Jaguar XJ40 Headlights Not Working
A dead or dim headlamp on these cars is usually a corroded or chafed wire in the front loom, or an oxidised fuse feeding that one beam. The front bulb-failure modules switch the headlamps too, and their solder joints crack. I'd start with the fuse for that beam, then swap the front modules side to side.
How bad is it · Garage, carefully
Fine by day — don't drive at night on one headlamp
The engine and brakes aren't affected. At night a single dipped beam is easily mistaken for a motorcycle, and it's an inspection failure.
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.
A corroded or chafed wire between the fuse box and the headlamp
The feed for each beam runs through the thick loom along the inner wing and through the plugs at the A-post. On these cars the headlamp loom cooks hard and corrodes, or a wire rubs through on a sheet-metal edge; the result is no voltage, or low voltage that changes as the loom is moved.
One beam on one side is dead or dim, the bulb is good, and there may be no bulb-failure warning at all — main beam isn't watched by it.
Pull the fuse and check continuity from its socket to the module plug; then feed 12 V straight to the module pin. If the lamp lights, the wire is the fault. Look closely where the loom crosses metal edges and along the top of the inner wing.
Cut back to bright copper, solder in a new section and seal it with heat-shrink. On the right dipped beam, check the white/purple wire at connector RS23 as well.
An oxidised fuse or a cracked joint behind its socket
Each headlamp beam has its own fuse in the box on its side. A fuse whose blades have oxidised, or a cracked solder joint behind the socket, passes too little current — the fuse looks fine and the lamp stays dark.
One beam dead, odd voltages on its wire, and often a circuit-failure message from the trip computer.
On 1988–89 cars main beam is fuse 4 (15 A) and dipped beam fuse 14 (10 A) in the box on that side; on 1990–91 cars they are B8 (15 A) and C7 (10 A). Later years — check the chart for your car. Test both blades with a lamp, not by eye.
Pull and refit the fuse, or swap it with its twin from the other side, to clean the blades. If it comes back, take the box out and resolder the socket joints.
One headlamp dim on dipped or daytime running: lamp unit or the dip relay
On cars with daytime running lights the dipped beams run at reduced voltage through a relay, DBC 10005, with a red plug just in front of the right-hand module. Poor contacts in that relay, or a tired lamp unit, leave one side dimmer.
Both lamps work but one is clearly dimmer on dipped beam, with no warning on the dash.
Compare voltage at both headlamp plugs on dipped beam. Equal voltage points at the lamp unit; lower on one side points at the relay or the wiring.
Clean the relay contacts; fit a good lamp unit if the voltage is equal. Re-aim both lamps afterwards.
The front bulb-failure module behind the headlamp
The headlamps are switched by relays inside the front modules, mounted behind and slightly inboard of each headlamp. Most module faults are cracked joints on the relays and the sub-board, not failed parts.
Pushing the relay inside the module lights the lamp; or one side has lost its dimmed dipped beam.
Swap the front modules left to right. If the fault moves, it's the module; if it stays, the signal from the switch isn't arriving.
Resolder all relay and connector joints on the main board and sub-board, or fit a second-hand module.
The switch pack or the lighting logic module
The light switch earths a control wire for each beam through the 24-way green connector under the left dash panel. If that signal doesn't reach the modules, nothing lights whatever the fuses say.
Headlamps dead from the switch while main-beam flash still works, or a beam that won't switch off.
At the 24-way green connector, pin 5 (blue/pink, right dipped) and pin 6 (blue/slate, right main) should read 12 V with the lights off and drop to earth with them on.
Worth trying: resolder the light logic module under the fog-lamp switches, or fit a used switch pack on 1988–89 cars.
After a quad-headlamp conversion: front modules that don't match
Rectangular-lamp and quad-lamp cars use different front modules. A mismatched module can upset the bulb-failure warning and even the indicators.
Warnings or a fast-flashing indicator appeared right after round lamps were fitted, or the plugs had to be changed.
Check the front module part numbers against the headlamp type.
Worth trying: fit the quad-lamp modules; DBC5245 is reported as a better module than DBC3271.
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 what's dead
Which side, dipped or main, and does main-beam flash still work. One beam on one side is a fuse, wire or module; everything at once is the switch.
freeTest the fuse for that beam live, both blades
In the box on the same side. Pull and refit it to clean the blades.
a test lampSwap the front modules left to right
If the fault moves, resolder or replace the module.
freeFeed 12 V to the module pin, then trace the wire
Lamp lights on a direct feed — repair the loom between the fuse and the module.
freeCheck the switch signal at the 24-way green connector
12 V with the lights off, earth with them on. No change — the switch pack or the light logic module.
freeWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- XJ6 (XJ40) fuse boxesWhich fuse feeds each beam on your year — other years from the same page.
- Jaguar XJ40 FUSE 1 or FUSE 2 FAIL WarningIf a circuit-failure or fuse warning comes with the dead headlamp.
- Jaguar XJ40 Alternator Not Charging, Battery LightIf all the lights go dim together with the engine running.
- Jaguar XJ40 Bulb Failure WarningIf the bulb-failure warning stays on with every lamp working.
- Jaguar XJ40 Turn Signals Not WorkingIf an indicator on the same side is dead or flashing fast.
- 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.