Alarm going off during the night

I realise it might be a bit confusing when I’ve posted the same question on two different forums, but I do find I get response from a wider variety of people that way… but I’m also aware that some poor sod like me with a similar problem may do a search here some time in the future, so I’ll try to keep updating them both when anything happens.

So… I’ve been having problems with my battery (turned out to be corrosion on the terminals, so easily fixed). However, the boot was opened and closed a few times to get to the bonnet pull – and, last time when I closed the boot and switched the ignition on to go for a little blast, the boot-open warning LED on the dash came on.

I knew I hadn’t left the lid up. So I went round the back of the car, and watched through the rear screen as I pressed down harder on the boot lid… the light went off.

Now, just to recap a bit, you guys have told me lots about the switch, the wiring etc, and I’ve done my best to check them – opening up the cable where it attaches to the hinge, taking the switch apart etc. I am as sure as it’s possible to be that the cable is OK. Although it’s possible, as has been suggested, that the cable has a fault deeper inside the car, I take the view that that’s probably less likely than a problem within the boot somewhere.

There’s a lot of circumstantial evidence pointing at the boot switch. It’s a fairly simple thing, with a bit of metal on a spring making contact with another bit when the boot’s opened, switching on the dash led and triggering the alarm when armed.

I can see all sorts of ways in which the alarm could be triggered if it was a normally-closed connection – corroded switch contacts, broken wires etc. But to trigger a normally-open circuit is I think less easy.

So what I plan to do in the light of this latest evidence is to disconnect the boot-open switch wire completely – then, whatever the cause, if it’s in the boot it should be prevented.

Let’s see… will report back when anything happens…

The alarm shouldn't go off if the boot is opened, there is a wire that runs from the boot lock to the alarm control unit to recognise that the boot has been opened.

talkingcars wrote: The alarm shouldn't go off if the boot is opened, there is a wire that runs from the boot lock to the alarm control unit to recognise that the boot has been opened.

Aw, I'm totally confused now... I thought the point of the connection was to trigger the alarm? Why woudl the alarm want to know if the boot was open if not to sound?

- Confused of Carnmoney

So you can access the boot without disengaging the alarm.

The alarms main trigger is a voltage drop caused by opening a door or bonnet and switching on the light, thus one can open the boot but pulling the lever for the bonnet triggers the alarm as the under bonnet light is switched on. I believe in some models there is no under bonnet light but instead a switch that gives a path to earth from a circuit from the alarm ECU.

talkingcars wrote: So you can access the boot without disengaging the alarm.
The alarms main trigger is a voltage drop caused by opening a door or bonnet and switching on the light, thus one can open the boot but pulling the lever for the bonnet triggers the alarm as the under bonnet light is switched on. I believe in some models there is no under bonnet light but instead a switch that gives a path to earth from a circuit from the alarm ECU.

Cheers for that – so I’ve spent the last year agonising over something (he boot) which can’t even be causing the problem! Oh well, back to the drawing board… so really, it’s only the doors, bonnet, and volumetric sensor. Or some people say, the battery running down…

Well, I had the volumetric turned off via T4, so it’s not that; cleaned up and greased the contacts in the bonnet switch, so not that either.

I’m not going to start pulling the door locks apart, so will have to hope it was the battery problem or something…

Will reconnect the horns now the battery's working OK, and see if it happens again...

Alarm in the car reacts on voltage drop. That voltage drop is provided by putting wire onto ground/earth voltage. If wire is put onto ground - earthed, alarm will go off.

There are two switches in the boot. Boot lid switch and boot lock switch. If alarm is armed and you put your key in the key barrel and unlock your boot by key, boot lock switch will provide information to alarm that boot is opened with the key, assuming that is legitimate process and by that alarm will suspend its behaviour when boot lid switch voltage is dropped to ground by putting wire onto earth connection and will not triggere the allarm off when boot lid is opened.
However, if you open boot lid without putting key into boot lock then alarm assumes boot lid is opened by force and alarm will go off.

This means that if boot lid switch wire is earthed by coincidence or by error or by unwanted state, when alarm is armed alarm will go off.

Roverlike wrote: This means that if boot lid switch wire is earthed by coincidence or by error or by unwanted state, when alarm is armed alarm will go off.

Aaaaahhhh right. Finally it makes sense. You've probably tried to explain it to me before, but I just didn't get that there were two switches which interacted in that way!

So I just need to disconnect the right one...

You will be sure that you disconnected correct one if there is no light in the boot when you open it.

I know people (like me) often search through the topics when they have similar problems - so, if anybody's reading this years later, I have tentatively come to the conclusion after all the deliberation ove door switches, boot switches etc etc etc that the main culprit was two faulty LION batteries.

When I got the first one replaced under warranty, the alarm problem went away for a few weeks, then returned. A the same time, the car got harder to start when it had been sitting for a few days.

I got the battery replaced again - no problems since, and the car starts easily after even a week or two.

Thanks again for everybody's help.

Thanks for the update! Now all makes sense because I was intrigued with odd problem which was not possible to describe/diagnose in full by all we tried before.

Roverlike wrote: Thanks for the update! Now all makes sense because I was intrigued with odd problem which was not possible to describe/diagnose in full by all we tried before.

Yeah, exactly. Of course, knowing my luck, now that I've posted that, it'll go off at 5:00 tomorrow morning :rant:

Hogweed wrote:

Roverlike wrote: Thanks for the update! Now all makes sense because I was intrigued with odd problem which was not possible to describe/diagnose in full by all we tried before.

Yeah, exactly. Of course, knowing my luck, now that I've posted that, it'll go off at 5:00 tomorrow morning :rant:

Well, it hasn't (so far).

I think it's worth updating this old post because I now somebody else out there will be having the same hair-tearing problem... so it's now 18 months later, been through a winter etc, and still no recurrence of the problem. So if your alarm is going off for no apparent reason, maybe before tearing the doors and boot apart to replace switches, running diagnostics, and generally driving yourself insane, check your battery... and don't fit one from Lion