Jaguar XJ40 Rear Suspension Problems
On the XJ40 most rear suspension trouble comes back to the self-levelling system or to a half-done conversion away from it: short self-levelling springs left in, or the spring pan and isolator missing, make the rear sag. Knocks are usually the crumbled foam top bushes, and a roar that rises with road speed is usually a worn diff output bearing. I'd first find out whether the car still has self-levelling, measure the ride height and rock the rear wheels, then work from there.
How bad is it · Garage, carefully
Drivable, but not with a heavy load
A sagging or bouncy rear bottoms out on dips and grounds the exhaust when loaded, and a damper with a loose top can snap. A rear wheel that rocks at the top needs the output bearing seen to before a long drive: left alone it can collapse into the diff.
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.
Wrong spring stack on a converted rear
Self-levelling springs are short and soft because the hydraulic struts carried part of the load. When the system is taken out and the old springs stay in, or the spring pan MMD3563AA and isolator MMD3565AA that self-levelling cars never had are left off, the rear sits low and runs onto its bump stops.
Rear sits lower than the front, bottoms out over dips, scrapes the exhaust or boot floor loaded, and new dampers made no difference.
Look for a spring pan and isolator under each spring, then measure the spring: the standard one is roughly 1.25–1.5 in longer and carries one more coil. Paint marks only pair springs from a batch, they don't tell you the length.
Fit standard road springs (Jaguar kit JLM657 with packers) with the spring pan and isolator; the missing pan and isolator alone are worth at least 2 in of height.
Crumbled foam top bushes and spring isolators
The big foam isolator and the top bushes of the rear unit perish with age and collapse. That leaves a gap, so the damper rod and collar knock against the top plate, and a dry isolator creaks against the spring.
A clunk or ka-clunk from one rear corner over small bumps, a bang over potholes with normal ride height, or a creak and squeak after new shocks went in. On the XJ40 the spring and damper carry the car's weight, so these bushes take a pounding and can wreck the top hat itself.
The parts can't be judged in place: take the spring and damper unit out (four bolts at the top, one at the bottom) and look at the foam and the socket in the top plate. Check the shock isn't bent and that the top nuts were fully tightened when it went in.
Renew the isolator (CAC944960, the same part as MMD3565AA) and the top bushes, greased with copper grease; stay with soft foam rather than hard aftermarket rubber.
Worn diff output shaft bearings
On the XJ40 the half-shaft is also the upper suspension link, so it runs on a big ball bearing where it leaves the diff. When that bearing wears, the wheel rocks in and out at the top and the bearing growls at road speed; low diff oil from leaking seals is what usually finishes it.
A low roar, growl or whirr from the back that rises with road speed, often from about 35 mph, and doesn't change on or off the throttle. Worse with a full tank or a loaded boot.
On the road at 60–70 mph slip into neutral: if the noise stays the same, it's the output bearings. On stands, push the wheel up at 6 o'clock and watch the output shaft collar at the diff — it should not move in and out. Look for swarf on the drain plug magnet.
Renew both output bearings together, keeping the shims behind them, or fit exchange output shaft assemblies — four nuts and five bolts, with the diff left in the car. Check the diff oil level and refill afterwards.
A worn pinion bearing inside the diff
The pinion runs on pre-loaded tapered roller bearings at the front of the diff, and these diffs are prone to wearing them. Once one collapses, the back of the car whirrs like a turbine.
A high whine that wails under power and quietens on the overrun — though a collapsed bearing can whirr at every speed whatever the load. New output shafts change nothing.
With the half-shafts and propshaft off, turn the pinion flange by hand: a ratchety feel is the bearing. Fine swarf on the drain plug magnet goes with it.
The diff comes out with the rear suspension and goes for a full rebuild, after which it's quiet again. A used diff is the alternative: 1989–97 XJ40 and X300 diffs interchange, while early 1988 ones have a different ratio and a U-joint input.
One side sits lower than the other
Rear height on a car without self-levelling is set purely by the springs and the spacers above them. Uneven old springs at either end, or a different spacer each side, leave the car leaning a fraction of an inch.
Arch-to-ground differs by about 1/2–3/4 in side to side; the car otherwise rides and handles normally.
Measure all four arches to the ground unladen on level floor; 1/8–1/4 in between sides is normal on a used car.
Raise the low side with a spacer between the body and the top mount, or trim the spacer on the high side; the arch moves more than the spacer is thick, roughly one and a half to three times, so start thin. Keep spacers thin so the top hat still locates in the body.
Self-levelling parts left live on a converted car
On a car converted to springs the ride height sensor still asks for lift if it stays plugged in. The solenoid then opens and the pump pushes against a blanked pipe, which bangs in the valve block and can push fluid out of the system.
Heavy hydraulic banging or clicking from the bulkhead area when the car is loaded, or a pulsing pipe from the pump to the valve block, on a car with plain rear dampers.
Look under the rear for plain dampers, then check the rear port on the valve block is blanked and the sensor above the right rear lower arm is unplugged.
Disconnect the ride height sensor and seal its plug, or pull the ride-height relay; blank the valve block port, for example with bleed screw CAC8293.
Air in the self-levelling circuit
The self-levelling struts work on mineral oil under pressure. Air in the circuit compresses, so the rear loses its damping and floats.
Soft, bouncy rear with no fluid leaks, sometimes after the car has stood for a while.
Check the mineral oil level first, then bleed at the nipple near the top of the rear strut.
Bleed with the car standing on its wheels and the engine running, until no more air or foam comes out; the rear firms up straight away.
Wrong rubber stack in the rear top mount
The rear damper top needs a set order of washers, foam bushes and a hard plastic guide bush CBC2933. Without the guide bush, or with old rubbers reused, the rod moves in the mount, rattles and can snap at the threaded end.
A rear damper breaks at the top stud, more than one in a short time, or the rod rattles in the mount on a converted car. On a converted car the hole in the self-levelling top hat is slightly larger than the standard one, so the rod can wander and bang against it.
Strip the unit and compare it with the parts diagram for the rear suspension; check the guide bush is there. Make sure the distance tube is on the upper rod and the nut is torqued onto the dished washer. Look at the top hat hole for bright metal and an elongated edge.
Rebuild the top mount to the diagram with new rubbers and the guide bush; many replacement dampers come without the full three-part bush, so buy it separately. If a cheap replacement damper keeps snapping at the stud with the stack correct, change brand.
Rear hub bearings or dry fulcrum bearings
Each rear hub runs on taper roller bearings set by a selective spacer, and the hub carrier pivots on the lower arm on small fulcrum bearings that are greased only on assembly. Worn hub bearings moan or pulse at speed; dry fulcrum bearings squeak and can rust apart.
A pulsing moan or grinding at speed, play felt at the brake disc, or a squeak from the rear corner as the suspension moves.
Rock the wheel at 12 and 6 o'clock and then at 3 and 9: play in every direction points at the hub bearings, movement only at the output shaft collar points at the diff. A wheel bearing gets louder in a long bend on the loaded side.
Renew the hub bearings with a new one-use hub nut and the correct spacer, since tightening the nut harder won't take up the play. For a squeaky fulcrum, loosen its nut and turn the bolt a few times to spread the grease; rusted fulcrum bearings need new races.
Self-levelling not rising: sensor, lever or charge valve
The ride level sensor above the rear lower arm tells the valve block when to let pressure into the struts. A broken sensor, a lever popped off its bracket, a stuck charge valve or a weak brake accumulator stops the lift, because the system feeds the brakes first.
Rear stays low under load, does not rise after start-up, or keeps rising to full height and hunting.
Check the lever is on its bracket and the reservoir sight window shows green; jack the rear, free the sensor arm and test the six-pin sensor: with 12 V on pins 5 and 6 and earth on pin 4, lever up should light a test lamp on pin 3 and lever down one on pin 2.
Refit the lever or replace the sensor; a stuck charge valve or solenoid on the valve block costs far less than a conversion.
Worn self-levelling struts
The struts have a gas bladder at the top that can rupture, often after the empty car rises hard over a big bump. They wear inside without leaking, so the rear turns rock hard or bounces, and the system may pump to full height and stay there.
Rear rock hard over bumps, or bouncing with no visible leak, sometimes rising to the top and dropping suddenly. A rear that bounces with no damping at all is the struts gone.
Rule out air and the sensor first; a hard ride that bleeding doesn't change points at the struts.
Replace the struts or convert to standard springs and dampers; a repaired system on worn parts often fails again within months.
Worn rear dampers on a standard car
On a car without self-levelling, a tired rear damper lets the corner bounce and bang. Worn damper bushes usually mean the damper is nearly done too.
A gentle continuous bumping from the back, a corner that wheezes and keeps bouncing when pushed, or a bang over bumps.
Push each rear corner down three or four times and let go: it should settle with one more bounce.
Worth trying: new rear dampers in pairs with their mounting parts. Slim-jawed spring compressors are needed, and the new unit may have to be pressed together to start the top nut.
Perished rear subframe (A-frame) bushes
The diff and rear suspension hang in a cage mounted to the body through rubber A-frame bushes. When those go soft, the cage moves and knocks over bumps, the rear steers itself a little, and normal diff hum is passed straight into the body.
A thump from the rear over big bumps with good top bushes, a wandering rear after potholes, or a diff whine that new diff parts didn't cure.
The bushes can look fine and still be finished; look for the cage touching the body and for anything wedged under the bush, such as a stone.
Worth renewing the A-frame bushes before spending on the diff for a whine; they can be pressed out and in off the car with a 10-ton press, noting their orientation.
A loose pinion nut: whine on the overrun
The forward nut on the pinion shaft sets its end play and the gear contact. If it works loose, a whine starts on the overrun.
A sudden, loud whine when you lift off, for example between 40 and 60 mph.
Have the pinion nut checked for tightness before anything else in the diff is condemned.
Worth trying: re-torquing the nut cures it. Don't drive far on it loose, or gear wear leaves a permanent whine.
A dry half-shaft universal joint
The half-shaft is the upper link of the rear suspension. A universal joint whose bearing has failed clunks as drive is taken up.
A clunk from the back going from drive to reverse, or taking up drive.
On stands, look for play in the half-shaft universal joints and at the rubber coupling at the diff.
Worth trying: a new joint before the bearing housing eats into the shaft.
Something loose that isn't suspension
A knock from the back over bumps can come from anything that moves. Rule out the cheap things before the suspension comes apart.
A knock that stayed after new dampers and bushes.
Check the tools and jack behind the spare wheel and the spare's clamp, the exhaust rubbers, and a transmission mount that lets the propshaft touch the tunnel. Have someone ride in the back to pin it down.
Worth trying: secure what's loose. A dry rear door lock pin can knock just like suspension; a squirt of lubricant silences it.
Worn lower spherical joint on late-1994 cars
The very last XJ40s, from about VIN 696460, carry the X300-style rear with the spring on the lower arm and a spherical joint in the lower damper mount. Wear there gives a fast knock that bushes don't cure.
Rapid knock from the rear over uneven roads, up to several times a second, on a late 1994 car.
Check the VIN first, then look for play in the lower damper joint with the wheel loaded and unloaded; it can look fine at a glance.
Replace spherical joint CCC6782 or the damper; buy dampers for the right rear style, as the early and late ones don't swap.
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.
Find out what is fitted
Self-levelling is still there if a pipe runs from the valve block on the inner wing to the top of the rear struts and a sensor sits above the rear lower arm. A boot tag only tells you what left the factory; many cars were converted since.
freeMeasure the ride height
Unladen, half a tank, level floor: arch lip to the ground at all four corners. The rear should sit about 10–15 mm higher than the front; a rear lower than the front on a converted car means short springs.
freeLoad test a self-levelling car
With the engine running put heavy weight in the boot; the rear should come back to height. A rise of an inch or two a few seconds after start-up is normal.
freeRock the rear wheels and listen at speed
Car on stands, grip each rear wheel top and bottom and rock it. Movement at the output shaft collar by the diff is the output bearing; play in every direction at the hub is the wheel bearing. On the road, a low growl that doesn't change in neutral is the output bearings; a high whine that comes and goes with the throttle is inside the diff.
freeOil, bleed and the sensor
The system takes green-yellow mineral oil, never ATF. Bleed with the car on its wheels and the engine running, then test the ride level sensor.
cheapCheck the diff oil
The filler sits behind a bung under the boot carpet behind the spare wheel. Low oil from leaking seals is what wears the bearings out; swarf on the drain plug magnet means a bearing is going.
cheapCheck springs and spring seats on a converted car
Measure free length and count coils; look for the spring pan and isolator. Make sure the old sensor is unplugged and the valve block port blanked.
cheapStrip the top mount for knocks
Take the unit out, check the foam isolator, top bushes and guide bush, and grease the isolator on refit. Tighten the lower damper bolt with the arm jacked until the half-shaft is about level. If the top bushes are good, look at the A-frame bushes that carry the rear cage.
cheapConvert with the whole parts list
Jaguar's delete procedure is TSB 64-14. Fit standard springs, pans, isolators, guide bushes and dampers for a non-levelling car, not dampers alone. Late-1994 cars with the X300-style rear take different dampers.
dearWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Dashboard Warning LightsIf the brake or ABS lamp comes on with the levelling trouble — on these cars the accumulator feeds both.
- XJ6 (XJ40) fuse boxes and relaysWhere the relays sit before you pull the ride-height relay on a converted car.
- Jaguar XJ40 ABS and Brake Accumulator ProblemsIf the brake and ABS warnings come on too.
- Jaguar XJ40 Front Suspension Knock, Worn BushesIf the knock comes from the front of the car instead.
- Jaguar XJ40 Power Steering Problems, Steering RackIf the steering reservoir leaks on a 1990–92 car, the self-levelling hydraulics sharing it are often the source.
- Jaguar XJ40 Vibration at Speed, Prop ShaftIf the rear shakes at one band of speed rather than knocking — the prop shaft.
- 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.