Jaguar XJS Transmission Problems, GM400 and ZF Gearbox
On the V12's GM400 I'd start with the vacuum modulator hose, the kickdown switch and the fluid level before anyone quotes a rebuild: no change up, or no drive after a long lay-up, usually comes from there. On the 4.0's ZF 4HP24 the warning light and limp-home in third are mostly dirty contacts in the controller or its connector, or old fluid; a 1996 car stuck in third with P0758 needed a used valve body. Noise after mount work is the rear mount bush, not the gearbox.
How bad is it · Garage, carefully
Drivable with care in limp-home; stop if it slips
Limp-home holds the 4.0 in one gear to protect it. A box that slips, runs low or is driven with the selector half in gear burns its clutches, and a split modulator keeps drawing ATF into the engine.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
4.0: dirty contacts in the gearbox controller or its connector
The ZF 4HP24 on the 4.0 is run by a separate transmission controller through one multi-pin connector at the gearbox. A poor contact on a solenoid wire looks exactly like a worn gearbox to the controller, and it drops into limp-home.
Transmission warning light, only third and reverse with a jerk on engagement, cleared by switching off and on; or a long delay engaging reverse from Park with the light on.
Remove the controller and clean its contacts with switch cleaner. Then inspect the gearbox connector terminals one by one: the orange/green wire to the pressure control solenoid can leave the factory uncrimped.
Clean the controller and connector contacts; repair a loose terminal. The connector is only sold with the whole rotary switch, so a repair is usually the cheaper route.
4.0: old, contaminated fluid
Much of the 4HP24's fluid stays in the torque converter, so one pan drain leaves most of the dirty oil behind. Dirty fluid upsets the solenoids and valves, and limp-home comes back after a few hundred miles.
The fluid drains clean, then looks filthy and smells after a few miles; limp-home fades for a while after each change.
Look at and smell the fluid after a short drive, not just at the drain.
Change the fluid two or three times, or flush gently through the cooler line: idle through the gears, let about half a gallon out, put half a gallon in, repeat. Avoid a power flush.
4.0: valve body or a pressure solenoid
The 4HP24's pressure-regulating solenoid is a known weak point, giving pressure too high or too low. Worn check balls, springs and valve bores in the aluminium valve block do the same.
Hard shifts as if stuck in Sport, the warning light on, sometimes fine for a while and hard again after the car sits. On a 1996 car: stuck in third with P0758 (shift solenoid B) or P1775, cleared by switching off and on, then back again on the motorway.
Drop the pan, look for debris, then remove the valve body and clean both solenoid strainers. Don't test the solenoids with 12 V, it can damage them. With P0758, deal with any crank sensor code first: the controller compares gearbox speeds with engine rpm. ATF inside the gearbox connector means the case connector is leaking, and it also feeds the valve body solenoids.
Clean the pressure solenoid, or fit a known-good used valve body. A whole used valve body costs about the same as one solenoid and replaces the worn valves too. On a 1996 4.0 that had a new controller, a crank sensor and a cleaned connector with no change, a complete valve body from a 1997 XJ6 cured P0758: about $260 plus a day's labour.
4.0: torque converter
A converter that fails inside leaves no hydraulic pressure even though the pump gears are fine, so nothing engages. Swapping the gearbox alone doesn't help if the old converter goes back in.
Upper gears go first, then no drive and no reverse at all, with no codes and no warning light.
Disconnect a cooler line and run the engine: strong flow means the pump and the converter drive are working. Prove the controller's power, earth and inputs with a voltmeter before condemning the converter.
Fit a good converter, filled before fitting.
Low fluid, a clogged filter or air at the pick-up tube
If the pump can't draw fluid, nothing engages. Over a long lay-up fluid seeps past the pan seals and drains from the converter, so the level drops below the pick-up; the GM400 filter can also set rock hard, and a pick-up tube that isn't seated, or old O-rings at its case end, let the pump suck air.
After months laid up: drives a few yards in D and stops pulling, reverse doesn't work either, no clunk when selecting a gear. A whine loudest in first, or reverse slow to engage when cold.
Check the level first, warm, engine running, on level ground: on a V12 back from four years' storage, under a litre through the dipstick tube was all it needed. Then drop the pan, check the magnet for metal and look at the filter. There is no drain plug, and the pan sits behind the rear mount.
Top up a pint at a time. If it still won't pull: a new filter, the pick-up seated with two new O-rings, a new pan gasket, and plain Dexron/Mercon ATF. Brown varnish in the pan means the box may only be saved for a while.
GM400: rear transmission mount assembled wrong
The rear of the gearbox floats on a bush in a large plate; the bush only damps clunks. Fitted on top of the plate instead of pressed into it, or off centre, it carries gearbox noise into the body.
A loud whirring whine or vibration that starts right after the mount bush was renewed.
The bush must sit inside the hole in the big plate and the post must be centred. A poly bush, or a gearbox touching the body, transmits rumble too.
Press the bush into the plate and reassemble; to centre the post, loosen the four brace bolts and tap it straight with a rubber mallet.
GM400: selector cable out of adjustment
A gear that isn't fully engaged bleeds pressure and wears the clutches.
The lever positions don't match the gears, for example reverse engaging well before the lever reaches R.
Check the cable at the gearbox lever: it must be secure in its retainer and not left floating.
Adjust the cable at the gearbox end so each lever position matches the gear.
GM400: stripped governor drive gear
The GM400 knows road speed from a governor driven off the output shaft by a small plastic gear. When the gear strips, the governor never turns and the box thinks the car is standing still, so it stays in first.
Pulls away normally but never changes up in Drive; often after the speedo sender beside it has been out.
Rule out the kickdown switch and the modulator hose first. Then lift the carpet on the right front of the transmission tunnel, open the access panel and pull the governor out to look at its gear.
A new TH400 governor gear with its pin, fitted through the tunnel access hole: about an hour's work, and it shifted properly again. Often sold as a rebuild.
Fluid level read cold, the wrong fluid, or coolant in it
The dipstick is only accurate fully warm. Overfilled cold, the box engages sluggishly; thick non-ATF fluid makes the GM400 pump cavitate and whine. On the V12 the heater flush tee sits beside the dipstick tube: when it leaks, coolant runs down the tube into the gearbox and attacks the clutch linings.
Sluggish engagement or missed shifts after a top-up; a whine after a service with a heavy-machinery oil. With coolant: after 20 minutes' driving a whine above 2000 rpm in the gears but not in neutral, a dipstick too hot to hold that steams, shifts still fine.
Read the level in Park, engine running, fully warm, on level ground. Look at the fluid: pale pink and watery, or a strawberry milkshake, means coolant. Check the heater flush tee for a leak and pressure-test the ATF cooler in the radiator.
Drain the excess at running temperature and use conventional Dexron/Mercon, not synthetic. With coolant, fix the leak first, then exchange the fluid through the cooler return line until it runs clean, several gallons: on one V12 the whine went and the next drain was clean red.
Kickdown switch, or the throttle sensor on the 4.0
On the GM400 a switch at full throttle powers the kickdown solenoid; on the 4.0 a switch under the pedal works with the throttle sensor. A switch wired wrong or stuck closed holds the GM400 in permanent kickdown, so it never changes up; one the pedal never reaches, or one operated too soon, upsets full-throttle shifts.
GM400: revs to 3500 rpm at 25 mph and won't change up, often straight after work near the throttle or fuel rail; or no downshift when floored. 4.0: floored, it shifts twice then holds about 3500 rpm, and is normal as soon as you lift.
GM400: the switch (C33509, green and green/white wires) sits under a plate at the left rear of the engine; check it is connected correctly and isn't stuck closed. At about 40 mph in third, push past the detent and it should drop to second. 4.0: check the switch, the throttle sensor and the wiring between, and that a floor mat isn't sitting on the switch.
Correct the switch wiring: one HE that wouldn't change up after fuel rail work cruised at 2000 rpm again once it was reconnected. Otherwise adjust the switch to full cable travel, bend back and brace a spread pedal bar, free a sticky switch. On the 4.0, a new throttle sensor can cure repeated transmission faults; it needs the Jaguar PDU to set up.
GM400: vacuum modulator or its hose
The V12's GM400 (TH400) sets its shift points from engine vacuum: a rubber line runs back from the right-hand inlet manifold to a modulator on the gearbox's right side. The hose hardens and cracks near the exhaust, and a split diaphragm stops the 2-3 shift.
Won't change up past second, revs high at 30 mph. A failed diaphragm also sucks ATF into the engine: white smoke from the right tailpipe, low fluid, ATF in the right air filter box.
Inspect the hose along its whole length, then pull it off the modulator: ATF in the hose means the diaphragm has gone.
Worth trying first: a new hose and modulator. The modulator can be changed without removing the exhaust.
Worn clutches and bands
Black, burnt-smelling fluid means the clutch packs are burnt. Fresh fluid can even make a worn box stop gripping.
Slipping in reverse or in all gears, no drive in D while manual 1 and 2 still work, slip that starts cold and spreads.
Look at and smell the fluid; a line-pressure test separates worn clutches from a pump, converter or valve body fault.
Rebuild by a shop that knows the GM400: renew every clutch pack, flush the cooler lines and replace the converter. A used box must be the Jaguar GM400, whose case, bolt pattern and output flange differ from the Chevrolet one.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Know which gearbox you have
A V12 with a vacuum modulator and kickdown switch has the GM400 (TH400). The 4.0 has the electronic ZF 4HP24, with a transmission warning light and limp-home.
freeCheck the level hot
Park, engine running, fully warm, level ground. Note the colour and smell: black and burnt is bad news, milky pink means coolant.
freeGM400: modulator hose and kickdown switch
Trace the vacuum line along the right side down to the gearbox. ATF in it, white smoke from the right tailpipe or ATF in the air filter: renew the modulator. Won't change up at all: the kickdown switch first, then the governor gear behind the tunnel access panel.
free4.0: clean the controller and connector
Clean the transmission controller contacts and the gearbox connector, and check the transmission relay in the boot, right rear wheel well.
free4.0: read the codes
1993 cars: round connector in the boot behind the right tail light; 1995: an OBD2-shaped socket under the dash that isn't OBD2, so it needs the PDU. A 1996 car reads P-codes such as P0758: clear crank sensor codes before blaming the gearbox.
a specialist with the PDUSelector cable and rear mount
Lever positions must match the gears. Noise or vibration after mount work: reassemble the bush first.
freePan, filter and fluid
New filter, pick-up seated with new O-rings, plain Dexron/Mercon. On the 4.0, change the fluid two or three times.
a filter kit and ATFPressure tests
Stall test: brakes on, gear selected, full throttle, at least 1300-1500 rpm. Cooler-line flow and line pressure separate pump, converter and valve body.
a gauge, or a shopValve body, converter or rebuild
Only once everything above is clear.
hundreds to thousandsWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS Won't Crank, Starter RelayIf it won't crank in Park: the starter relay is earthed through the gear selector switch.
- Jaguar XJS Misfire, Rough IdleIf the vibration in P and D is the engine, not the gearbox.
- Jaguar XJS Electrical Problems, Bad EarthsIf the 4.0's transmission light comes and goes with other electrical faults.
- XJS fuse boxesWhere the relays sit before you look for the transmission relay.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.