Jaguar XJ6 (X300) Gearbox Problems, Gear Selector
On the X300 most selector trouble is electrical and cheap: a brake switch that won't release Park, or a sticking not-in-park microswitch. A gearbox warning light with limp mode, especially on the XJR and XJ12, usually means broken wires in the loom plug at the back of the box. I'd check the brake lights, read the codes and look under the car before anyone quotes for a gearbox.
How bad is it · Garage, carefully
Limp mode gets you home; a car that won't hold in Park doesn't stay put
Limp mode holds one gear with full line pressure, so drive it gently and not far. If Park doesn't hold on a slope, use the handbrake every time until the cable is sorted.
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.
The brake pedal switch
The lever stays locked in Park until the brake pedal switch tells a solenoid under the J-gate to let go. The same switch feeds the brake lights, so when it sticks or loses its plug, both stop working together.
Brake lights dead, and no click under the gate when you press the pedal with the ignition on. It can refuse to start as well. After work behind the driver's knee panel, suspect the plug: the climate sensor uses the same type, and if the two are swapped the climate display beeps and shows an error. The opposite failure shows too: brake lights on all the time together with a lever stuck in Park is the same switch sticking.
Ignition on, foot on the brake: listen for the click under the gate and have someone watch the brake lights. The switch sits at the top of the pedal arm on a white four-way plug, CA72; DBC 10800 on cars with cruise control, DBC 10799 without.
A few hard stabs on the pedal can free a sticking switch; clean it or replace it, or put the plug back in its own socket. To move the car meanwhile, take out the plastic Torx plug in the selector surround, press the latch below it with a small screwdriver and select N.
The not-in-park microswitch on the selector
A small microswitch on the selector side plate tells the car the lever has left Park; it also covers neutral starting and the reversing lights. When it sticks, the car thinks you're still in Park and fires the interlock solenoid every time you brake.
A click or clunk from the gear lever at every touch of the brake while driving, worst first thing in the morning and getting more frequent. The clunk itself means the solenoids are fine. Cycling through the gears quietens it for a while in the morning.
Rock the lever firmly against the Park detent a dozen times. If the clicking stops for a while, it's the switch. Removed, it reads intermittent on an ohmmeter.
A new switch. Jaguar supplies it only with the side plate (MMD4940AA), but a generic submini lever microswitch with the same hole spacing fits — for example Cherry DB1C-A1LB, Maplin N96AQ or RadioShack 275-016A — fixed with the original rivets or M2 bolts once the ski-slope trim is off.
Broken wires in the gearbox loom plug, pulled by a sagging rear mount
The loom from the gearbox computer ends in a round multiplug at the back of the box. When the rear gearbox mount sags, the box moves and stretches the loom, and wires break inside the plug where a meter can still read them as good. This is the classic on the XJR and XJ12 with the GM 4L80-E.
Gearbox warning light and limp mode — held in 2nd or 3rd, or 4th lost — that clears after a restart and returns more and more often. A fault that comes and goes when the loom is moved. The connector itself can simply be loose: the locking ring on the multiplug not engaged, or a wire pulled out of it. In limp mode the 4L80-E starts in 2nd at maximum line pressure, so every selection lands with a bang. A gearbox temperature warning at sustained high speed can come from the same stretched loom rather than real heat.
Unplug the multiplug and test each wire for continuity while bending it. From under the car, look at the rear mount: crumbled foam or tired springs mean it has dropped. Before anything else, pull the plug, clean the pins with contact cleaner, refit it and make sure the locking ring clicks home. Tightening the Torx bolt in the rear mount reduces bounce, but too far changes the prop shaft angle.
A new external gearbox harness (LNA 3380-AD) and a new rear mount together; a new loom on an old mount gets stretched again. Wires that pull out of the plug can be re-soldered. A loose plug cured by cleaning and engaging the locking ring; broken wires at the pins can be repaired without a new loom.
The selector cable has come off at the gearbox
The cable from the lever pushes onto a ball pin on the gearbox lever and is held by a small nylon clip. When the clip breaks or the ball socket wears, the cable drops off and the lever no longer moves the gearbox.
The lever feels loose and wanders round the gate, but the box stays in whatever gear it was in; the P lamp may stay lit. Sometimes after a car wash or work underneath.
From under the car, look at the cable end on the gearbox lever and at the bracket that holds the outer cable.
Refit it with a new clip — a generic one fits — and clean any rust from the linkage. A worn ball socket isn't sold on its own, so it's a new cable (MNA4830CA); so is a cracked lower bracket.
An engine fault upsetting the gearbox computer
The gearbox computer shifts on data from the engine management: throttle position, engine speed, torque. A failing throttle position sensor, an arcing coil or a dying crank sensor can push the box into limp mode with nothing wrong in the gearbox.
Limp mode alongside a high or hunting idle, a misfire, stalling at junctions, or a light that only comes on when coasting. The whole dash may flicker. P1775 among the codes means the engine side asked for limp mode. An engine that stalls in gear sends the box into limp mode by itself.
Read the engine codes first and cure any misfire or throttle fault. If the rev counter stays at zero while cranking, suspect the crank sensor.
A replacement throttle body (used ones are cheap, for example about £35–100), a new coil where one was arcing, or a new crank sensor.
An aftermarket gear knob screwed down too far
A replacement knob without a lock nut can wind itself down onto the gate surround, so the lever can no longer travel far enough to work the Park switches.
Stuck in Park, or the key won't come out after parking; a knob that isn't the original.
Look at the gap under the knob before taking anything apart.
Back the knob off slightly and lock it with a nut.
The selector cable out of adjustment
If the cable has slipped out of adjustment or worn at the gearbox end, Park doesn't fully engage the parking pawl.
The car rolls in Park on a slope while the handbrake holds it.
Test facing up and down a slope. Follow the cable for anything it's caught on — the exhaust heat shield, for example.
Free and readjust the cable; a worn inner cable means a new one.
A stored fault holding limp mode
Limp mode can stay latched after its cause has gone, for example after a repair or a single electrical glitch.
Gearbox fault lamp and limp mode, nothing else wrong. A light that comes on mid-journey with 4th lost and hard shifts, and is gone after a night's rest, may never come back.
Turn the key on and off five or six times before starting, or clear the codes with a scanner.
Worth trying first; it has brought back normal shifts. If the fault returns, one of the causes above is underneath. If it does come back, clean the gearbox connector first.
A stone wedged between the sump and the gearbox support
The gap between the sump cover and the transmission support is only a few millimetres. A pebble thrown up from the road can lodge there and transmit the drivetrain's vibration into the body.
A low growl from under the middle of the cabin in Drive with your foot on the brake, constant under load, worse after a long fast run.
Look into the gap between the sump and the gearbox crossmember from under the car before suspecting bearings.
Pop the stone out.
Low or tired fluid
Both boxes fitted to the X300 — the ZF 4HP24 behind the six and the GM 4L80-E in the XJR and XJ12 — are touchy about fluid. Even half a litre low can upset engagement.
Stalls when braking to a stop from cold, a slow Park-to-Drive engagement in the morning, or a noisy, harsh 1–2 after a long fast run.
Check the level fully warm, engine running, on level ground.
Worth trying: fill to the top of the dipstick range rather than anywhere within it, without overfilling. Fluid that has gone almost clear wants draining and refilling.
4L80-E shift solenoids and speed sensors
On the XJR and XJ12 the GM box has known weak points: the A and B shift solenoids and the speed sensors. A failed output speed sensor puts it into limp mode; a failed temperature sensor gives harsh shifts instead.
Harsh or early shifts, shudder, no kickdown, or limp mode with speed-sensor codes. A sensor can read normal cold and open-circuit hot. A slow engagement into Drive or Reverse after the car has stood, with slip, points to low line pressure.
Read the gearbox codes, then measure the speed sensors cold and hot. A cold line-pressure reading at idle in Drive and Reverse needs a 300 psi gauge on the 3/8 in plug on the driver's side of the case, about a quarter of the way up from the pan. A stuck pressure-control solenoid (force motor) is replaceable with the pan off, like the filter.
Worth trying: upgraded GM solenoids, fitted with the pan off at a filter change; GM parts counters know this box well.
Poor power to the gearbox computer
The gearbox computer shares its ignition-on feed and relay with the instrument pack, ABS and cruise control. Burnt relay contacts, a bad splice or a chafed feed or earth make it drop out, and the box goes into limp mode.
Limp mode together with the dash blacking out or the gauges dropping to zero; P1777, a torque reduction circuit fault; faults after heat soak. A replacement gearbox computer changes nothing.
With the ignition on, measure the computer's supply under load and its earth. Look at the bulkhead power studs, the main earth and the wiring near the computer; then the ignition relay and the ignition switch.
Worth trying: on a car this age, renew the relays that feed it rather than chasing one; repair the supply or earth wire.
Shift adaptation relearning after a battery change
The gearbox control adapts shift pressure and shift points to how the car is driven. On the 4L80-E in the XJR and XJ12, with the battery off for more than about 12 hours the learned data is lost, and the box relearns from whatever driving follows. Lock-up points on the six-cylinder box shift with conditions too.
Shifts that turned spongy — or suddenly violent — within days of a new battery or a disconnection, with no warning light; lock-up at a slightly different speed after new parts.
Ask what was done to the battery recently before suspecting the gearbox.
Worth trying: drive normally, with ordinary throttle, for a number of trips so the box relearns before chasing a fault.
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.
Check the brake lights
Lever stuck in Park and no brake lights: it's the brake switch. Ignition on, press the pedal and listen for the click under the gate.
freeRelease the lever by hand if you must move the car
Take out the Torx plug in the selector surround, press the latch with a small screwdriver, select N. Avoid Park until it's fixed.
freeCycle the key and read the codes
Five or six key cycles may clear a latched limp mode. Then read engine and gearbox codes — engine faults first.
freeCheck the fluid hot
Fully warm, engine running, level ground. Top of the range, not over.
freeLook under the car
The cable end and its clip on the gearbox lever, the round loom plug at the back of the box, and the rear mount. Check that the plug's locking ring is engaged, and look for a stone between the sump and the gearbox support if it growls.
freeThen the switch, the loom or the solenoids
Clicking at every brake press: the not-in-park microswitch. A light that comes and goes: the loom and the rear mount together. Only after that, solenoids and speed sensors.
parts from a few pounds to a few hundredWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 (X300) Won't Crank, Just ClicksWhen the lever is in Park but the car still won't crank.
- Jaguar XJ6 (X300) Rough Idle or High IdleA throttle body fault can also put the gearbox into limp mode.
- Jaguar XJ6 (X300) Cranks But Won't StartThe crank sensor behind stalling and limp mode.
- XJ6 (X300) fuse boxes and relaysFor dead brake lights. 1996 and 1997 from the same page.
- Jaguar XJ6 (X300) Vibration at Speed, Steering Wheel ShakeIf there's a vibration at a steady speed rather than at the shift.
- 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.