Jaguar XJ-S. Manual - part 85

 
 

335

BAND ADJUSTMENT:  Huh?  There is no band adjustment on the GM400!  Pekka Eklund clarifies:  “You can adjust 
the bands on GM400.  You just drop the pan and the valve block.  Then you have access to the accumulators.  Open 
them carefully and you'll find piston and "conrod", I don't know the right word for that stick. 

“Jaguar did offer at least 3 different length sticks for GM400.  By making this stick longer (I welded mine) you can 
adjust the band.  When you have the accumulator open, consider a change in the piston ring material too.  That should 
not be made of teflon, because it will leak after few thousand miles and allows the band to slip. 

“The stick has the right length when the drive shaft turns when you push the accumulator piston up.  And you should 
not be able to turn the drive shaft by hand then.” 

 

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. 

 

KEEPING THE TRANSMISSION COOL:  Greg Meboe:  “One other thing I did to prolong the life of this box was to 
insulate the metal oil cooler lines which run 1 cm. away from the RH catalyst.  I used a multi-ply aluminum foil 
wrapped and stapled over loose fiberglass cloth weave.” 

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 ‘box...” 

In stock form, the GM400 is kept cool by a heat exchanger in the end tank of the radiator -- the same setup used in 
many other automobiles.  It seems plausible that this method is used to take advantage of the thermostatic control of the 
engine temperature; since the engine won’t run cold because the thermostats won’t let it, the transmission won’t run 
cold either.  If this is the case, then it might make sense to install any additional transmission coolers in the line before 
the heat exchanger in the radiator, so that the heat exchanger can still control the transmission fluid temperature -- by 
heating the fluid if necessary, which would mean that the added transmission cooler becomes an engine cooler. 

Bill Fernandez says this isn’t so:  “There is no such thing as too cold for an automatic, the colder the better.  Routing 
the trans lines through the radiator first and then through the cooler and back to the trans box will work just fine.  
Routing the lines from the transmission to the cooler only and back will also work just fine. 

“Some may think that a cooler is unnecessary in the XJ-S; I beg to differ for several reasons.  A trans cooler is a must in 
the XJ-S for long dependable transmission life.” 

Fernandez puts considerable blame on the catalytic convertors.  “These buggers generate a huge amount of heat and 
nearly lay on the sides of your transmission, cooking the fluid and baking the clutches.” 

Of course, one major factor in the durability of the GM400 in the XJ-S is undoubtedly the fact that the engine is prone 
to overheating -- which means the transmission is as well, since their operating temperatures are linked at the radiator.  
Getting the engine temperatures under control would go a long way towards making the transmission last longer. 

See the note on transmission overheating on page 323. 

 

 
 

336

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 (see page 326), boosting 
the line pressure (see below) or installation of a “shift kit” may be in order. 

 

RAISING THE LINE PRESSURE:  Tim Blystone sends this idea in response to the suggestion of using Type F fluid 
on page 326:  “The F-Type fluid is somewhat thinner and is supposed to allow the tranny to shift faster and more 
positively.  It does no harm and neither have I ever been able to see any good it does either.  I would rather use Dexron 
3, remove the pump and replace the pump pressure spring with a new red spring.  Used in Corvettes.  This will raise 
line pressure and make the whole thing shift more positive.” 

 

GM400 TRANSMISSION MODS:  Helpful sources include B&M Racing (page 711), TCI (page 718), and Turbo 
Action (page 718). 

 

GM400 TRANSMISSION SHIFT POINT MOD #1:  The 5.3 liter Jaguar V12 was designed prior to the fuel crunch of 
1973 and has “over-square” engine dimensions -- the bore (90 mm) is considerably larger than the stroke (70 mm).  
Such a design is conducive to high performance at high RPM and is therefore ideal for racing applications, but results 
in poor low end torque.  However, to optimize fuel economy, it is best if an engine is turning rather slowly, so the stock 
GM400 automatic transmission is set up to keep the V12 in the RPM range where its performance is poorest. 

The point at which the GM400 shifts is determined primarily by a governor that responds to engine RPM, and is varied 
by two things:  1) a vacuum modulator that makes the car upshift promptly when vacuum is high (light throttle); 2) an 
electrical switch on the throttle linkage that forces the tranny to downshift, and remain in lower gears until 5000 RPM, 
when the pedal is pressed all the way to the floor (“kickdown”). 

The vacuum modulator is a bolt-on device on the right side of the transmission.  Some of these modulators are 
adjustable by removing the vacuum hose and inserting a straight blade screwdriver into the vacuum fitting.  Turning the 
adjustment changes the preload on the spring, therefore altering the shift points. 

The vacuum modulators are available in several sizes, and since they are a standard GM part, are found in any auto 
parts store for reasonable prices.  In general, the smaller they are, the more vacuum required to make the tranny upshift, 
so the longer the car will remain in the lower gears.  Of course, it is recommended that you make sure to buy one that is 
adjustable. 

Alan Jenks “fitted a B&M adjustable vacuum modulator (B&M #20234).  This fits all TH400’s.” 

Rather than buying a smaller modulator, Greg Meboe modified his existing one: “I needed to increase the spring 
pressure against the diaphragm.  I did this by collapsing the modulator body around part of the spring in a vise, 
effectively shortening the working length of the spring.  I imagine I took about 30% of the travel out of the spring.  This 
had the effect of increasing the effective spring rate.  The vacuum modulator modifications produce an upshift between 
500 and 1500 rpm’s higher than before, depending on throttle position.” 

 

GM400 TRANSMISSION SHIFT POINT MOD #2:  For absolute maximum performance, shift points should be 
selected that surround the peak on the horsepower curve -- without exceeding the redline, of course.  The engine should 
be allowed to run past the power peak, so that when shifting the engine doesn’t drop back too far below the power 
peak. 

 
 

337

The published stats rate the Jaguar V12 at 5000 RPM.  With the large steps between gears on a three-speed 
transmission, the engine should be taken to the redline of 6500 before shifting to optimize performance. 

The GM400 slushomatic transmission, of course, was designed with the 1950’s-design pushrod V8 in mind.  Not only 
does it normally shift at relatively low rpm, it absolutely refuses to allow the engine to rev beyond 5000 rpm; it will 
shift at this point even if you have selected low gear with the lever.  The above modification to the vacuum modulator 
will not affect the limit where the GM400 will force a shift. 

The governor assembly is contained under a sheet metal cover on the right rear side of the transmission, and there is an 
access cover under the carpet of the XJ-S to get to it.  If you remove the governor and grind a little weight off of each of 
the counterweights, the engine will shift at higher RPM under all conditions.  This modification will allow the engine to 
run faster than 5000 RPM, where the stock GM400 will force a shift no matter what. 

It may make sense to modify the governor as described, and install a large vacuum modulator to keep the light-throttle 
shifts at a reasonable RPM. 

According to Chad Bolles, the GM400 in the 1991-92 XJ-S was programmed differently than earlier cars, providing 
higher RPM shifts under power, better downshifting, and holding low gear forever if the shifter is held in 1st.  It may be 
possible to retrofit the changes into earlier cars (replace the governor, etc.) or to buy the later transmission and retrofit 
it. 

 

GM400 TRANSMISSION SHIFT POINT MOD #3:  If you want serious performance from your XJ-S, install one of 
the many available “shift kits” for the GM400.  Keep in mind when you select your kit that your objective is to allow 
higher RPM shift points at full throttle (when the kickdown switch on the throttle cable is activated) or when manually 
shifting;  High RPM shifts when taking it easy are of little benefit and will hurt fuel economy and passenger comfort.  
Many shift kits offer more firm shifting, some shift at redline at all times (!) and some even render your car a 3-speed 
manual. 

Most shift kits can be installed by simply lowering the pan from the transmission and removing the valve body within.  
Replace a few springs and the like, and reassemble.  See the notes on page 325 regarding the differences between pre- 
and post-1988 GM400 transmissions.  

Alan Jenks reports on a “‘Shift Improver Kit’ from B&M Racing to the Turbo Hydromatic 400 automatic transmission. 
 There are two kits, one for pre-1988 TH400 (B&M #20260) and one for post-1988 (B&M #20261).  The kit can be 
fitted in two forms, either ‘heavy duty’ (stage 1) or ‘street/strip’ (stage 2).  Stage 1 provided firmer shifting without 
compromising the quality of the ride, which is what I wanted.” 

Per Chad Bolles:  B&M makes a product called a Transpak, “with this you can set up the trans to shift like you want, 
from very hard, to normal street.”  B&M also makes a Racing Kit, “this is a race very hard shift kit (neck snapping).”  
Finally, in what should probably be listed as GM400 Transmission Mod #3A, there is a replacement valve body.  “The 
B&M valve body changes the shift pattern to 123NRP instead of PRN321.” 

Jim Hall says, “I installed a TransGo competition shift kit recently.  In addition to very positive shifting, it has the 
added benefit of allowing downshifts to 2 and L at any speed.  I'm delighted with it.” 

Jim Cantrell says, “The TH400 in modified form normally has a whine in first gear.  This is the planetary gears.  I don't 
really understand why, but installing a shift kit seems to increase the whining.  It is considered normal behavior and at 
least on the one I have, has gone nearly 100k miles with no problems.” 

Jan Wikström had his local tranny shop “lock up the freewheel on 1st gear, which gives me respectable engine braking 
and a better “feel” in the really low-speed twisty bits.  Caution: it’s now possible to engage 1st at any speed...” 

Paul Burke reports on “some information regarding "tweaking" of the GM400 box.  It describes removal of four of the 
six balls, shift improver plate (different valve body transfer plate) and plugging one of the passages.  The article was in 
Musclecar Review, February/March 1997 starting on page 58.”  Sean Straw adds, “The publisher's (lackluster) website 
is at: 

 

 

http://www.d-p-g.com/musclecar/

 

 
 

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The back issue can be ordered by calling:  01 (941) 683-8807.” 

 

TORQUE CONVERTOR REPLACEMENT:  First off, you might wish to note Roger Bywater’s comments on the 
stock GM400 torque convertor on page 32. 

There are torque convertors available that have a higher “stall speed”, which sorta means they engage at a higher RPM. 
 This is wonderful for the Jaguar V12, but does require removing the tranny to change.  According to Thomas E. 
Alberts, “B&M tells you to try for 500-750 RPM less than the RPM at peak torque.” 

Michael Kenrick reports that the torque convertor from a 3.8 Buick Regal Turbo can be used to provide a more suitable 
stall speed. 

According to Chad Bolles, the later model XJ-S uses the HI-Stall convertor.  “It is the same converter used in the 427 
L-88 and LS6 and LS7 Chev, flash stalls around 2600-2800rpm, the difference being the Jaguar converter has 6 lugs on 
it where the Chev has 3.”  He adds that a Chevy convertor may be used in the Jag, leaving the other three bolt holes 
unused.  Reportedly, some GM torque convertors use six bolts as well. 

Bolles also explains the difference between stall speed and “flash stall”.  Stall speed is the RPM the engine runs at 
continuously if the brake is held and the accelerator is floored in gear.  Note:  this is very hard on the equipment, and 
causes a lot of heat in the transmission; such tests should only be done for a couple seconds, and the car should be 
driven around at normal speeds for a while afterward to allow the transmission fluid cooler in the radiator to cool the 
transmission back down.  “Flash Stall, hold the brake, trans in gear, quickly mash the acc pedal, watch the Tach, it will 
flash to about 2500-2800 rpm, but if you just hold the brake and ease the acc pedal down the converter will not reach 
the same rpm before it tries to spin the rear tyres.  Don’t ask me why, I just know that that’s how it works.” 

 

GM400 RATIO CHANGE:  Thomas E. Alberts says, “...I agree that the standard gearset is poorly matched to the 
XJ-S... Note that the modified gearsets, like TCI’s, lower the second gear ratio as well as first.  Observe: 

 

 

TH 400 

TCI gearset 

 1st 

2:48 

2:75 

 2nd 

1:48 

1:57 

 3rd 

1 

1 

I think it would make a very nice improvement and without any sacrifice of top end speed.” 

 

ADDING OVERDRIVE TO THE GM400:  John Goodman has heard about “overdrive units manufactured by 
American Overdrive Inc. designed for autoboxes, GM400 included?  This effectively changes your GM400 into a six 
speed. 

“Two possibilities here would give much better acceleration.  The unit can be supplied with the internals reversed so 
you have underdrive first, underdrive second and underdrive top.  Or install a lower axle ratio and leave the overdrive 
in and switch it out for “Sport Mode”.” 

 

 

ALTERNATIVE TRANSMISSIONS 

 

TH700R4:  One way to deal with the shortcomings of the GM400 or BW12 is to replace it with the more modern 
TH700R4.  One large benefit is that the TH700R4 will provide much better performance from a standing start: the 
torque convertor provides a larger effective reduction, and first gear is much lower than the GM400’s 2.40:1.  Plus, it