Web site for technical info on bearing preloads.

Bolt the case back to the bench, assemble the pinion shaft, including the new crush sleeve, install it and fit the output drive flange. The threaded part of the shaft must be dead clean, as well as the nut. You are going to need an inch-pound torque wrench, Loctite 273 (I couldnít find this so I used 271 which is almost identical), a clock or watch because you are going to be working against the adhesive cure time (get the data sheet from Loctite), and a torque multiplier. This last tool is expensive (around $400), so try to rent one if you can find one. I work on farm stuff, so this gave me an excuse to buy a used one on e-Bay. I used a 4-to-1 multiplier, and I still showed about 125 ft-lbs on the torque wrench I used to drive it, which means the actual torque on the nut was over 400 ft-lbs. This is the torque it took to crush the crush sleeve. You could do it with a large pipe on the end of your socket wrench, but you wonít have the necessary control over the pinion bearing preload, which is what the crush sleeve is for- the idea is to tighten the pinion shaft nut just enough so the frictional drag on the bearing is between 35 and 55 in-lbs. To achieve this requires precise control over the amount the pinion nut is tightened, and checking the turning torque after each 2-5 degrees the nut is turned, which would be very difficult with a long breaker bar. To see how much you are going to turn the nut, measure the difference between the old crush sleeve and the new, and divide this by the pitch of the shaft threads. For example, if the pitch is 13 tpi, then one turn is one-thirteenth (.077) of an inch, and if the crush sleeve difference is say .040 inches, then you will turn the nut approximately one-half a turn to get ďin the zoneĒ of the required preload. But for a number of reasons you will start measuring preload early, in fact, right after you have tightened the nut far enough to take up any slack (you will feel it when this happens). I started measuring about every 5 degrees of tightening. Tighten, remove the multiplier setup, attach the in-lb torque wrench and measure, repeat. With about 30 minutes of cure time for the Loctite, you have canít waste a lot of time. As soon as you get the preload you want, stop! If you have consulted the Timken site you will have noted that too little preload (as long as it is positive) is a whole lot better than too much.

After this things are easier. Stretch the case again and insert the crownwheel and bearing assembly, and then everything else is pretty much cut-and-dried, just reverse what you did to take it all apart. Be sure all shims are in their correct positions and that you have followed your alignment marks. And donít forget to fill the case with new lubricant!

I now have over 600 miles on the rebuild and it is still dead silent, so Iím pleased. It wasnít easy, and if I had to do it again I would seriously consider replacing it, especially since a couple of our well-known dealers have brand-new Power-Lok versions available for a great price. But it was interesting and I ultimately saved enough to do the SLS conversion at the same time and then some. And a few new tools for the box.

8.2 - Transmission Mount ( )

When the transmission mounts fail often the first thing noticed is a knocking noise when cornering as the propshaft hits the tunnel sides.

The tranny mount cross member has large round bucket hanging down. Put hand over the top of cross member towards the right and attempt to find middle of mount. You won't because the top cup of the mount (around 4" diameter) is in the way. Feel out from this and downwards and you should find smooth round steel ridge about 7 " in diameter (same as bucket). This is ring that hold bucket in place.

Between these two items is foam rubber ring set into top of bucket. This is the bit that rots and fails.

If you do manage to get your bearings when feeling mount you should be able to feel sideways slop if mount is shot. There should be no free sideways motion as foam should be snug fit (of course if you push hard you will compress foam).

Replacement is fairly easy as long as you take the weight of the transmission (2x4 on jackstand across sump will do). Remove the 4 bolts carefully as the spring will still be compressed. Try using a trolley jack to hold it and ease it down. The spring force is not huge but will give you nasty smack in the teeth if you're careless.

The rear of the crossmember won't come all the way down and it will start swinging towards rear. This is OK as the spring is almost uncompressed here. Just let it down and the spring will either stay put or pop out gently. There is a wire strap between the rear of the crossmember and the transmission mount. Remove clip and pin and the cross member and mount comes free.

Mount has a ring which holds assembly to cross member. Remove 3 bolts to reveal all the parts.

The foam retainer loses inner surface which allows sideways movement and allows the inner spring carrier to rise up out of the assembly. Because of this it's not obvious where the bits should be.

When putting new one in you have to put the bits in this order:


2...spring cup(large)

3...rubber spring seat

4...large spring

5...foam ring

6...small cup with balance weight mounted in it

7...big ring with travel limiter pin mounting

8...small spring(goes inside small cup)

9...rubber spring seat

This sits in a cup on the transmission, and is stopped from moving to far by the travel limiter cable thatís held in place using a pin

8.3 - Transmission Warning Light Troubleshooting ( John Ping,  )


The ZF 4HP24E9 transmission is a strong and reliable, electronically controlled, four speed automatic transmission with lock-up overdrive. It is of German design and manufacture, which is used by several automotive manufacturers. The electronic control system is by Bosch. Although Jaguar utilizes other transmissions on XJ40 lineup, this design is the mainstay for 90-94 model years. It was also carried over for use in the 4.0 X300 models of 1995-1997. The ZF 4HP22 is quite similar mechanically, but it does not utilize any electronics for control purposes. The ZF 4HP24E9 has approximately 30% greater engine torque handling capacity than the ZF 4HP22.

The ZF 4HP24E9 has the following gear ratios: 1st - 2.48 : 1, 2nd - 1.48 : 1, 3rd - 1.00 : 1, 4th - 0.73 : 1 and Reverse - 2.09 : 1. With a differential ratio of 3.58 on the 4.0-liter engine equipped 90-94 model years, the XJ40 has a road speed of 29.2 mph per 1000 rpm in overdrive. The transmission has an electrical "kick down" switch located behind the accelerator pedal for throttle-manipulated downshifts. The transmission sump capacity for a simple fluid and/or filter change is approximately 3.0 liters (3.2 quarts). The entire system holds approximately 9.5 liters (10 quarts) which is approximately one liter greater than the ZF 4HP22 models. The correct fluid is Dexron III or improved upgrade.


The transmission warning light is designed to "self-check" illuminate when the ignition key is turned to the "ON" position. It should immediately extinguish (along with the other warning lights) a few seconds after the engine in started. Failure to turn "OFF" after an engine startup is an indication of an "electronic" problem with the transmission control system. It does not mean that a mechanical failure has occurred. If the transmission warning light illuminates while driving, pull over at the first reasonable opportunity, shut off the engine and immediately restart. If the light extinguishes on the restart, the problem was likely a control system malfunction (computer glitch). If the light remains illuminated, the transmission will likely default to "Limp Home" mode. In this operational mode, the transmission is electronically locked into third gear. It will not shift properly while in the "Drive" position or by manual J-Gate action. The vehicle is drivable in this condition, but the fault should be investigated and rectified as soon as reasonably achievable.

Note: This transmission has no pressure or temperature sensors. There are no low / high pressure and/or high temperature alarms. The transmission has only a single eight-pin connector providing the interface with internal solenoids or speed sensor.

The following is a step-by-step method for troubleshooting the problem. It's reasonably easy to perform and the cost is minimal. Copy and print this section of the manual so that you have the information at hand when you begin the troubleshooting process. Keep in mind that failure of the transmission control module (or transmission ECU by other names) should not be the first thought in your mind. Like most modern electronic control processors, they have a reputation for reliability and long-life. The fault is typically an ancillary component, but microprocessor failures are not impossible especially since these vehicles are well over ten years old. The following steps assume there is NO pre-existing check engine light illuminated.

Tools Required For Trouble-Shooting:

Typical hand tools for removal of upholstery / sheet metal panels to access the electronic rack and rotary position switch on the side of the transmission. Any good quality digital or analog multi-meter for performing voltage and resistance checks as suggested by trouble-shooting process.

Step #1

Re-boot the Transmission Control Module (microprocessor) by disconnecting the negative battery cable for a few minutes. Re-connect the cable, start the engine and check if the transmission warning light is extinguished. If not, go to Step #2.

Step #2

Check the Transmission Control Module battery power supply fuse. It's typically found in the Left Side Fuse Box in position C2. Remove the five-amp fuse and visually inspect. Replace with a known good fuse of the same amperage if you have any doubt as to its integrity.

Start the engine and check if the transmission warning light is extinguished. If not, go to Step #3.

Step #3

Check the transmission fluid level in accordance with the method described within the owner's manual. Ensure the fluid is at operating temperature by driving the vehicle for at least ten miles prior to the check (further in cold weather). Start the engine, move the gear selector through its range a couple of times to ensure the system is primed. With the engine idling in park, check the fluid level. The level should be between the minimum and maximum "hot" level indications. Add carefully (half liter at a time) to bring the level into the proper operating range. This transmission is reasonably tolerant of high fluid levels. If not certain as to the fluid level from dipstick observation, drain and refill the sump with the appropriate volume of Dexron III.

Re-start the engine and check for transmission warning light illumination, if still present, go to Step #4.

Step #4

Check the throttle position potentiometer. Throttle position is a major input for the control system. The throttle position sensor (located underneath the throttle assembly) provides this input value. On the 90-92 models, the TPS has its own potentiometer supplying a voltage signal to the transmission control module (TCM). On the 93-94 models, the engine management system provides the throttle position information to the TCM. In this case, only one of the two potentiometers is utilized within the TPS.

Locate the throttle position sensor electrical connector (in front and slightly below the throttle assembly) and remove it from its mount. While pressing inward on the side mounted barrel connector tabs, pull the connector apart to expose the male and female halves. Inspect the pins and sockets for corrosion and residue. Clean the pins and sockets with either a good quality electrical contact cleaner or a brake system/electrical parts cleaner. Thoroughly spray the pins and sockets to ensure they are clean and free of residue Ö do not use any petroleum-based solvent, which leaves a residue or could attack the connector seal.

If you have a wiring diagram available, connect a digital multi-meter across the "transmission" throttle position potentiometer power input and wiper to measure component resistance.

Note: For 90-92 models, the "white with blue tracer" conductor is the throttle potentiometer power input. The "green with blue tracer" is the throttle potentiometer wiper. A failure of the throttle potentiometer on the 93-94 models will result in a "check engine light" being illuminated.

With the throttle closed, the resistance should be approximately 5.5k ohms. With the throttle fully open, the resistance will be approximately 250 ohms. Ensure the resistance change is smooth while the throttle is operated. There should be no "abrupt" changes in resistance as the wiper moves. Re-connect the halves of the TPS connector while ensuring good alignment and seating of the pins.

Start the engine and check if the transmission warning light is extinguished. If not, go to Step #5.

Step #5

Inspect the transmission rotary switch located on the side of the transmission housing. Lift the vehicle in a safe manner; remove the debris shield located in front of the rotary switch (two 10 mm bolts). Initially, inspect the exterior of the rotary switch and its wiring for obvious defects. Remove the rotary switch cover (six T20 torx screws) to allow visual inspection. Do NOT touch the rotary switch mounting/locating bolts. Using a good quality electrical contact cleaner or a brake system/electrical parts cleaner, liberally spray the copper slides and runners to remove any corrosion film or dirt. Exercise the J-Gate shifter while cleaning to ensure all surfaces are sprayed. Re-install the rotary switch cover.

Further back (towards rear of car) on the transmission housing, locate the transmission electrical connector (round screw-on type) and inspect for any obvious damage to the connector or its wiring bundle. It is not necessary to remove this connector as it is quite difficult to re-install. Electrical checks can be performed on the transmission components from another more readily accessed connector.

Replace the rotary switch debris shield and lower the vehicle. Start the engine and check if the transmission warning light is extinguished. If not, go to Step #6.

Step #6

Check and/or Replace the "transmission relay". This relay is a simple four pin, 1" cube style design common to XJ40s. The relay is typically located in the front passenger footwell region. Consult your wiring diagrams, or post to the XJ40 List for location information. Its not usually necessary to purchase a new relay immediately as you can substitute any typical four pin relay for this application.

Disconnect the battery negative cable Ö do not forget this step as battery power is constantly supplied to many electrical / electronic devices. Remove the passenger side footwell upholstery cover. Remove the electronics and relay rack sheet metal cover.

Note: On early 90s models, the transmission control relay is mounted on the front of the passenger side climate control blower housing. You will need to remove the four (4) sheet metal screws that mount the "black" relay panel to the electronic rack. The lower two screws need not be completely removed as the panel has "slots" incorporated to slide over the partially fastened screws. Lower the relay panel to access the transmission control relay. It typically has a "purple" relay socket.

Re-connect the negative battery cable. But before replacing electronic rack panels, start the engine and check if the transmission warning light is extinguished. If not, go to Step #7.
Step #7

Again, disconnect the battery negative cable.

Inspect and/or replace the transmission decoder module. This module is a solid-state device containing two integrated circuits on a printed circuit board. Its purpose is to translate transmission gear shifter position from the rotary switch to the Transmission Control Module. It takes input from the transmission rotary switch and converts it to a two level signal (ground or open circuit) for the TCM.

The transmission decoder module is located on the rear side of the relay rack panel in the upper right side corner. It is directly behind the Door Lock Control Module. It's a rectangular "black" module with a "blue" multi-pin connector. It is mounted with two small machine screws secured with 7 mm locking nuts.

Remove the electrical connector and inspect for obvious problems. It is also possible to inspect the circuit board by removing the module cover and slipping out the board. If no defects are found, you may wish to simply replace this part as its reasonably inexpensive (new Ö $50 US, used Ö $25 US). Re-attach the electrical connector to the decoder module and re-mount to the relay rack panel.

Re-connect the negative battery cable. But before replacing electronic rack panels, start the engine and check if the transmission warning light is extinguished. If not, go to Step #8.

Step #8

Again, disconnect the battery negative cable.

Inspect and check the "Transmission Control Module (ECU). The TCM is located at the lower right region of the footwell area while the Engine Management ECU is located at the lower left. There is a schematic layout drawing on the electronic rack sheet metal cover providing component location. Typically, you will need to remove the front passenger side footwell climate control duct to access the TCM. There are no fasteners, just slip it off the black plastic outlet nozzle.

Disconnect the electrical connector to the TCM by lifting upward on it silver colored hasp. This action will pivot the connector off the TCM. Visually inspect and thoroughly clean both connector pins and sockets using a good quality electrical contact cleaner or a brake system/electrical parts cleaner.

The TCM electrical connector is a three-row type having 55 pins. Note that not all pin locations are utilized. Pin identifications are as follows: Viewing the connector in the horizontal plane with the wiring bundle on the right side, the top far right female pin female is #1 and continues across to the top far left female pin #19. The second row starts with #20 on the middle far right and continues across to the middle far left being # 37. The bottom far right starts with #38 and continues across the to the bottom far left being #55.

Electrical checks can be performed on the internal transmission components at this time if you have a quality digital or analog multi-meter. The three (3) internal solenoid valves should have an approximate resistance value of 34 ohms at 20 įC (68 įF). The resistance values can go as high as 60 ohms with the transmission at elevated temperatures. The main point is to verify a reasonable resistance value for each solenoid. The solenoids should have very similar resistance readings.

Resistance measurement points for solenoids MV1, MV2 and MV3 using the conductors provided by the TCM multi-lead connector are as follows:

Pin #19 (Solenoid Valve Power Supply) to Pin #5 (Solenoid Valve - Forward Drive)

Pin #19 (Solenoid Valve Power Supply) to Pin #24 (Solenoid Valve - 2nd)

Pin #19 (Solenoid Valve Power Supply) to Pin #42 (Solenoid Valve - Lockup)

Again, all three solenoids should have approximately the same resistance although this value will change as a function of temperature.

Resistance measurement points for the Pressure Regulating Solenoid using the conductors provided by the TCM multi-lead connector are as follows:

Pin #19 (Solenoid Valve Power Supply) to Pin #6 (Pressure Regulator Valve Solenoid)

This solenoid valve should have an approximate resistance value of 5-7 ohms. The value may be slightly higher with the transmission at elevated temperatures.

Resistance measurement points for Output Shaft Sensor using the conductors provided by the TCM multi-lead connector are as follows:

Pin #2 (Output Shaft Speed Sensor) to Pin #38 (Output Shaft Speed Sensor)

This sensor should have an approximate resistance value of 300 ohms (+/- 10%).

Check for proper TCM ground conductors by checking for minimal resistance between Pin #7, Pin #9, and Pin #26. There should be not significant resistance between any of these pins and a solid chassis ground.

Check for proper TCM power supply by performing the following voltage checks at the connector. Constant 12 VDC (battery power) should be supplied at Pin #39. Ignition switched power should be available at Pin # 1 when the ignition switch is in the "ON" position.

If any of the above resistance or voltage readings are abnormal or non-existent, problems are likely in the wiring harness or the solenoid(s) have failed.

Re-connect the TCM multi-lead electrical connector. Re-install all previously removed electronic components and replace the metal shield and upholstery panel.

Re-connect the negative battery cable. Start the engine and check if the transmission warning light is extinguished. If not, go to Step #9.

Note: The TCM also interfaces with the Engine Management ECU, but this type of diagnostics is best left to a franchised Jaguar dealer that has the necessary (proprietary) test equipment.

Step #9

Take your XJ40 to the Jaguar dealer of your choice and have the Service Department run a diagnostics check on the TCM. Franchised automatic transmission repair shops are NOT likely to have the necessary diagnostics equipment for your XJ40. If the dealer diagnostics indicates a major component fault, instead of purchasing a new transmission control module (ECU) from Jaguar (high expense) consider purchasing a good used component from a salvager or reputable used parts supplier.


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This section currently maintained by Pascal Gademer; questions, comments, submissions and suggestions, email


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