Jaguar XJ40 Immobiliser Problems, Won't Start
The XJ40 has no key transponder; what stops it starting is the security system's start disable or an add-on alarm with its own immobiliser. Most often the system has latched after a flat battery or a battery disconnect, and a reset clears it. I'd start by disarming it properly, then look for a hidden alarm box under the dash or bonnet.
How bad is it · Safe to drive
Nothing is damaged — the car is refusing, not broken
Once it starts, it drives normally. Don't cut wires to the alarm module: on 1993–94 cars the same electronics run the locks and indicators.
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 security system has latched after a flat battery or a battery disconnect
A deeply flat battery, a jump start or a battery change can leave the security system stuck in an armed or fault state. It then blocks the starter and the central locking even with full voltage back. Tripping the inertia switch can do the same.
Dead crank, locks that work only by key, the remote chirps but does nothing — and it started right after the battery went flat or was disconnected.
With a charged battery, press the remote again and listen for two chirps. Then switch the console security switch off and on. On 1993–94 cars, key in and ignition on and off three times quickly stops an alarm that has tripped.
Disarm with the remote, cycle the security switch, or disconnect the battery for a few minutes to force a reset. If the inertia switch was involved, reset it firmly again before cycling the security switch.
An add-on alarm with its own immobiliser
Many cars carry a dealer-fitted or aftermarket alarm, a small black box under the dash or near the bonnet with a wire to the starter relay. Silencing its siren doesn't stop its immobiliser, and many of them arm themselves after a battery disconnect by design.
A single click and no crank with the dash lit; the car cranks when the starter relay is bridged; a mystery box with loose wires behind the dash; full alarm after a battery reconnect.
Bridge pins 30 and 87 of the starter relay base: if it cranks, the starter is fine and the fault is in what switches the relay. Then trace the extra wire to the relay back to the alarm box. Some units have a key switch on the case or in-line fuses.
Disarm the box with its own fob (often off–on–off after a battery reconnect) or its key switch, or remove it and restore the original relay wiring.
A remote that isn't programmed, or the wrong procedure for the year
A new, used or freshly reset fob has to be coded to the security unit, and up to five fobs are coded together. On 1993–94 cars the battery-disconnect method doesn't work — coding is done with the console security switch.
The remote does nothing, or only changes the dash LED, while the doors and alarm don't respond.
Check the fob battery first: the infrared key takes a 12 V A23 cell. Then run the coding sequence: security switch on, ignition to I, security switch off, ignition off, then flick the security switch on and off five times as fast as you can — two to three seconds, not five. A tone and the overhead lights confirm programming mode; press each fob until five tones are heard.
Recode all fobs in one go with the console-switch sequence. If the car won't enter programming mode, the switch cycling was too slow.
Corroded battery terminals after a deep discharge
Dirty posts and cable ends read a normal voltage with nothing running but can't pass real current. The security and locking electronics then misbehave as if they were faulty: no remote unlock, no crank, a stuck selector interlock.
Several unrelated things died at once after the battery was run flat, and the meter says 12–13 V.
Watch the battery voltage while cranking and while operating the locks. A big drop points at the terminals or a battery damaged by the discharge.
Clean the posts and clamps until bright. If it fails to crank again within days, the battery itself was damaged by the deep discharge — replace it.
A failed security module, or bad joints and plugs at it
The security ECU sits in the left side of the boot near the fuel filler on most cars, behind trim or a foam cover; early cars may have it under the glovebox or in the right side of the boot. When nothing responds at all — not even a beep — the module itself is the prime suspect; 1993–94 relay modules also crack their solder joints.
No reaction whatever from any fob, after the fob battery, the coding and the wiring are all ruled out.
Unplug, clean and reseat the module connectors first. Resolder cracked joints in the relay modules.
Fit a used security module and code the fobs to it.
A door, boot or bonnet switch that reports open
The security unit watches the door latch, boot and bonnet switches. Any one that reads open stops it disarming, or sets it off and locks out the starter. The door switches are micro switches on a small board inside each latch.
The alarm won't disarm after a battery reconnect, sets itself off when a door opens, or locks and unlocks on its own.
On the security unit's 20-way plug, check each input against its resting state: the boot switch pin should go to earth with the boot open and open-circuit with it shut, and so on for each door and the bonnet. Check the front door handle micro switches for sticking.
Worth trying: repair the faulty switch or its wiring at the door connector before dismantling the latch.
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 clean the terminals
A weak or poorly connected battery makes the security system misbehave on its own.
freeDisarm properly
Press the remote, cycle the console security switch, or on 1993–94 cars turn the ignition on and off three times quickly.
freeReset by disconnecting the battery
A few minutes off clears a latched factory system. Expect an add-on alarm to sound on its own back-up battery.
freeBridge the starter relay
Pins 30 and 87 on the relay base. It cranks — look for an add-on alarm wired into the relay circuit.
freeRecode the fobs
With the console security switch, cycling it fast. Fit a fresh battery in each fob first.
freeTest the door, boot and bonnet switch inputs
At the security unit plug, against their open and shut states.
freeReseat or replace the security module
Only when nothing responds at all after everything above.
a used moduleWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Won't Crank, Just ClicksIf the alarm isn't involved and the starter only clicks.
- Jaguar XJ40 Central Locking Not WorkingIf the locks misbehave with the key as well as the remote.
- Jaguar XJ40 Battery DrainIf the battery keeps going flat and setting this off.
- Jaguar XJ40 Cranks But Won't StartIf it cranks normally and the alarm is quiet.
- 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.