Jaguar XJ6 Series 3 Battery Drain, Flat After a Week
Measure before you guess: an ammeter between the negative post and its clamp says whether anything is drawing and roughly what. About 20–35 mA is normal; around half an amp is typically a lamp left on, often the boot lamp. If nothing is drawing, the battery itself is the fault — even a new one.
How bad is it · Safe to drive
Safe to drive — the battery is what suffers
The car drives normally once started. But each deep discharge shortens a lead-acid battery's life, so find the drain rather than living on jump starts; a maintainer or disconnecting the battery buys time.
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 lamp that stays on: boot, courtesy, puddle or glovebox
These lamps run on small plunger or pillar switches. A bent contact, failed switch or chafed wire keeps a lamp fed — sometimes one that doesn't visibly light.
About half an amp; a warm bulb or socket; the draw stops when the interior lamp fuse is pulled.
Press the boot lamp switch at the hinge and watch the lamp go out; check the glovebox; feel each socket for warmth after the car has stood.
A new switch or bulb, or repair a puddle-lamp wire chafed in the door jamb.
The battery itself does not hold its charge
A battery can discharge internally with nothing connected. Under-bonnet heat and age shorten its life, and a new one can be faulty too.
The ammeter shows only the normal few tens of milliamps, yet it goes flat — for example from 12.6 to 11.4 V overnight.
Charge it, disconnect it, read it next day: a healthy battery stays a little above 12 V. Then load-test it.
A new battery.
A circuit with no fuse: the horn relay, and friends
Pulling fuses only finds fused circuits. The horn relay and several accessories are fed directly or through hidden in-line fuses.
The draw stays the same with every panel fuse out.
Ammeter connected, disconnect the horn relay feed, then the in-line fuses for the cigar lighter (behind the console side panel) and the electric seats, until the draw drops.
Repair the circuit found — for example a horn and steering-column earth fault behind the horn relay's green wire.
The trip computer
It keeps its memory alive with the key out, and a failing unit can draw too much. It also freezes at low voltage — then it's the victim, not the cause.
The draw falls when it's unplugged; the unit feels hot.
Slide it out, unplug and tape its connectors, and see if the battery survives. Nothing else depends on it.
Worth trying: leave it unplugged.
Leaking alternator diodes
A failed diode lets current flow back from the battery with the engine off. A draw of two amps or more points here.
A large draw no fuse changes; charging trouble too.
The small plug doesn't isolate the diodes — take the heavy cable off the output stud and see if the draw goes.
Worth trying once the draw is proven there: repair or replace the alternator.
An aftermarket alarm or other added equipment
Alarms, remote locking and amplifiers are rarely documented. A failing one draws steadily with the car locked.
An alarm box or extra wiring; the drain appeared after something was added.
Disconnect it and measure again.
Worth trying: remove 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.
Ammeter between the negative post and the clamp
Everything shut and asleep. About 20–35 mA is normal; around 0.5 A is typically a lamp; two amps or more, the alternator. Reads zero? Check the meter's fuse.
a multimeterNothing drawing? Test the battery
Charge it, disconnect it, read it next day. If it won't hold charge on its own, it's the battery.
freePull fuses one at a time
Interior lamps first — or measure the tiny voltage across each fuse so nothing wakes up.
freeEvery fuse out and still drawing?
The horn relay, the in-line fuses, then the alternator output cable.
freeUnplug the trip computer and any alarm
See whether the battery survives.
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, 1987 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:
On jagaux.com
- Jaguar XJ6 Series 3 Alternator Not ChargingIf it goes flat while driving too, start there.
- XJ6 Series 3 fuse boxesWhich fuse feeds the interior lamps.
- Jaguar XJ6 Series 3 Won't Start, Cranks But Won't FireIf it cranks but won't fire even with a good battery, the pump, relays and spark checks are here.
- Jaguar XJ6 Series 3 Turn Signals, Indicators Not WorkingIf the indicators died along with other circuits.
- 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.