Jaguar XJ-S. Manual - part 83

 
 

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Slightly different explanations -- but either way, the fix is exactly the same:  Drop the transmission pan and replace the 
filter, making sure to get the intake tube and O-rings installed properly when reassembling.  Continue reading for more 
concerns related to this same set of O-rings. 

 

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 and 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°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. 

Richard Chapman says, “I recommend using 2 O-rings on the oil pickup tube connection to the pump within the trans, 
this is a common point where air can creep in. Also check that the valve body bolts are not loose while you're in there.” 

Greg Meboe says, “The filter came with one O-ring only.  The B&M shift kit came with 2 additional rings, and it said 
that 2 rings should always be fitted to eliminate air being sucked into the pump.  I was able to get three on, and still 
have plenty of pipe engagement into the trans case.  The pick up tube is held in very securely now.” 

Mark D Young had similar problems, and watched as a mechanic worked on it:  “He had indicated that he thought the 
filter had dropped into the bottom of the pan and he was correct.  There is little but a push-fit O-ring used to secure the 
filter.  He used two flat washers and a circlip to install a new filter securely.  The O-ring fits over a pipe that goes up 
into the tranny and seats in a recess.  This chap cut a lip near the extreme lower end of the pipe and fiddled with a 
washer that cleared the pipe diameter (just) and then he pushed this up to past the recess and slipped the circlip into the 
recess.” 

Apparently sometimes it’s not the O-rings; sometimes it’s the plastic pickup tube itself.  Craig Sawyers says, “If the 
tube is cracked, it is dead easy to replace - it just pulls out.” 

Slipping is not good on the transmission, and the sooner it is corrected the less damage will be done. 

 

GM400 FLUID AND FILTER REPLACEMENT:  Before buying a new pan gasket, note the shape of the gasket 
needed; as noted above, they aren’t all the same. 

Dale Green says, “If the transmission filter is dirty, then when the fluid is cold, there is a delay in adequate flow through 
out the transmission, which in turn will delay the shift change.  By either changing the fluid and filter or by having a 
flush done at a shop with the flush equipment, you can then determine if this cured your problem, or if a rebuild is due. 

“As the fluid gets contaminated with deposits from the wear of the internal parts, the fluid becomes thicker.  As these 
deposits are collected by the filter, the filter becomes clogged.  These deposits also cause the internal parts to become 
sluggish as they plug the clearances around the parts.  Once the fluid is warmed up, there is a better chance of flow, but 
the conditions mentioned should be used as a sign to change the fluid and filter.” 

 
 

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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 -- see page 341.  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. 

Steve Kennedy adds worse news: apparently some shops have figured out that they can drain the fluid without 
removing the transmission mount by simply bending the pan down.  When all done, simply bend it back into place 
(probably with some sealant, since you can’t replace the gasket this way), reinstall the bolts and refill.  He suggests you 
question the shop closely to make sure this isn’t what they have in mind. 

This author once made a suggestion to replace the double-ended bolts at the rear end of the pan where the transmission 
mount is attached with bolts and spacers.  That way, perhaps the pan could be removed next time without disassembling 
the transmission mount; just remove the bolts and slide the pan forward until the spacers fall out and the pan clears the 
transmission mount.  Patrick MacNamara responded:  “When I got the Cat, someone had already replaced the double-
ended bolts with regular ones and spacers.  The first time I changed the filter and gasket I tried the trick you describe.  I 
ended up with a leak at the back of the pan from too much compression and twisting trying to fit it back in place.  There 
is definitely enough room to slide forward and down but the replacement of the cork gasket and pan into the small 
space spelled disaster.  I did the job again two days later with the complete mount diassembly...” 

If you’re doing this job at home, Brian Schreurs provides a procedure:  “Prepare to get messy.  I hope you have a nice 
big drain pan. 

“Remove the bolts along the back row.  Loosen the rest of the bolts progressively from there: back side bolts a lot, front 
bolts hardly at all.  At some point as you keep progressively loosening the pan, the weight of the trans fluid will break 
the gasket's seal and the pan will drop.  Of course you were loosening the bolts progressively so the back edge of the 
pan will drop rather further than the front, and most of the fluid will dump out the back where your drain pan is handy. 

“Keep removing bolts progressively and the pan will keep tipping down, depositing more and more fluid into the drain 
pan.  As you get to the last few bolts there will not be very much fluid left to spill on you.  (remember to do the bolts 
progressively, you do not want the weight to be concentrated on the front of the pan too early, or you risk bending the 
lip of the pan) 

“As you undo the last few bolts with one hand, hold the pan in position against the trans with the other.  Then carefully 
lower the pan, only to suddenly discover that you missed a bolt, which unbalances the pan and dumps the remainder of 
the fluid down your sleeve.  Startled, you let go of the pan which is heavy enough to pull that last bolt free of the caked 
gasket material; it drops into your drain pan and splashes the side of your face with Dexron III. 

“After cleaning yourself off, see to it that you scrub the inside of the pan clean.  Clean up the magnet too, and don't 
forget to put it back in.  Scrape the remains of the gasket off the trans and the pan.  A gasket scraper works much better 
than a screwdriver and is cheap.  Change your filter even if it looks ok -- it is not.  It never is.  No matter how often you 
change your trans filter, you do not change it enough. 

“If you find little bits of metal in your trans pan that are large enough you can actually identify them as little bits of 
metal, not just sludge, then count on something expensive needing to be repaced in the none-too-distant future. 

“Getting the new gasket aligned right may seem like an impossible task, what with its odd shape and the fact that it was 
no doubt balled up by the filter manufacturer.  You'll find that the bolt holes are slightly undersize to provide a good 
seal so if you jam 4 bolts (1 in each corner) through the pan & gasket ahead of time, the weight of the bolts will go a 
long way in forcing it to keep its shape.  Don't tighten any bolts till you have all of them started.” 

Richard Chapman has a suggestion:  “You can avoid alot of mess by pumping out the trans before you remove the pan; 
this method does not get out all the fluid and leaves a bit less than 1" in the pan.  This is enough to get it off cleanly if 
you can keep it level.  Method: disconnect the lower cooler line (beside the alternator) either at the trans pipe end or the 
radiator end.  Regardless the lower pipe from the trans is the outlet.  The other end will dribble a bit, so have a plug 
handy.  Do whatever to run a tube from the lower line into a 1-2 gallon can.  Run the engine at idle and the trans 
internal oil pump will pump fluid into the container, when it starts to splutter, you're done - takes less than 1 minute. 

 
 

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“Before doing this run the engine a few minutes if it has stood for a while.  This will ensure there is some lube in the 
trans.  Do not rev the engine, idle only as the fluid returning from the cooler is used to lube the bearings in the trans and 
with this all disconnected there is none.” 

The lack of a drain plug is apparently deliberate; the designers wanted to ensure that mechanics would not change the 
fluid without changing the filter.  But it does make it very messy for the do-it-yourselfer to remove the pan.  lan Jenks 
reports that B&M Racing (see page 711) 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 1¼” deeper for “extra capacity”.  They’re both chrome plated, and cost only about twenty bucks.  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.” 

Steve Chatman retorts:  “I ordered one of the chrome-plated over-size transmission pans from J. C. Whitney as 
mentioned in The Book.  At least with a Series II XJ12, it doesn't fit.  The pan contacts the exhaust pipe and, even if 
you try to insulate that, the bolts along one side would be inaccessible.  I regret not ordering the regular size.  Thinking 
back to the comment in the book, the author said that it didn't fit over the filter, right?  Well, neither does the original.  I 
believe that both push the filter up slightly.”  Aaaargh!  Maybe better just stick with the stock pan and add a drain plug 
kit. 

Peter Cohen says, “A few years ago, I was availing myself of the free half hour inspection that Jag dealers were doing 
to try to bring in service revenue.  The mechanic spotted the cork gasket on my oil pan, and commented on its 
"aftermarket" nature.  He said that the real Jag gaskets were better, because the cork tended to leak (mine was, in fact, 
leaking in a minor way).  Since then, I have used Jaguar trans pan gaskets, even though they cost more than the filter 
kit.  IMHO, it has been worth it.”  Of course, if the cork gasket does the job...  After all, there are a lot of Chevys out 
there with transmissions that don’t leak. 

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.” 

 

FLUID LEAKS:  Michael Pakonis looked for a leak for a while.  “At first it looked like the pan gasket, so I changed 
the gasket, refilled with fluid only to find it still leaking. After laying under the car with a rag and parts cleaner looking 
for the leak, I found it, it was the detent solenoid plug in the side of the case; bad O-ring and the plastic plug work 
harden, fell apart when I removed it.  I would suggest anyone changing the pan gasket to replace this plug, GM Part 
number 8629503, $10.00.” 

 

SHIFTING PROBLEMS:  If your car isn’t upshifting properly, Robert Warnicke says, “One thing to check is what 
happened to me.  The little plastic elbow in the vacuum line to the modulator got too close to the pipes and melted 
shut...” 

Or, the modulator itself could be bad.  Bill Freeman says, “the original vacuum modulators on the GM-400 
transmission get weak and will fail.”  Being near hot exhaust components doesn’t help; see page 168. 

 

GOVERNOR DRIVE GEAR FAILURE:  I’ll let George Balthrop explain both the problem and the solution: 

“PROBLEM:  While cruising at 70, suddenly & without any noise or warning of any kind, the transmission kicked out 
of top gear, and when I pressed the throttle, the engine just revved as if in Neutral.  I coasted to a stop on the shoulder 
and checked for transmission fluid leak -- level OK and nothing under car -- not wet.  Got back in, and car would back 
up normally, and pull forward normally, it just wouldn't upshift from 1st regardless of the selection: 1, 2 or D.  It would 
freewheel in D or 2 when let off accelerator (no engine braking) so I limped back home via back roads. 

 
 

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“CAUSE:  Nylon gear on shaft of governor had worn away in the middle of its length, where it rides on the metal gear 
on the output shaft.  This is believed to have been caused by rust buildup on the metal gear (above at-rest fluid level) 
from the car sitting unused for several years.  If the governor does not turn, its spool valve which is operated by 
centrifugal force will never open to send fluid under pressure to the shift valves, so no upshift. 

“FIX:  The governor may be removed without getting under the car, accessed as follows: 

1) Slide the right seat fully rearward & remove the carpet from the Right footwell floor; 

2) Peel up the carpet & pad on the Right side of the transmission tunnel to expose the access cover, located just ahead 
of the seat and approximately even with the shift lever. 

3) Remove the access cover which is attached to the transmission tunnel sheet metal by 3 screws and 2 studs/nuts @  
1/2".  The uppermost screw is at the top & hidden by the carpet just below the leather console cap, but may be removed 
with a #3 Phillips or Pozi-driv equivalent, without removing any part of the console. 

4) Place a drain pan on the ground (not in the car) under the access opening & remove the four 1/2" bolts securing the 
governor cover, on the right side of the transmission, above the right-rear corner of the transmission pan.  All four bolts 
may be removed using a 1/4" socket set with U-joint via the access cover opening.  Remove the cover, & some fluid 
may drain from the transmission case, but it shouldn't gush into the car. 

5) The governor is the part just under the cover; grasp the top of this part and twist without pulling.  If it turns, you have 
found your problem. Now remove it by gently pulling as you twist. The governor should pull straight out through the 
access hole.  There is a nylon gear on the inboard end of the governor shaft.  If this gear is worn so that the center is 
dished or smaller diameter than either end to such an extent that the teeth are nearly gone in the center, then the 
governor is not being turned by the output shaft gear and the shift valves never get pressure via the spool valve in the 
governor. 

6) Replace the nylon gear.  I found a transmission parts & repair facility (County Transmissions in Vienna, VA) where 
very nice & experienced personnel replaced my worn gear for $5 (no labor charge) took several minutes to explain how 
the wear probably occurred (rust on output shaft gear) and even explained how to replace the gear again if this one goes 
bad, and gave me a second replacement gear for another $5.  Now that’s service! 

7) Remove nylon gear by tapping out retaining split pin.  Clean spool valve beneath & entire assembly.  Fit new gear to 
shaft, driving it fully home against the shaft end.  Using a 1/8" drill bit, drill a hole for the split pin, half way through the 
shaft of the new gear, then rotate and drill from the other side to meet the 1st half and finish the hole -- this to avoid the 
shaft breaking.  Tap in the new split pin and peen the ends to prevent slipping out. 

8) Replace and top up with fluid as necessary, and smile when it upshifts.  Carry that spare gear though, because there 
are several possible causes for the original gear wearing, and the cause may still be present and do the same thing to the 
new one.  I figure several nylon gears ought to clean the rust off the metal output shaft gear though ;-)” 

Balthrop advises that if your car won’t shift out of 1st, “It takes less than an hour to get to the governor (the first time 
you do it--maybe 20 minutes once you've been there), so rather than mess with other tests, I would advise just pull it & 
check it out.” 

Martin Runneals adds:  “The transmission mechanic that fixed mine said that it probably was caused by a worn bushing 
on the output shaft.  This allows the metal gear to skip and wear the plastic gear faster.  The permanent fix would be to 
replace the bushing.  My '84 has been running fine for almost a year with the simple fix.  Since the plastic gear is only 
$5, I'll definitely pick up a spare gear. 

“He also told me that the worm gears were made of brass at one point, but they would also strip.  Then you would have 
to clean the brass out of the teeth of the corresponding gear on the output shaft.” 

 

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.  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