Alarm Does Not Reset on Starting Engine

[Paul6348]

Hi
Recently, several times, my alarm triggers when car is unlocked with the remote. Once started, the alarm cannot be stopped - even by starting the engine (which always worked with previous accidental alarm triggers). It is a bit embarrassing driving up the road with the alarm sounding! It does stop after about five minutes. Any thoughts on the cause and, more importantly, the solution would be most welcome.
The fault was not there today - so I'm hoping that it was a 'glitch' and not an intermittent fault. I would like to be prepared should it happen again.

[mike020150]

There are of course plenty of sensors associated with the alarm system and any one of them could be producing a false signal and setting off the alarm. It would probably be most cost effective to have the system scanned to give you the best clues. The task will require the Autologic Jaguar specific software or full Jaguar dealer diagnostics. The door latch microswitches are likely culprits since they are well known failure items. Mike.

[Paul6348]

Thanks Mike
Would a dodgy sensor account for the alarm not stopping when the engine is started?

[mike020150]

I admit that's an unusual combination of symptoms but the alarm and the vehicle disabling are separate facets of the anti theft system. Without testing the vehicle, it will always be impossible to distinguish between component errors (sensors or the siren) and wiring faults. The system uses voltage drops to sense unwanted entry to the car so battery (main car battery and the siren internal battery) faults can also cause alarm triggering often accompanied by other battery issues, like loss of window drop settings. These cars are simply too complicated, compared with their predecessors, to adopt the old diagnostic approach of progressively replacing components until you get to the faulty unit or, by a process of elimination, deciding it must be a wiring fault. In the hands of a good technician like David Marks or Ian Kelsall (XJK), paying for an hour's diagnostics becomes the sensible option. Mike.

[ibnct]

Paul,

A couple of days ago. I experienced a problem with my Alarm system sounding off for no apparent reason shortly after returning from a drive. There had been no strong winds, no heavy rain, or any other obvious reason for this, so I just reset it by unlocking and re-locking. The alarm went off again about 1/2 hr later, so I unlocked the car with a view to investigate further in the morning. Just as I was climbing into bed, off it went again even though the car was unlocked. I whipped my Dressing Gown on, grabbed the keys and tried to disarm it by pressing the lock/unlock buttons without success. By this time I could sense curtains twitching and eyes boring in all around me, so decided the best thing to do was to disconnect the battery, hand lock the car and investigate in the morning.

The following morning I removed the battery and placed it on charge, knowing that a weak battery is often the cause of many electrical faults. I decided to look at the security module to see if there was any obvious cause of the problem. It was then that I noticed an aerial cable screwed into and looping out from the security module, adjacent to where the battery is fitted. It looked to me like the battery had been pushed up against the cable putting a sharp right angled bend in the cable at the point the insulation reinforcement starts Thinking this could be the cause of my problem, I straightened out the cable into it's natural loop and replaced the battery, ensuring it was positioned as far forward in the boot as possible before clamping in place.

It's early days yet, but so far my Alarm has since, behaved itself

In view of my experience, I'd like to bring this to the attention of all XK8 owners (mine is a 2004 model). If yours is anything like mine there is the potential for the battery to damage the security module aerial cable if the battery is seated in its rear most position. - Could this be your problem

[Paul6348]

Thanks for the thought!
Since the day after I posted, the car has behaved perfectly. So, I will probably never know for certain what caused the problem.
I will certainly check the battery position and the aerial just in case - but I hesitate to do anything much in case the problem starts again.
I'll post again if I find anything.