Jaguar XJS Electrical Problems, Bad Earths
On the XJS most electrical faults are bad connections rather than failed parts: a missing or loose engine-to-body earth, tired glass-fuse contacts and the blue relays at the front. Start with whatever was last worked on, then the earths and the fuse boxes. Buy parts only once a measurement points at them.
How bad is it · Garage, carefully
Don't drive it if anything smokes or gets hot
A poor engine earth makes the starter current find its way through thin wires and cables, which can melt. A dead fuse or lamp alone is usually safe to drive on with care.
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 missing or loose engine-to-body earth
The engine is earthed by straps low at the left front, from the engine to the subframe and on to the body. If a strap is loose, missing or refitted in the wrong place, starter current returns through whatever it can reach: throttle and kickdown cables, the gear selector cable, the thin console harness.
Wiring in the console or under the dash smokes only while cranking, a throttle cable melts, or gauges swing about during cranking. It often starts right after work that disturbed the engine or the console.
A voltage reading at the coil doesn't prove the earth is good. Clip a jumper cable from battery negative to a solid point on the engine and crank: if the fault stops at once, it's the earth. The factory straps hide under grime near the left engine mount; on the V12 the air cleaners have to come off to see them.
Add a heavy earth cable you can reach, from the engine (alternator bracket, an intake manifold bolt or the sump) to the body itself, for example a torsion bar mount or near the radiator. Don't use the subframe: it sits on rubber. Keep it metal to metal, avoid a highly loaded brace bolt, then check the cables that carried the current for damage.
A fuse that blows: a wire pinched during recent work
One fuse often feeds several things you wouldn't group together: indicators, brake lamps and the gearbox kickdown switch share a circuit, and the instrument fuse also feeds the speedometer sender on the gearbox. A wire crushed against metal anywhere along that circuit takes the fuse out, so the dead item may be far from the fault.
The fuse goes at key-on, or after a minute with the brake pressed, and it began after work on the throttle linkage, the gearbox or cruise control.
Look first where the last work was done: the kickdown wire beside the throttle pedestal, the sender wires where they enter the cabin at the upper left of the gearbox tunnel, any leftover cruise control parts. Pull all brake bulbs and press the pedal: if it still blows, it's the wiring or the switch. Then open the circuit's connectors one at a time.
Free and repair the trapped wire, or disconnect the abandoned cruise electronics. Fit the correct fuse rating, never a bigger one to see what happens.
Dirty glass-fuse clips and a tired fuse block
Earlier cars use glass fuses in clip holders. The clips and the crimped joints under the block corrode, so a fuse can look fine and still break the circuit. A failing block can also heat up and feed one circuit into another.
A lamp or wiper comes and goes, or one side of the headlamps works and then the other. A radio or other item stays lit with the key out.
Test each glass fuse across its end caps with a meter, not by eye. Rolling a fuse in its clips brings the circuit back for a while: that's your answer. Look at the back of the block for heat marks and failing crimps. On blade-fuse panels a glow-when-blown fuse picks out the dead one with the key on.
Clean the clips, throw out doubtful fuses and resolder failing crimps under the block, or change to a modern blade-fuse box. If one feed is live with the key off, move that item to a feed that switches off.
A failed blue Hella relay
On later cars a row of blue Hella relays at the front driver's side switches the lamps; the side-lights relay also feeds the instrument and console lighting. Jaguar issued bulletin 86-178A for an inoperative blue relay.
A whole group goes at once: side lights, plate lamps and side markers on the first click of the switch, with good bulbs.
Find the relay from the electrical guide for your year: several positions are left empty depending on market, so the count won't match the manual. Swap in a known-good relay of the same type.
Replace the blue relays as a set. The later brown relays from the XJ8 and XK8 era are a known substitute.
The bulb-failure unit's earth and plugs
The bulb-failure units are earthed through their mounting screws and measure the current in their own circuit. A loose screw or dirty plug keeps the warning lamp on and can kill the lamp it watches.
A side lamp is out and the bulb-failure lamp stays lit, often after work on the dash or blower.
Check for 12 V along the circuit with the lights on: switch, bulb-failure unit, fuse, lamp, including the inline plug by the bulkhead. Then wiggle the unit's mounting screw near the boot lid opening on the driver's side.
Clean and tighten the unit's earth screw and spray its plugs with contact cleaner; clean the side-lamp fuse clips at the same time. The lamp should go out about 30 seconds after the lights are switched on. A feed wired around the unit lights the lamp but won't clear the warning.
Switches and Lucas connectors carrying the full load
From the factory the window motors, wipers and headlamps take their full current through the dash switches, the column stalk and Lucas bullet connectors. Every worn contact drops voltage, so motors crawl and lamps go dim.
Windows slow or sticking part-way, switches that burn, wipers that drag, headlamps dim on main beam. Under 10 V at the headlamps is a clear sign.
Measure at the lamp or motor against a reliable earth, to tell the feed side from the earth side. On the headlamps, look at the tubular connector in the main feed under the thin plastic cover at the left front of the engine bay, behind the coolant filler cap.
Clean the original window switches inside before buying new ones, which often fit badly. Replace melted bullet connectors with larger spades. For a lasting cure, fit relays so the switch only drives a coil; on the convertible the quarter windows then have to be parked by hand.
The column stalk switch and its earth wire
The stalk only switches the headlamp relay's coil to earth. It is known for losing its own earth wire under the column cover, and its contacts corrode.
Headlamps won't change between dip and main. The click you hear is the relay, not the stalk.
With the headlamps on, disconnect the wire from the stalk to the relay and touch it to earth (terminal 31B, brown/black, at the relay in the engine bay). If the relay clicks and the beam changes, the relay is fine and the fault is the stalk or its earth. Later cars also have a separate dim/dip control unit that can fail.
Worth trying first: refit or clamp the loose black earth wire to the column tube, then strip and clean the stalk switch contacts.
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.
Start where the last work was done
A fault that appears right after a repair is usually an earth or plug that work disturbed. Look nearest to whatever was last taken apart.
freeFind the right fuse boxes
The handbook in the glovebox may be for another year. On late facelift cars the main fuses sit behind the driver's knee bolster, which drops once you turn the slotted retainer at the door end a quarter turn. Use the electrical reference for your model year.
freeTest the fuses with a meter
Check glass fuses across the end caps, not by eye, and roll each one in its clips. If rolling it brings something back, clean the clips and the block.
freeProve the engine earth
Run a jumper from battery negative to a solid point on the engine and repeat the fault. If it goes away, fit a permanent heavy cable from the engine to the body.
about the price of a jumper cableIsolate a fuse that blows
Take out the bulbs or unplug the loads on that circuit and add them back one at a time. If it still blows with everything off, open the connectors along the wiring until it stops.
freeSwap suspect relays
When a whole group of lamps dies together, swap its relay for a known-good one before chasing wiring.
a few relaysMeasure voltage at the load
With the circuit working, read the voltage at the lamp or motor. A big drop means connectors or switch contacts; measure the earth side too.
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.
Circuit descriptions from the Jaguar fuse tables, 1988 edition. Drawing by jagaux.com from the factory electrical guide. Fuse layouts change between model years — read the legend on the lid of your own box before pulling anything.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS Won't Crank, Starter RelayIf the key gives a click or nothing at all: the starter relay and its feed.
- Jaguar XJS Battery DrainIf the battery is flat by morning.
- Jaguar XJS Alternator Not Charging, Battery LightIf the voltmeter reads low or the battery light stays on.
- XJS fuse boxes and relaysWhere the fuses and relays sit; other years from the same page.
- Jaguar XJS Brakes and ABS ProblemsIf the ABS and BRAKE lights came on together with other electrical faults.
- Jaguar XJS Headlights Not WorkingIf the headlamps themselves are dead, dim or won't dip: the dip relay, headlight relay and beam fuses.
- Jaguar XJS Window Motor, Windows Slow or Not WorkingIf the electric windows are slow or dead — relay, breaker, switches and motors.
- Jaguar XJS Brake Light Switch, Turn Signals Not WorkingIf the brake lamps stay on or the indicators and hazards die together.
- 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.