Jaguar XJ40 Battery Drain, Flat After a Few Days
On the XJ40 the battery is rarely at fault — something in the car stays awake, most often the door lock heaters on early cars, then a hidden lamp or a window circuit held on. I'd start with an ammeter in the battery lead, doors shut, then one fuse at a time.
How bad is it · Safe to drive
Safe to drive — the battery is what suffers
The car drives normally once started. Repeated deep discharge ruins even a new battery, so charge it with a charger rather than a jump and a drive, and find the drain before fitting another.
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.
Door lock heaters stuck on
Early cars heat the front lock barrels. Lifting a front door handle latches a relay that runs the heaters until thermal switches in the locks open; a stuck handle switch or thermal switch keeps them on.
The relay at the base of the right A-post is warm; the draw sits on the always-live auxiliary fuse, shared with the radio. 1994 cars don't have the heaters. A drain on that fuse looks like the radio or the aerial, but it's usually the heater relay that shares it.
Ammeter in line, pull the heater relay behind the vinyl at the right-hand fuse box. If the reading drops to normal, you've found it.
Leave the relay out, unplug the heater in each door, or fix the handle switch. The draw falls to normal — for example from about 170 to about 40 mA. A door-handle flap switch mounted too close to its pivot sticks on and keeps the heater live — worth checking after a door or handle has been changed.
A lamp left on where you can't see it
Seat-back, headrest and rear side lamps, glovebox and boot are easy to miss. The seat-back lamps have a permanent-on position, and a worn switch can light them by itself.
About half an amp that vanishes when the interior lamp fuse is pulled, with every visible lamp off. The bonnet and interior lamp circuit is the usual one; a lamp that doesn't switch off can pull a lot — for example over 300 mA on top of a normal 35 mA.
Walk round the car in the dark, doors shut; look under seat covers and open the boot slowly. The boot switch is behind the right-hand hinge.
Switch it off, reseat it, or replace a switch that lights in the off position; straighten a bent boot switch tab.
A window circuit held on
A stuck relay in a rear-door window switch box, or a chafed 'up' wire, keeps a window motor powered. Its thermal cutout cycles, so the draw comes and goes — easy to miss fuse by fuse.
Several amps that come and go, clicking near a door, a window that rises by itself or not at all. Faint ticking from a door with its window switch stuck in the up position does it too.
Watch each fuse for a minute. When the window fuse stops the cycling, unplug each door's switch box in turn.
Replace the switch box (a used one does) or repair the chafed wire in the B-pillar loom. The draw can be several amps — for example 5 A, or over 9 A. A stuck switch sometimes only needs tapping back.
The lighting logic module or light switch
The light switch left of the steering wheel carries a small module with its own electronics. When it fails it can hold current with the key out.
Instruments glowing with the key off, or a warning sounder when the driver's door opens.
Look at the dash at dusk, key out; unplug the module and read the meter again.
A replacement module, used is fine, or a new dimmer switch.
An add-on: CD changer or alarm
Aftermarket kit is often on a permanent feed and never sleeps.
A changer cycling with the key out; an old alarm nobody remembers.
Disconnect it and leave the car a week.
Leave it disconnected. An aftermarket alarm can draw enough to flatten the battery — for example about 2 A.
The battery itself, when the draw is normal
A battery can self-discharge internally and still pass a quick load test. If the settled draw is normal and it still goes flat, the battery is the fault. A dying battery also brings odd symptoms: cutting out on deceleration and a false bulb-failure warning.
The draw settles at a normal figure, yet the car is dead after a day or two; a battery that has been run completely flat once rarely holds a full charge again.
Charge it off the car and leave it 12–24 hours: about 12.7 V is full, 12.45 V three-quarters, 12.24 V half. Then have it heavy-discharge tested on a proper load tester, not a quick electronic one.
A new battery of the right size and cranking rating. Charge a flat one with a charger rather than letting the alternator do it.
Battery post touching the bonnet
A battery too tall, or on a thick pad, puts the positive post against the bonnet. It shorts only with the bonnet shut.
A burn mark on the bonnet frame above the battery.
Look at the underside of the bonnet above the battery.
Correct battery size, a thin pad, insulate the spot.
The central processing unit (1993-on)
It switches the ground for the starter relay, washer motor and key-lock solenoid. An internal fault can leave several on at once.
Unmistakable: the key won't come out, the washers spray at switch-off, the starter engages in position II.
Unplug its connector with the ammeter in line; if the draw collapses, it's the unit.
A replacement unit; a used one does.
The wiper motor park switch
A cam-worked micro-switch in the wiper motor keeps it powered until the blades park. If it sticks, it holds a small draw with the key out while the wipers still work normally.
An extra draw of around 50 mA that goes when the wiper fuse in the left footwell box is pulled; the wipers themselves are fine.
Wipers parked, stalk off: pull the in-line park fuse to the wiper motor. If the draw drops, it's the park switch; if not, look at the wiper logic unit, the CPU or the stalk switch.
Replace the micro-switch in the wiper motor gearbox — it's still made. The motor cover on the bulkhead comes off with a Torx socket.
The alternator's leaking diode
The alternator lead reaches the battery unfused. A leaking rectifier diode drains it with the engine off, yet charging looks normal.
Every fuse out and the draw remains; the alternator warm on a cold car; speaker whine that rises with revs.
Take the output lead off the alternator and read again. A big drop points at the diodes.
Repair the alternator's rectifier.
Motorised seat belts that never park
Some North American cars have belts running on a motor. If a belt sticks short of home, the motor keeps trying.
Soft clicking in a door; a belt just off its end stop; a drain that comes and goes. Flat after working on the car with the doors open for a while.
Pull the belt system fuses with the meter connected.
Clean and grease the tracks.
A seat lumbar switch or seat relay stuck on
A stuck or shorted lumbar switch keeps the lumbar pump trying to run, and a seat relay can stay energised with the key out. Either can drain the battery hard.
A hot relay under the glovebox, or the lumbar bladder motor running with the ignition off.
With the meter in line, feel the relays under the glovebox for a warm one and pull it; unplug the lumbar motor and read again.
Worth trying: a new lumbar switch, or the stuck relay replaced. There can be more than one drain, so measure again afterwards.
A welded relay: fuel pump, ABS pump or A/C clutch
Welded contacts keep a load running after switch-off. If the wiring has been altered, it may bypass every fuse.
The fuel pump whirring after switch-off, or the compressor clutch engaged with the A/C off. On 1990-on cars with Teves ABS, a stuck pump relay runs the pump every few seconds to top up the accumulator — a draw of several amps that cycles on and off.
Listen with the car locked; swap or pull the suspect relay. The ABS relay sits in the boot.
A new relay.
A circuit off the fuse boxes, such as self-levelling
Some feeds run straight from the battery distribution posts, not through the footwell boxes.
Every fuse out and the draw remains.
Disconnect the distribution post wires one at a time; try the ride-height fuse and the connector under the glovebox.
No proven cure yet: isolate the circuit, then repair it.
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 and see if it holds
Fully charged and rested it should read 12.6–12.8 V and stay above 12 V next day. If it drops hard on the bench, it's the battery — even one that passed a load test.
free with a chargerMeasure the draw — correctly
Meter in series in the negative lead, 10 A range first. Latch the doors shut and wait up to five minutes. A normal settled reading is roughly 30–80 mA; 1993-on cars can read lower. Never crank with the meter in line.
a cheap multimeterEarly car? Pull the lock heater relay first
It's the most common cause and the quickest test.
freeWalk round the dark car
Headrest, seat-back and rear side lamps, glovebox, boot.
freeThen one fuse at a time, a minute each
Both footwell boxes; the console box is ignition-fed, so skip it. A cycling draw means a motor with a thermal cutout.
freeStill drawing with everything out?
Disconnect the alternator output lead, then the distribution post wires one by one.
freeStopgap while you look
Fit a battery isolation switch. Keep the radio code to hand.
a cut-off switchThe 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
- XJ6 (XJ40) fuse boxesWhich circuit each footwell fuse feeds — other years from the same page.
- Jaguar XJ40 Dashboard Warning LightsIf the instrument lights stay on with the key out — usually the dimmer module.
- Jaguar XJ40 Central Locking Not WorkingIf the central locking stopped after the battery went flat.
- Jaguar XJ40 Won't Crank, Just ClicksIf the battery is fine but it still only clicks, start there.
- 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.