Jaguar XJS Battery Drain
On the XJS it's usually the power aerial: its motor sits on a permanent feed and keeps running when its board corrodes or its cord breaks. Then boot lamps that never go off, a tired battery, a radio memory feed drawing far too much and motorised belts on US cars. I'd pull the aerial fuse first.
How bad is it · Safe to drive
Safe to drive — but it ruins batteries
The car is fine once started; letting a good battery go flat again and again kills it. Charge with a charger, and pull the aerial fuse as a first free test.
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 power aerial won't switch off
The motor is always live so it can lower the mast after switch-off. Water from a leaking seal and blocked drain corrodes its board, which keeps driving; a broken cord leaves it running for ever.
The aerial stuck or moving oddly; its motor unit warm in the boot with the car off. It can drain the battery in complete silence, with only the motor housing hot to the touch; or you hear it trying to wind down every minute or so with the mast already home.
Pull the aerial fuse and leave the car a few days. If it starts, you've found it.
Repair the board, replace the cord or the aerial, and clear the drain tube under the boot. A wet board can sometimes be revived, but a repaired one often drains again within a day. A good board out of a scrap Hirschmann with a broken mast swaps straight in. An aftermarket power aerial that fits the stock base, about $50–100, does the job just as well for years; only the timed delay is lost.
Boot lamps left on
A worn switch pad or a lid not fully latched leaves them on; 1995-on cars hide a second lamp above the CD changer. A switch plunger that no longer releases fully when the lid shuts does the same.
A steady few hundred milliamps.
Pull the boot lamp fuse and read the meter again. Every fuse means every one: on a 1992 the boot lighting fuse sits tucked at the end of the left-hand fuse box in the boot and is easily missed in a fuse-by-fuse test.
Fit a rubber pad on the lid so the switch is fully pressed, or a small tab over the switch that holds the lamp off for good. A number-plate lamp that pulses rather than glowing steadily afterwards points at a second short in the same circuit.
A worn-out battery
Deep discharges ruin it, and a 'new' one may be shelf-old. It can show voltage with nothing behind it.
No draw found; it loses charge even disconnected; voltage collapses under load. A charger that keeps pushing two amps or more long after it should have tapered off, with the battery getting warm, means the battery itself, not a drain.
Charge it off the car: it should hold above about 12.6 V. Load-test it.
A new battery — even a young one can fail. Even without a fault the standby draw flattens a battery in about three weeks parked; a battery conditioner, or a cut-off switch that keeps the clock and the coded radio alive, saves it from deep discharges.
The radio memory feed drawing far too much
The radio keeps a permanent feed for its memory and security code. A faulty set can pull several hundred milliamps through it, enough to flatten the battery overnight. Leftover wiring from a removed aftermarket alarm in the console makes it harder to see.
An overnight drain that survives the aerial and boot lamp tests; a fuse-by-fuse search that never seems to find it, because the memory fuse sits apart from the main boxes.
Measure the current through the radio's memory fuse on its own rather than assuming it's negligible. On these cars it can sit behind the console, so it's often the last place anyone looks.
Pulling the memory fuse brought the overnight draw down to almost nothing; the price is re-entering the radio code each time. A new or different radio is the lasting cure.
A short in the starter feed or a chafed wire
The starter feed is always live. An oil-soaked starter or a wire rubbed through leaks to earth.
Completely dead overnight; oil on the starter; added lamps or accessories.
Disconnect the starter feed at the firewall post and measure again.
A new starter if it's oil-soaked; re-insulate chafed wiring, such as fog lamp wires.
Motorised seat belts cycling (US cars)
A belt that doesn't reach its stowed position keeps its motor trying.
A click every few seconds from the belts with the car parked.
Pull the belt fuses and measure again.
Worth trying: free the belt that won't park, or convert to manual belts.
The alternator leaking through a diode
Its battery cable is unfused. A shorted diode drains the battery with the engine off.
No fuse changes the reading; flat within a day.
Disconnect the alternator's battery lead and measure again.
Worth testing: an alternator shop can renew the diodes in a few minutes for little money.
A blower that keeps running
A failed diode in a blower's control unit can leave it running with the key off.
Air still blowing after switch-off.
Find which side is running.
Worth trying: repair that side's unit.
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.
Charge the battery off the car
If it falls overnight on its own, it's the battery.
free with a chargerPull the aerial fuse, wait a few days
The cheapest and most often decisive test.
freeMeasure the draw
In series, high-amp range first, boot switch held closed, doors shut. About 20–50 mA is normal. The alarm draws more for a minute after locking. Anything much above 50 mA is a real drain; 140 mA will flatten it in days. Leave each fuse out long enough for the alarm to settle, or the reading lies.
a multimeterUS car? Listen for the belts
A click every few seconds means a belt not parked.
freeFuses one by one, then the heavy leads
Under both knee pads and in the engine bay. Then the starter and alternator leads at the firewall posts — battery off first.
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, 1989 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 Overheating, Cooling Fan ProblemsIf the aux fan keeps running after the engine is off.
- XJS fuse boxesWhich fuse feeds the aerial and boot lamps — other years from the same page.
- Jaguar XJS Blower Motor Not WorkingIf a blower keeps running with the key out.
- Jaguar XJS V12 Won't Start or Starts and DiesIf a weak battery leaves the V12 cranking but not firing on a cold morning.
- Jaguar XJS Electrical Problems, Bad EarthsIf fuses blow or lamps and gauges act up as well: bad earths and tired fuse contacts.
- Jaguar XJS Alarm Keeps Going OffIf the alarm sounds or the fobs stop working after the battery was flat.
- 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.