Alarm siren sounding for no reason

[ChrisB]

I wonder if anyone has any ideas? My alarm siren is sounding continuously for no apparent reason. It first happened whilst driving down the road, and I unplugged it to silence. A week later, plugged it back in and all is well again.

Fast forward 2 years to last week and Disco is on the drive and alarm siren starts sounding. No indicators flashing or anything else to show the alarm has been activated, just the siren. No amount of locking or unlocking with the fob stopped it. I unplugged it again. Today tried to plug it back in again, but it sounded instantly, so it's currently unplugged. I've done lots of forum searching and can't find anyone describing a similar fault. The disco is a 98 300Tdi Auto (EDC).

Cheers

Chris

[Boydie]

Defective spider or your keyless entry is crook

[ChrisB]

Thanks Iain. Reading some posts on here, I thought that the EDC-equipped vehicles don't have a spider though? Is this correct? Not 100% sure what you mean by keyless entry, but the two-button fob works fine, and the vehicle starts and drives with no problem, even if the siren is sounding.

[teabag]

As you have a 'battery backed up sounder' BBUS which has a voltage drop circuit that's become faulty, its rare but not unknown,

although the issue may go away as easy as it came.

Also as unplugging the sounder silences it, that's an indication the battery within the sounder is faulty anyway as normally the sounder would sound.

So bottom line is u have an iffy sounder.

The spider immobiliser circuit has no association with the sounder, which you don't have as your engine is fitted with EDC .

[ChrisB]

Thanks teabag. Do you know if they are repairable? I'll also keep an eye out for a replacement.

[Fatboy]

Many moons ago, I had a random alarm problem on my '96 Discovery. It turned out that the plunger had got broken off the bonnet position sensor. IIRC, it was at the rear of the bonnet and was supposed to be depressed when the connect was shut. With a chunk broken off, I guess it was at the brink of making contact so occasionally would trigger the alarm.

Easy thing to check.

[teabag]

Thanks teabag. Do you know if they are repairable? I'll also keep an eye out for a replacement.

No really, the battery is replaceable but there's no diagnostics or circuit info for the PCB to be able to determine if there is an issue with it, so replacement is the best option although if second hand the battery will now be useless..

The battery in my unit only lasted four years from new so 15 years without this function, I've never bothered to replace it, although I'm in the security industry. The BBUS was only installed to meet the upgraded Thatcham security regs by LR for the 300 series and later on vehicle security.

Building security systems have a similar battery back and the same issues with battery's failing.

[ChrisB]

So I took the siren unit out and opened it up. The inside is potted with black silicone. The two NiCd cells were both leaking, one to the point where the whole cell fell apart in the middle when I was digging around in the silicone. The whole thing has now gone in the bin.

It would be nice to have some kind of sounder on the alarm. I can wire in something to the existing connector, but does anyone know the pinout?

[teabag]

There's no reason why u can't install a horn style alarm sounder.

I never found the pin-out info but the wiring to the existing sound could follow the simple format of a permanent supply to charge the battery, a switch live in an alarm condition and the ground connection, plus there could be an additional wire to provide the mis-lock sound a facility that u may loose.

All you need to do is see with a multimeter, what condition u have on each wire at the plug, and the same again with the system an alarm state to find the 12v you require to supply the replacement sounder in that condition. Simples really.