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Alarm (developed a life of its own)

Gimme Shelter

Gimme Shelter

PCGB Member
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So it's finally happened: the alarm on my 996 TT has started going off all by itself. I recall an item a few years ago saying it's the tilt sensor that lets water in ... I've had a look at it, and indeed, the orange connector's contacts have corroded, so it looks like it's been rather wet. Richard, your excellent Ultimate User's Guide tells me that later cars had the sensor under a seat. Can I update, or do I simply have to replace like-for-like? I've tried to access Alarm Issues on this forum, but for some reason can't.
Many thanks
 
Ideally it would be best to get a diagnostic check done to confirm it is the tilt sensor which is setting it off, but it does seem the likely culprit if is filled with water and the connectors are corroded. Disconnecting it completely should prove the point. You'll probably get a beep on locking/unlocking, but that's all. If it isn't the tilt sensor causing the false alarms, the next most likely culprit is the interior monitoring sensor.

They don't make the old tilt sensor, so you have to update it with a new sensor under the passenger seat. There is a Technical Bulletin covering this, and I can send you a copy if you PM me your email address.
 
As ever, the plot thickens. This morning, I disconnected the orange connector, ie the tilt sensor. For a few hours, all was well, and I thought that was it. Then this afternoon there were two alarms - the first one an undulating siren; the other, maybe an hour later, a rapidly-repeating chirp. So now I've disconnected the siren as well and taken the unit out of the car.
For interest's sake, the part numbers are:
Tilt sensor: 996.618.265.00
Siren & battery unit (Hamilton & Palmer): 996.618.237.01
and the metal bracket has two numbers on it: 996.637.219.01 and 996.637.419.01. It is also marked with the Hamilton & Palmer logo.
I've tried to open up the tilt sensor, but it's very well sealed (it appears glued/bonded); and in any case, I now suspect it's not the culprit. I've opened up the siren & battery unit, and inside there is not only the siren and battery, but also a printed circuit board with two chips and a handful of other components - capacitors, etc, i.e. it looks like this is the full alarm circuitry.
Is it OK to leave it off the car for now? As far as I can tell, the car starts and runs normally without it connected.
Richard, I'll PM you with my email address and take you up on your offer of the tech bulletin.
 
Yes, it is fine to leave the siren off too. The circuitry is independent of the main alarm module under the passenger seat. You will still have the alarm horn connected (the horn on the other side of the battery) so will still hear a (rather pathetic) beep-beep if it goes off.

The alarms are quite sensitive to battery voltage - is the battery in good shape?
 
As far as I know, the battery is in good shape. I bought it in 2015 and use a C-Tek trickle charger as a matter of course. (I've checked: the alarm sounds independent of whether the trickle charger is connected or not.)
Incidentally, I've tried disarming the interior sensor by locking the car using a double-plip. The alarm still goes off at random, so I presume it's not the interior sensor.
Your comment about the alarm horn prompts a thought: during all these random alarms, the siren sounds, but the alarm horn doesn't. It does make me wonder if it's the siren & battery unit.
 
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