Jaguar XJS Headlights Not Working
On the XJS dead or dim headlamps are nearly always a relay at the front left of the engine bay: the latching dip relay, or the headlight relay with corroded contacts. Facelift cars add a logic module that can drop the lamps. I'd start by noting what still works, then test the blue wire at the relays and bridge the fuse for the dead beam.
How bad is it · Garage, carefully
Fine by day — don't drive at night without them
The engine isn't affected. A tired dip relay can stick on main beam or drop the lamps altogether, so sort it before a night drive.
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 dip relay that flips between main and dipped beam
Dipping is done by a latching changeover relay, a larger silver Hella unit (001836057) at the back of the relay bank on the left inner wing. A pulse from the stalk flips it and an internal latch holds the beam. Inside it a fine coil wire or a solder joint corrodes or breaks, and with 100 W bulbs the plastic inside can melt.
Dipped beam only comes on after flashing main beam a few times, the lamps go dark for a few seconds when you dip, the beam sticks on main, or both beams are dead with no click from the relay.
With the lights switched on, test the blue wire from the switch at the relay. 12 V there and no beam on either setting points at the relay itself.
Open the relay, clean the contacts and resolder the broken wire or joint; a used relay is the alternative. With higher-wattage bulbs, add two plain relays for main and dipped so the original only switches coil current. Don't hold the flasher on for long: the relay coil can burn out.
The headlight relay with corroded contacts
The headlight relay carries the lamp current switched on from the dial. On these cars its contacts corrode inside the sealed case, so the beams get too little current or none at all.
Headlamps dead from the light switch while the main-beam flash still lights them, or everything drops to a faint glow the moment main beam is selected.
With a 12 V test lamp and the lights on: the blue wire from the switch should be live; the blue/red wire is the relay output to the dipped beams. Live on blue but not on blue/red means the relay.
Cut the sealant round the base, open the case, clean the contacts, spray them and reseal with RTV silicone. Otherwise fit a replacement; on a 1989 car the part is C38616, shared with the Series III XJ6.
A fuse that reads good on a meter, or loose fuse clips
Each beam on each side has its own fuse at the front left of the engine bay. A fuse can show continuity on a meter and still not carry the lamp current, and tired clips in the old glass-fuse boxes do the same.
One beam on one side is dead with a new bulb in, or only about 2 V arrives at the headlight fuses, or one fuse runs very hot with the lamps on.
Bridge the two terminals of that fuse with a jumper wire with the lights on. If the beam lights, the fuse or its clips are the fault. Clean the spring terminals under the fuse box as well, not just the visible contacts.
Fit a new fuse, sand the fuse ends and the clips, and tighten loose clips. Use full-size 1/4 in glass fuses; 6 mm ones are too small for the clips.
Dim headlamps because the switch and thin wiring carry the full load
As built, the lamp current runs through the column switch and long thin wires. On these cars that loses enough voltage to make the headlamps noticeably dim, and a poor earth makes it worse.
All the headlamps work but give a yellow, weak light; well under 12 V at the lamp on main beam.
Measure the voltage at the headlamp plug on main beam, first against the lamp's own earth and then against a good chassis earth. A difference between the two means the earth side; a low reading both ways means the feed.
Add relays near the headlight fuse box, triggered from the existing headlamp relay and fed from the power post on the left of the bulkhead, so the switch only carries the coil current. Brighter bulbs on top of that make a large difference.
After LED bulbs the main-beam warning works backwards
An LED bulb has its own UP mark, which isn't where the halogen bulb's latch sits. Fitted by the latch, the beams come out reversed.
Since LED bulbs went in, the main-beam warning lights on dipped beam and goes out on main.
Find the UP mark on the LED bulb and see where it sits.
Refit the bulbs with the UP mark at 12 o'clock rather than the latch. Try this before buying a beam-reversing module.
The headlight logic module on facelift cars, DAC 6902
On facelift cars a small black module at the end of the relay rail in the front left bank does the logic: dipped beam by default, main and dipped swapped on a stalk pull, and the flash. It is two relays and some electronics, and dry solder joints inside are common at this age.
The headlamps drop out completely when you switch between main and dipped, come back if the module is tapped, or switch themselves on with no key in.
Tap the module firmly with the fault present. If the lamps come back, it's the module.
Worth trying: resolder the joints inside, or fit a replacement module. Aftermarket units work but are built lighter than the original.
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 works
Dipped, main, the flash, one side or both. One beam on one side points at a fuse or bulb; both sides at once points at a relay or the switch.
freeTest the blue wire at the relays
Lights on: 12 V on the blue wire from the switch, then on the blue/red output. Live in but not out means the relay. No voltage on blue means the switch or its feed.
a test lampBridge the fuse for the dead beam
Don't trust a meter on these fuses. On 1987 cars it's F2 to F5 in the headlight fuse panel; on 1989 to 1991 cars F2 to F5 in the black under-bonnet panel; on 1992 to 1996 cars main beam is fuses 5 and 6, dipped beam 9 and 10 in the left front panel. Clean and tighten the clips.
freeOpen and clean the relay
Dip relay or headlight relay, whichever the test pointed at: clean the contacts, resolder broken joints, reseal the case.
freeTap the logic module on facelift cars
If the lamps come back, resolder or replace the DAC 6902 module.
freeMeasure voltage at the lamp for dim headlamps
Against the lamp earth and against the chassis. Then fix the earth, or add relays so the switch stops carrying the lamp current.
freeThe fuses named above — where they sit
The highlighted position is the one the procedure talks about. Pulling a fuse is a stop-gap, never a repair: check what the circuit feeds before you pull it.
We have no drawing of this box for this car — find them on the legend printed on the lid of the box itself.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS Electrical Problems, Bad EarthsIf other circuits are misbehaving too: earths, fuse clips and the blue relays.
- Jaguar XJS Alternator Not Charging, Battery LightIf all the lights go dim together with the engine running.
- XJS fuse boxes and relaysWhich fuse feeds each beam on your year; other years from the same page.
- Jaguar XJS Brake Light Switch, Turn Signals Not WorkingIf the brake lamps or indicators are out too: the bulb-failure unit in the boot and the flasher 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.