Jaguar XJ-S. Manual - part 82

 
 

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DRIVETRAIN

 

 

The XJ-S was originally fitted with a Borg-Warner Model 12 automatic transmission, but in 1979 this was replaced 
with the GM400 automatic.  According to Ed Sowell, “If it's a BW12 the dipstick/filler neck will be at the left side of 
the engine bay.”  The dipstick/filler is on the right side with the GM400. 

In the mid-70’s reportedly 352 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. 

 

OVERHEATING:  An automatic transmission generates waste heat as a result of the “slipping” of the torque convertor. 
 It always slips; more at low rpm and less at higher speeds, but the only way it won’t slip at all is if there’s a “lock-up 
torque convertor” fitted -- popular on some modern transmissions, but not on the GM400 or Borg-Warner 
transmissions used in the XJ-S.  There is a cooler provided to deal with this waste heat; it’s in the end tank of the 
radiator, and therefore rejects the transmission heat into the engine cooling system. 

If there are faults in the automatic transmission, it can greatly increase the amount of waste heat generated.  David 
Hodges had a transmission that he knew was malfunctioning, and before he fixed it he did some careful testing.  With 
calibrated temperature sensors in each bank of the engine, he drove 125 miles at various continuous speeds.  Then he 
installed a small transmission cooler and made the return trip under nearly identical driving and weather conditions.  He 
showed conclusively that, with known transmission faults, the small transmission cooler lowered the engine 
temperatures significantly -- from just a few degrees at low speeds to as much as 18°C above 100 mph.  Hodges adds:  
“At idle with the separate transmission cooler the engine cooling fan (electric) runs for shorter periods of time before it 
switches off and the gap between switching back on is most definitely longer.” 

Hodges’ conclusion:  “Bearing in mind the V12's tendency for overheating and the trouble many have in curing it, a 
faulty transmission could be leading many up the garden path.  I only noticed the initial problem with the gearbox 
whilst driving very hard.  It makes you wonder if a gearbox that is 'iffy' and has gone undetected could be contributing 
to the overheating problem so many have experienced.” 

There is much discussion on keeping the GM400 cool on page 335.  Much of what is said may apply equally to any 
automatic transmission. 

 

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. 

“We took part of the console apart and 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.  The current chose the next best available route, 
which was through the shifter cable to the body.”  This problem is reportedly common enough that some parts clerks 
are familiar with it. 

Avoid this problem and install an additional ground strap.  See page 553. 

 

 

BORG-WARNER MODEL 12 TRANSMISSION 

 
 

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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.  Robert C. 
Warnicke:  “I have a '73 pre-H.E., '84 H.E., T400 and BW sitting in my carport.  The bolt patterns appear to be the 
same.  The H.E. has two dowels on the back that are much bigger than the dowels on the pre-H.E.  At least one of these 
dowels is in a different place as well.  I don't know how important the dowels are with all the bolts, but one will have to 
be removed off the pre-H.E. for the T400 to fit, and the other will be loose in the T400 hole unless something is fabbed 
to make it fit tight.” 

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:  If you have the early XJ-S with the Borg-Warner automatic, the owner’s handbook specifies Type F fluid.  
However, there are reports that some Jaguar dealers refill these transmissions with Dexron anyway, because they feel 
their customers appreciate the smoother shifts resulting from the slipperier fluid.  This might not be a good idea, since 
firmer shifts cause less wear. 

 

 

GM400 TRANSMISSION 

 

REPAIR MANUALS:  Craig Sawyers says, “I rebuilt my GM400 using just the ROM.  However, I wish I'd bought 
Ron Session's book on "The Turbo Hydra-matic 400" - tells how to take all variants to bits and overhaul using "real 
world" tools.  Includes performance tuning too.” 

Richard Chapman adds, “I have it and it gave great insight into the internals.  Some of Ron's suggestions do not seem to 
get acceptance in the industry it would appear.” 

George Balthrop suggests “Haynes Techbook 10360 "GM Automatic Transmission Overhaul", which covers both the 
TH400 and the TH700, provides information on the function of this transmission, special tools, in-car repairs, testing & 
trouble-shooting, full overhaul & modification.” 

 

 
 

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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 called the 4L80E; the E means it is controlled 
electronically.  It cannot easily be retrofitted to earlier cars.  The earlier basic 3-speed GM400 was posthumously 
renamed the 3L80. 

 

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.” 

Besides these functional differences, there are also physical distinctions -- including the shape of the pan and therefore 
the shape of the gasket that fits.  John Himes:  “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.” 

 

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 

 
 

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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: 

 

http://www.mobil.com/

 

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

 

FIRMER SHIFTS IN A BOTTLE:  The owner’s handbook specifies Dexron 2D or Dexron III automatic transmission 
fluid for the GM400 transmission.  Draining this fluid and refilling with Type F fluid will result in firmer shifts.  The 
Type F fluid is less slippery. 

Alternatively, there are aftermarket fluids available that are less slippery than either Dexron or Type F, resulting in 
firmer shifts yet.  

 

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, but it probably had to do with not fully draining the torque convertor or the 
transmission cooler. 

 

DIPSTICKS:  Sean Straw says, “I snapped pix of the dipsticks on both my 1985 and 1988 XJ-S', and I was surprised to 
find that the dipsticks are not the same length.  The top handles are the same (except for the colour of the plastic), and 
both are a spiral metal spring shaft, but the 1985 has a flattened metal tab at the bottom, while the 1988 has a plastic 
gauge. 

“The Jaguar part number for the complete dipstick is EAC 8313, which according to my local dealer is superceeded by 
EBC 4037, which is listed as the part going through 1996.  The parts catalogue does not list a separate part component 
for the gauge at the end. 

“As a comparative reference, the 1985 dipstick shows the HOT min mark at 27 1/8" (end of measure to the base of the 
dipstick handle), and the max mark at 26 9/16", while the 1988 shows the same marks out at about 1 1/32" longer.  
Cold on the 1985 is 27 1/2" and 27 1/4" (min, max), vs the 1988 at 28 11/16" and 28 5/16" (min, max). 

Straw can’t say for sure that somebody didn’t put the wrong dipstick into one of his cars.  It certainly wouldn’t be 
difficult to find an incorrect dipstick; “My Buick and the Jaguars all have the same TH400 trans, and the dipstick on the 
Buick is a good 3 inches or so shorter than on the 1985.” 

 

PLUGGED FILTER:  George Balthrop says, “If you have a turbine-like whine that is audible at idle when you bend 
down to listen under the front door, that is most likely the transmission, caused by a plugged filter & gummy fluid.  
This is particularly likely if the car has sat without regular use for months.  The whine (from the transmission pump) 
will vary with engine speed as you drive the vehicle.” 

 

MOANING SOUNDS:  In response to complaints about cars that will “moan” for a minute or two after cold startup, 
Jim Schultetus says, “A trans filter that is partially clogged will make a moan noise when cold as the fluid is thick and 
doesn't flow well.  This was common complaint on the 400 turbo regardless of vehicle make.” 

On the other hand, Stefan Schulz says, “Trans oil pump sucking air and cavitating.  Can be alleviated by renewing the 
o-rings and if necessary the intake tube.”