After its 2 month winter holiday on axle stands, my 147 alarm is playing up intermittently. The other day it was OK for almost 24hrs. Yesterday it went off twice so I locked the car on the key. This morning it it went off within 2 minutes of locking the car on the fob. Then again a few minutes later. Tedious. Every time, the reported cause was 5 flashes = tilt/movement. At least today it went off whilst I was watching it and was able to see there wasn't a passing vehicle or gust of wind, so it looks like an electronics issue.
There seem to be a lot of threads with similar, and a lot of uncertainty about where the tilt sensor is, and frustratingly none reach any resolution. Some say it's in the roof module along with the ultrasonic transceivers, others that it's in the main alarm/siren unit under the passenger wing, and others that it's a sensor unit bolted to the front suspension.
Since it's dead easy to pry the ultrasonic panel from the roof, I started there. I pulled the unit from its housing and removed the PCB. There is what looks like a single miniature non-mercury tilt switch. It's the little metal can about 10mm x 4mm dia, with 2 solder connections and siliconed to the board.
No matter how I positioned this, it was open circuit when tested with a meter, which of course a tilt switch shouldn't be.
As a test I pulled it away from the silicone and twisted it so that the leadout wires shorted each other, which meant it now is 0R regardless of position and stuck it back in the car. The alarm armed OK, now with a second beep that signals a system check failure. But I have had no further false alarms.
However, the existence of a check failure with the sensor shorted says to me that the system expects a resistance value xR (ie not ~0, with the sensor shorted as it is now) which it then takes as baseline. If R varies sufficiently, the system knows that tilt has occurred and the alarm goes off. I think it just can't cope with open circuit at all. That doesn't give an error beep, but the system has no reference R from which to determine tilt. Maybe any voltage change, eg due to temperature, will set off the alarm in that condition.
I am sure the sensor isn't a simple on/off switch, but gives a range of resistance values depending on its attitude. Of course I don't know the range. If I was more patient, I'd try substituting different values of resistor for the tilt sensor, and find out what the system doesn't think is an error, or patch in a variable resistor to figure exactly what is needed. But where's the fun in that? I've ordered one of these, which varies between 5R and 10M, and physically fits. http://www.ebay.co.uk/itm/121743222769
I'll follow up and say whether it worked or not. If it doesn't, I'll try different resistors to see what the system accepts as a non-fault, and forget about having a tilt sensor.