Jaguar XJ-S. Manual - part 84

 
 

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inch or so.  It is intended to be a distinct enough feel that the driver can easily choose whether 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 turntable.  A photo of this microswitch can be seen at: 

 

http://www.jag-lovers.org/xj-s/book/Microswitches.html

 

After the throttle is wide open and the turntable is against the stop, further pressing of the pedal can move the turntable 
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 GM400.  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 vacuum modulator will cause the transmission to downshift (gently!) under heavy 
throttle, they presume that everything is OK.  It is easy enough to check the kickdown operation, though. 

The first thing to do is learn what that snap at full throttle feels like -- which can be tough if it doesn’t work!  So, you’ve 
got to check that operation first.  With the car shut off, make sure pressing the accelerator hard operates the microswitch 
on the throttle cable as it should. 

If not, the mechanism might be jammed, and you’ll have to correct it before you can go any farther.  David Peek says, 
“This was quite an easy job given that I am a dedicated pen pusher with no socket set (yet).  At the back (facing the 
windscreen) of the housing holding the taper unit there is a small phillips head screw; remove it (be careful not to drop 
it).  This will release the spring which holds the taper in position (nothing falls out).  A good dose of WD40 and a few 
pulls on the throttle cable outer soon released it for me.  Push the throttle cable outer back towards the housing to 
compress the spring and put the small screw back in to retain. 

“Apparently the best way of keeping it in working order is regular use of the kick down - what fun!” 

Dennis Roberts says, “Flooring the throttle caused the sheath to slide and the switch to close, so I didn't understand why 
it wouldn't downshift unless there was a problem with the transmission.  Hoping to find a simpler solution, I put an 
ohmmeter across the switch and saw that when the switch closed it still had a small resistance.  Using my finger to 
move the switch would cause the resistance to drop to zero.  I concluded that the switch was really not closing all the 
way until I pulled with my finger.  So I examined how to adjust the movement of the sheath.  First I lubed the sheath 
and groove to see if this would cause the switch to close tighter; no change.  So I adjusted the large nuts that attach the 
cable/sheath to the bracket (same bracket that holds the switch). I moved the nuts closer to the end of the sheath, 
tightening the cable. This caused the throttle to open farther with the same pedal position; and it caused the throttle to 
hit the full open position before the pedal hit the floor.  Further pressing on the pedal then caused the sheath to move 
farther than it had before the adjustment.  This caused the switch to close tighter than before and the resistance to drop 
to nearly zero without help from a finger.  Now the transmission downshifts every time I floor it without sticking my 
foot through the floor. 

“One word of caution:  Be sure not to overtighten the cable since this will not allow the throttle to fully drop back to 
idle.  On mine this was a fairly close adjustment: Very little adjustment would cause the cable to be too tight to idle or 
too loose to cause a downshift.” 

There is, in fact, a common problem here.  The throttle pedal consists of a steel bar that is bent at an angle with a pivot 
at the corner.  You push on one end of the bar with your foot, and the other end of the bar pulls the throttle cable 
vertically downward.  Unfortunately, spirited use of the kickdown feature has been known to bend that bar at the 
corner, so that it is a wider angle than it should be.  The result is the problem Roberts was facing above: pressing the 
pedal all the way won’t operate the kickdown switch, because the pedal hits the rubber button on the floor first.  If you 
just keep adjusting the cable the way Roberts describes, you will eventually run out of adjustment altogether; it will 
become impossible to get kickdown and idle with the same adjustment. 

 
 

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The solution is simple enough: bend that throttle pedal back the way it’s supposed to be.  Just crank the angle closed a 
bit.  Precision is not required, but it might help to take it out of the car and put it in a vice.  It’s easy to remove.  Of 
course, once you’ve got it bent properly, it might just bend itself out of shape again.  So, the author simply tied a piece 
of steel wire between the two ends, positively preventing the angle from opening up. 

Once you have the throttle pedal bent properly, Roberts’ contention that the cable adjustment is “a fairly close 
adjustment” goes away.  You’ll find you can adjust the cable all over the place and still get both idle and kickdown with 
ease.  It’s a good idea to adjust it so you get kickdown just as the pedal hits the button on the floor so you don’t 
overstress anything stomping the throttle. 

Oh, one other concern: sometimes you can’t get kickdown simply because the carpet has gotten out of place and is 
keeping the throttle pedal from moving as far towards the floor as it should.  Deal with it. 

Once you’ve got that cable working properly, we can move on to actually road-testing the kickdown.  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 you have a hard time telling the difference between kickdown or no kickdown, it ain’t working.  
There is nothing subtle or gentle about kickdown; it should provide a swift kick in the pants. 

If the cable is moving the switch properly but the kickdown still doesn’t work, check fuse #5; there should be power to 
the kickdown switch on the LG/W wire when the ignition is on. 

Next, 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.  Obviously, the microswitch itself 
may be defective; check it with a VOM.  If it’s NFG, you can find a replacement microswitch -- complete with the little 
roller on the lever -- at a good electronics supply house.  It’s actually a standard configuration microswitch. 

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. 

Believe it or not, it can pass all those tests and still not work.  It’s possible for the solenoid within the GM400 to audibly 
click and yet not provide a kickdown.  Tony Bryant says, “The solenoid in my trans would click, but the screen inside it 
was blocked.  A rinse in kerosene fixed that.  Also gave an opportunity for a well needed fluid and filter change.” 

Mark Johnson says, “Seems there's a small screen and a seal in the solenoid assembly, and on mine, lots of hot stop-
and-go traffic before I bought the car apparently had cooked the seal.  The result was that the seal cracked and fell out, 
allowing fluid to seep past it all the time.  I finally found the problem and bought a new part at the Chevy dealer - 
presto!  Like getting a new transmission.  If you get the click but still have kickdown problems, you might want to 
check this out.” 

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.”

 

 
 

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PLAYING GAMES WITH THE KICKDOWN:  Hey, that kickdown switch just connects two wires together and the 
transmission drops into a lower gear.  Give you any ideas? 

Al Askevold says, “My kickdown on my '90 never worked most of the time and clicked back off when it did engage.  
Throttle cable stretch or bad machining on the indent for the switch roller I do not know but I gave up on adjustments 
and tried a mod that another lister sent in. 

“Very simple mod, just mount the switch on top of the mounting plate and run the switch off the throttle rod.  Very 
adjustable and I have my kickdown at just over 3/4's so I now have a normal passing gear when needed and a solid full 
throttle kick in the pants.” 

Matthias Fouquet-Lapar has another idea: “Simply add a button in parallel to the kick-down switch of the GM400. You 
can mount it where the two switches of the power windows are, and if your arm is on the centre console, you hand 
really "falls" on it. 

“If you push it, it's like a full-throttle kick-down for the transmission, so the normal protection should work. You don't 
need the shifter to find you pushing it into 'R' at 60mph :-) 

“And it's really nice.  If you want to bypass, click (and hold down) just 1 second before and then push.  It really makes 
a lot of fun.  In normal driving situations you have almost always enough time for this gear change, and then you really 
have the power.  I'm even thinking to get this into the steering wheel, so I don't need to take the hands of the wheel.  
Maybe I'll use the horn contaacts.  Hey, this car really has power!!” 

Steve Gallant suggests putting a switch in parallel with the kickdown switch -- the kind of switch that you turn on and 
leave on, not just a momentary button.  “The kickdown solenoid changes the shift characteristics of the transmission.  
The reason most people think that the kickdown solenoid always forces a downshift is because to engage the switch in 
the first place, you have to depress the throttle all the way to the floor.  And this does indeed force the downshift, but 
only because vacuum is sufficiently low to do so. 

“Activating the kickdown solenoid all by itself (i.e., without depressing the throttle) does not cause a downshift, 
because vacuum pressure is unchanged.  You still must hit the throttle, thus reducing vacuum to force a downshift.  
However, activating the solenoid changes the pressure at which a downshift will occur, so closing the parallel 
kickdown switch makes the throttle more sensitive.  Meaning that downshifts will occur with smaller throttle changes, 
or at a lower 'rate'.  The downshift does not occur at a fixed point, because it depends on the how fast you increase the 
throttle.  If you accelerate slowly, pressure does not change too fast and normal shifting occurs, but hit it, and you cross 
the pressure threshhold resulting in a downshift. 

“And if you do everything gently, you get upshifts (and no downshifts) just as you would without the switch 
engaged...but it will downshift with less movement and hold a lower gear to a higher rpm.  When you get tired of this 
'sportiness' you can just disengage the switch.  It's that simple. 

“Anyway, best thing for you to do is just give it a try.  Close the switch with a jumper wire and drive around.” 

Justin Pashley:  “I would just like to tell you how to make a neat job of the button.  I got the bits from a local scrap yard 
who currently have a 1989 XJ-S convertible.  I used the electric window plastic switch plate that has space for an extra 
button for the convertible roof (the XJ-S with a sunroof has the same plate).  I also used the cruise control button from 
the same year of car as the switch to enable / disable the sport mode.  Note, this will only work from 1987 car onwards 
as the earlier car did not have this switch surround. 

“The whole job cost me £2.50 and took 10 minutes to fit.” 

 

VALVE BODY REMOVAL:  Tom Bennett passes on this tip, which he claims to have gleaned from a Chilton’s:  “If 
you leave the solenoid bolted and then undo the valve body bolts about an 1/8" and gently push on the steel spacer 
edges so that it just separates from the valve body, it is possible to remove the transmission valve body without the 
check balls flying out!  When you remove it completely the steel spacer stays in place and so do the balls.” 

 
 

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GOT BALLS?  Some GM400 transmissions have six check balls in the valve body, and some have seven.  Note that 
the versions with seven balls, apparently 1987 on, also use different valve body gaskets.  Compare your new gaskets to 
the old gaskets very carefully to make sure you have the right ones. 

 

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.” 

Greg Meboe had this problem:  “As I was lowering the valve body, I could feel there was some spring force trying to 
push it down on me.  When the last thread disengaged, the valve body sprung down and stuff went flying.  Turns out 
the plastic accumulator piston had been broken, which is why there was a spring force pushing down on the valve 
body.” 

Steve Barker tells all about doing this repair yourself:  “NAPA supplies a plastic replacement.  The GM dealer said he 
can get the part, (parts departments don't actually stock parts anymore) the GM part is 8626883 about $20.  I called a 
transmission shop and he had the aluminum part, a valve body gasket and a 6 ball and 7 ball valve plate gasket and 
photocopied a ball location sheet and gave me some hints, all for 21 bucks. 

“When you drop the valve body, leave the two bolts in that hold the solenoid.  This will hold a plate in place that holds 
the balls.  The two tubes on the valve body just slide into two drillings in the casing, there are no seals to replace.  
Because the plate that is held in place by the solenoid bolts will have separated from the case, it is possible that the 
gasket will be damaged.  There are two different gaskets, one for a 6 ball unit and one for a 7 ball unit.  Mine (89) is a 7 
ball. 

“Holding the plate up, remove the two bolts, let the solenoid dangle and keeping the plate level, put it in a clean level 
spot.  Now remove the old gasket from the case and discard.  Put the 6 or 7 ball gasket on the plate and to hold the balls 
in place while you reinstall the plate, put a dab of petroleum jelly that has been in the freezer on each ball.  Then you 
can replace the plate and hold it with the same two solenoid bolts.  Keep those two bolts loose and run a couple of bolts 
into their holes to align the gasket before you tighten the solenoid bolts. 

“The piston is mounted in the back of the valve body.  It's kind of confusing in that there is a servo piston assy on page 
44-20 that goes in the case; this isn't the one.  (The page numbers refer to The XJS Service Manual 4 volume set with 
the Ed2 supplement.)  The one we're after is item 34 of the Control Valve Assembly diagram on page 44-18.  You push 
the piston all the way in (watch it or it will spit fluid at you), and remove the c-ring and withdraw the piston. 

“Installation is the reverse.  To put the valve body back in, it helps to put two of the 5/16" bolts through and hold them 
to keep the valve body gasket in place. You also might have to jiggle it a little to get the two tubes into their bores.  I 
managed to do the job without dropping the balls but you would definitely want a tub under there to catch them if you 
tip the plate.”