Jaguar XJS Alarm Keeps Going Off
An XJS alarm that goes off when you get in is most often just the disarm sequence: opened with the key, the car gives you about 15 seconds to switch the ignition on. After a battery change the fobs may need resyncing. A siren that won't stop whatever you do, or no response at all, points at the security module, which also holds the starter enable.
How bad is it · Safe to drive
Safe to drive if it starts
The alarm doesn't affect how the car runs. It can stop the starter, so don't switch off far from home until it behaves.
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.
Opened with the key: the 15-second disarm
Unlocking with the key instead of the fob doesn't disarm the alarm. You get about 15 seconds from opening the door to turn the ignition on, and the system ticks while it waits. Miss it and the horn and lights start, and the alarm can kill the ignition. With the valet switch on, it arms itself about 30 seconds after the door shuts, locked or not.
The alarm goes off every time you get in with the key, often after the fob battery has died; turning the key later does nothing.
Look in the owner's manual for the disarm sequence for your year. Fit a new fob battery (an A23) and see whether the fob disarms it normally.
Let the siren run its cycle, about 90 seconds, then open the door again and turn the ignition on within 15 seconds. On some cars switching the ignition on and off three times inside that window resets it; starting the engine silences it on others. Expect the first turn of the key to set off the full alarm: the system is counting the turns, so carry on.
The security module: siren won't stop, or no response at all
On 1993-96 cars the security module (DPP1050 in the US, DPP1051 in the UK) runs the siren, the locks and the starter enable: it supplies the ground that reaches the start relay through the gear selector switch in P or N. A failed or confused module can sound the siren endlessly, or ignore the fob after a battery change.
The siren and flashing lights keep going until the battery is disconnected, sometimes starting in heavy rain; or after a new battery the fob and security switch do nothing and the car won't crank.
On 1994-95 cars pull the security fuse F11 (7.5 A) in the rear fuse panel: the alarm falls silent but the car won't crank, which confirms the module holds the starter. The start-enable wire is black/green, pin 5 on the module plug.
To keep using the car, leave the fuse out or unplug the module and ground the black/green start-enable wire. Ground it on the selector switch side, not straight at the start relay, or the engine will crank in any gear. The alarm function is lost until the module is replaced. A new module is wired the same way; with a switch on the yellow/white wire to ground left on, it works as keyless entry and panic only, with no start inhibit. The black/green and black/light-green wires run side by side: on a convertible the wrong one leaves the car starting but the hood won't go down, so check the colour against the diagram for your year.
The immobiliser still holding the starter after a lay-up
The starter inhibit is part of the alarm. After the car has stood, or after the alarm has tripped, it can stay armed and cut the auxiliary load relay, so the car looks like it has a relay fault.
No start after storage, but bridging the relay next to the starter relay lets it start; or a park/neutral message after the alarm stopped responding.
Before replacing relays, lock and unlock the car with the fob and try again.
A lock and unlock cycle with the fob releases it, and the car starts normally.
A battery run down by standing
After weeks parked the battery sags below what the security module needs. It either sounds by itself or stops answering the fob, while the red LED keeps blinking.
The car has stood for weeks; a fob with a fresh battery does nothing, and the engine may barely turn.
Test the battery before blaming the alarm: if it can't crank the engine briskly, start there.
Unlock the driver's door with the key: a few short beeps, and the fob works again. Then charge the battery fully; if the alarm is still confused, disconnect it for half an hour and reconnect, with the radio code to hand.
A door, the boot or the bonnet not seen as shut
The alarm watches every door, the boot and the bonnet. One switch that doesn't make, or a door lock that hangs and stays unlocked, and it goes off on its own or chirps and counts down when you arm it.
It sounds at random when parked, or gives one low chirp and a countdown when you press lock.
Close the bonnet, the boot and each door firmly and arm again. Lock and unlock with the key any door that didn't lock with the others.
Closing every lid properly resets it. A lock that hangs needs lubricating; where a movement sensor is fitted, it can also trigger the alarm by itself.
After a total power loss: the alarm waits for its fobs
Up to about mid-1993 the alarm has its own small 6 V backup pack. When it and the main battery have both gone flat, the module wakes up in learning mode: a deep beep about once a second, or chirping and flashing, until fobs are pressed.
Regular beeps or chirps every few seconds after a flat battery or a long disconnect, and the emergency disarm does nothing.
Press your fob up to five times, a second apart; each accepted press chirps, then a different tone says it's armed. Only the fobs pressed now will work afterwards.
Without a working fob it stays in bursts until one is paired, by a dealer if need be. If the backup pack is dead it will keep coming back: a 6 V 1 Ah sealed lead-acid battery beside the module, or five 1.2 V NiCd cells in the old case, ends it.
The fob itself: a broken contact inside
A fob that stops arming the car with a fresh battery often has a broken contact: the metal arm holding the battery snaps off the circuit board. Early fobs can also drift off frequency.
New battery, still nothing, and no light on the fob.
Open the fob and look at the battery contact and the solder joints next to it.
Re-soldering the arm brings it back. For drift, turn the small adjusting screw inside gently with a fingernail, and keep metal tools away from it while testing.
A dealer-fitted alarm of another make
Before the factory system arrived, many cars, US cars especially, left the dealer with a third-party alarm: its own box, its own valet switch, its own fobs. The Jaguar disarm sequences won't touch it.
The fob and the siren don't behave as the handbook describes; a box about 5 in square under the dash, or a maker's name on the siren.
Look under the dash and on the siren for a maker's name, and get that maker's instructions.
Typically: key to II, flick the small valet toggle under the column and the siren stops at once. To re-code fobs, disconnect the battery or the unit's harness, reconnect, wait for the chirp and hold each fob's button until it chirps.
Fobs out of step after the battery was disconnected
A correctly programmed system keeps its fobs through a battery disconnect, but a reconnection can trip the alarm and leave it confused. The indicators flash and it beeps on its own, and the fobs stop arming it properly.
Straight after a battery change or winter storage: beeping, flashing indicators, the fob sounding panic instead of locking.
Lock the doors from inside with the chrome paddle, then unlock with the fob; that alone can reset it.
Worth trying: press the fob five times. Otherwise resync: on cars with the two-wire plug behind the console tray (yellow/white and black), bridge it to switch security off, toggle until short beeps start, press each fob in turn until the beeps stop, then lock and unlock with each fob and switch security back on. Where a valet switch is fitted, flicking it on and off quickly four times puts the system into learning.
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.
Disarm with the ignition inside 15 seconds
Wait for the cycle to end, open the door, key to position II at once. Fit a new A23 battery in the fob.
freeThree beeps: check every door, the boot and the bonnet
The system won't arm cleanly if one of them isn't seen as shut.
freeAfter a battery change: resync the fobs
Paddle lock then fob unlock; then five presses; then the full resync. Before bridging any wire, make sure it isn't the cigar-lighter lamp connector.
freeFind the module and, on older systems, the backup battery
On 1991-93 cars the module and its 6 V 0.5 Ah backup pack sit at the right rear of the boot near the aerial; dealer-fitted early alarms put them under the driver's seat. Late federal cars have no backup battery. With a backup pack, disconnecting the main battery may not silence it. On a 1989 convertible the unit can be in the storage box beside the hood relays; the sounder sits behind a front wheel-arch liner.
freeSiren won't stop: pull the security fuse
F11, 7.5 A, rear fuse panel on 1994-95 cars. Expect no crank until the start enable is restored.
freeReplace the module
Match the part number and market: the radio frequency differs between US and UK cars.
a used moduleThe 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.
We have no drawing of this box for this car — find them on the legend printed on the lid of the box itself.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS Central Locking Not Working, Door Won't OpenIf the doors won't lock or unlock with the fob.
- Jaguar XJS Won't Crank, Starter RelayIf the alarm is quiet but the car still won't crank.
- Jaguar XJS Battery DrainIf the battery keeps going flat and the alarm plays up each time.
- XJS fuse boxes and relaysWhere the security fuse sits; other years from the same page.
- 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.