Jaguar XJS Brake Light Switch, Turn Signals Not Working
On the XJS, lower brake lamps that die while the high-level lamp still works usually come down to the bulb-failure unit in the boot or its plug; lamps that won't go out point at the pedal switch cam. Indicators and hazards failing together point at the shared flasher relay or its feed through the hazard switch. I'd start by noting exactly which lamps are dead.
How bad is it · Garage, carefully
Get the lamps working before you drive far
The engine isn't affected, but drivers behind get no warning when you brake or turn. Brake lamps stuck on hide your braking and can melt a lens.
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 stop-lamp bulb-failure unit in the boot, or its plug
The two lower brake lamps are fed through a bulb-failure check unit. The wiring diagram puts it behind the dash, but on some facelift cars it sits in the boot, on the back of the left support strut near the stop-light relay and the ABS unit. Its three-pin plug is easily knocked off or crushed when the fuel tank comes out, and both lower lamps go dark together.
Both lower brake lamps dead while the high-level lamp in the rear window works; tail lamps fine; a new bulb and a new fuse change nothing.
A working high-level lamp clears the pedal switch, the stop-light relay and their fuse. With the handbrake on, press the pedal and watch the dash bulb-failure lamp: it should light. Then look for 12 V on the green/purple wire into the unit at pin B and out at the L pins to the lower lamps.
Straighten the pins, push the plug fully home and clean the contacts.
A loop plug or a pinched harness in the boot
On facelift cars the rear lamp feed passes through a loop plug in the boot next to the fuse rack. Bulbs, relay and earths can all test good while one pin of that plug passes no 12 V. The harness leaving the boot can also be squeezed between the fuel tank retaining plate and the body.
Tail lamps dim and then dead, or the lamps in the boot lid out while the outer lamps work; 12 V at the fuse but less at the bulb socket; the dash bulb-failure lamp lit.
Measure at the fuse, at the loop plug and at the bulb socket in turn. Full voltage at the fuse and a low reading at the socket puts the fault in between.
On a 1996 car, changing the affected pin of the loop plug to carry 12 V brought the tail lamps back. Free the harness from under the tank plate while you're there.
No feed to the flasher: a bad fuse contact or the hazard switch
The flasher relay is fed through the hazard switch: brown wire in, green wire out. A poor contact at the fuse or inside the switch cuts the feed, and the indicators and hazards die together, often on and off.
Indicators and hazards dead together, or working intermittently and sometimes coming back after a few minutes; the flasher relay tests good on the bench.
Look for 12 V at the flasher supply terminal 49. If it's missing, check the brown and green wires at the hazard switch: brown live and green dead puts it in the switch or the fuse feeding it. Wiggle the wiring as you test: a fault that comes and goes as it moves is the supply wire.
Clean the fuse clips and fit fuses of the correct rating. Working the hazard switch many times can restore contact for a while. If the feed wire to the switch is intermittent, run a second supply to it.
Melted wiring that crosses the indicators into another circuit
The wipers and the indicators have separate fuses and different wire colours, with no factory link between them. Over-rated fuses, an old fire or mouse damage can melt a junction where the two runs meet, and one stalk then works the other circuit.
Moving the indicator stalk starts the wipers, and the hazard switch does nothing.
Try every wiper setting with the indicator held each way, then the other way round, to map which position feeds which wire. Then look where the two circuits run together, including inside the steering column.
Repair the melted junction and replace over-rated fuses with the correct ratings.
The brake-pedal switch: its adjusting cam, or the switch itself
On late cars the pedal switch housing holds two microswitches side by side: one for the brake lamps, one that releases the selector from Park. A serrated ratchet cam sets how hard the pedal at rest presses the plunger. Ratcheted fully in, it holds the plunger in the lamps-on position, so the lamps burn until the pedal is pressed. The original switch is also a known weak part in its own right.
Brake lamps lit with the pedal untouched and going out when you press it, and a brand-new switch behaves exactly the same. Or no brake lamps at all, the high-level one included. With the sidelights on as well, a lamp stuck on can get hot enough to distort its lens.
Look at the cam bracket the switch sits in before buying anything: pushed all the way in is the fault. If the high-level lamp works, the switch is fine and the fault is further back. Before fitting a new microswitch, check whether it is normally open or normally closed: a 3-terminal type has both and can be wired either way.
Worth trying first: ratchet the cam back out to full extension, so pressing the pedal closes the switch. A dead switch can be replaced with a generic microswitch with a long springy lever, on a small bracket so the pedal arm presses it. An interchange list names a 1980 Pontiac Phoenix brake-lamp switch as a substitute.
The flasher relay DAC1731, or a wrong replacement
One flasher relay, DAC1731 (Hella TBB44), drives both the indicators and the hazards, and its fourth pin feeds the dash arrows as a pilot circuit. A generic 4-pin flasher from a parts shop has that pin wired for a trailer and hyper-flashes on these cars.
Indicators and hazards misbehave together; the relay clicks but the dash arrows stay dark; or rapid flashing straight after a relay change.
Swap in a known-good relay of the right type. If everything comes back at once, it was the relay.
Fit a DAC1731 or a compatible unit: Lucas, Rover and Flosser flashers bench-tested compatible. None of the flashers tested works with LED bulbs.
A failed or wrong-spec indicator bulb
The relay senses the current the outside lamps draw. A failed bulb, or a replacement that draws too little, upsets that sensing, and the dash arrow stops flashing. Low voltage through corroded connections does the same.
Outside indicators flash but the arrow on the dash stays dark, on one side or both.
Check every outside indicator bulb on the dead side and compare its rating with the original.
Worth trying: fit bulbs of the correct specification and clean the bulb holders.
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.
Note exactly which lamps are dead
Lower brake lamps only, all brake lamps including the high-level one, brake lamps that won't go out, indicators and hazards together, or just the dash arrow. Each points somewhere different.
freeLower brake lamps dead: the unit in the boot
With a working high-level lamp, go to the bulb-failure unit and its three-pin plug in the boot, then the loop plug by the fuse rack. Check behind the dash only if it isn't in the boot.
freeBrake lamps stuck on: the pedal switch cam
Look at the ratchet cam before replacing the switch, and reset it to full extension.
freeIndicators and hazards dead: test the feed
12 V at flasher terminal 49? If not, brown and green at the hazard switch, then the fuse clips feeding it.
a test lampSwap the flasher relay
Use a DAC1731 or a compatible unit, not a generic trailer-type flasher.
a relayCheck the outside bulbs for the dash arrow
Fit correct-spec bulbs on the dead side. Arrows still dark with good bulbs and a good relay: see the instrument cluster page.
a bulbWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS Instrument Cluster, Gauges Not WorkingIf only the dash arrows or the bulb-failure lamp misbehave, or after an LED conversion.
- Jaguar XJS Headlights Not WorkingIf the headlamps or the dip are out as well: relays at the front left of the engine bay.
- Jaguar XJS Electrical Problems, Bad EarthsIf other circuits fail with the lamps: shared fuses, earths and fuse clips.
- XJS fuse boxes and relaysWhich fuse feeds the brake lamps and indicators on your year; other years from the same page.
- 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.
