207CC alarm trigger

[MixerFistit]

Hi, I've got a hyperactive alarm and just wondering if there's any common cause.
Alarm started false alarming a few weeks ago the first time I thought I'd left a window open a crack, then the next time when we had extremely heavy rain and hail. There's also a slow drip on the top of passenger window if rain is very heavy so I initially assumed it was either the pounding or the drip setting it off and locked it with the key.

However, the alarm continues to set itself off regardless of weather. It also seems to occur within ten to fifteen minutes of me locking it.
I haven't noticed it sounding if I press the remote to cancel a currently sounding alarm (similarly if I unlock and let it auto lock). I also haven't noticed alarm sounding once it's silenced itself although this hasn't been tested much.
I've tried sun visor stowed in case that was causing it, there's no spiders or flies etc I'm aware of.
I've seen a few posts on other forums but they are mostly made up of people jumping in with "how did you solve this, mine has just started doing the same" etc

So just wondering if anyone else has a solution other than cancelling the ultrasonic sensors.

[GiveMeABreak]

Hi

So the prime suspect is the battery in the siren module. This has a service life and when it start to go off like this, it needs replacing. It's akin to there being a cut in the power supply - as if someone were trying to disarm it, the siren goes off. When it gets towards end of life it can't hold as much charge, triggering it going off randomly, but will get worse.

Item (10), Anti Theft Alarm Siren, part: 6680W7, £217.45

[MixerFistit]

Ah, excellent suggestion! Thanks for the diagram, I wonder how serviceable that unserviceable part is, I'm pretty good with a soldering iron if nothing else!
I haven't managed to get it to trigger if the ultrasonic sensors are off though and would think a bad battery would still alarm but it's certainly a great place to start. I suppose the ultrasonic sensors are the most energy intensive part of the alarm circuit except for the siren itself and probably rely on a good power source for a stable pressure reading.
Thanks again

[ozvtr]

The siren module (#10) is self contained and has a battery in it (as Marc said). That battery only powers the siren.
The siren module is glued together (not screwed), so you will need to break it open some how if you want to change the battery!
If the main battery is disconnected while the alarm is active (the car locked) the siren will go off. If you disconnect the siren module while the car is locked, the siren will go off!
The alarm module (#1) is powered from the car battery.
The bonnet switch (#18) detects if the bonnet is open. The bonnet switch is only connected to the alarm module. The alarm relays the bonnet status to the BSI and the BSI relays the status to the central display. Typically, if your car doesn't have an alarm, there is no bonnet status!! FYI the bonnet switch on my C2 was not clipped in correctly and caused random alarm triggers. I don't think that's the problem your car is having.
The BSI relays the door and boot status to the alarm.
Opening a door with the key should set the alarm off and starting the car should stop the alarm. This bypasses the remote locking de-activation (as a test).
Marc is probably right, BUT if switching off the volumetric (ultrasonic) sensors stops random triggering you might tend to suspect the sensors.

[MixerFistit]

Thanks Ozvtr. I haven't witnessed an alarm when sensors are deactivated so it's possible but not conclusive.
I don't have a bonnet sensor as far as I can tell. My C8 has (a problematic) one near the bonnet catch which I unplugged but nothing on this one.

I'm fine with cutting, epoxy to reseal and solder if necessary but would avoid that if it's unnecessary.

With faulty sensors is it simply a case of swapping out the actual UsS. I'm assuming they have connectors on the end of tails if they're like my c8. I could unplug one at a time and maybe locate a faulty sensor that way. If no luck I'll get the dremel out lol

[MixerFistit]

Ah, I just found the very sneaky bonnet switch hiding in the catch mechanism, not like the c8 one at all. Gonna focus on that first as I've had the front recently disturbed when I was changing the turbo's oil feed pipe

[ozvtr]

With faulty sensors is it simply a case of swapping out the actual UsS. I'm assuming they have connectors on the end of tails if they're like my c8. I could unplug one at a time and maybe locate a faulty sensor that way.

There are two sensor on one loom which plugs directly into the alarm module. I'm not sure, but I think that one sensor is the transmitter and the other is the receiver. The loom runs up one 'A' pillar, across the top of the wind screen to the other 'A' pillar.
I have not tried it, but LEXIA might be able to tell you what is setting the alarm off? I know that the "volumetric sensor off" light will flash to tell you how many times the alarm has been set off.

[ozvtr]

I know that the C4 has the bonnet switch attached to the radiator support, I guess your C8 is the same. My C2 has the switch attached to the bonnet catch apparently the same as your 207.

[MixerFistit]

Looks like it's solved.
I've disconnected the bonnet sensor and had no alarms since.
Not sure which way the switch works as haven't tested yet but I've put a jumper wire in the connector and had no problems. Probably won't bother with replacing or repairing the switch

*Edit*
For anyone following this for whatever reason, the jumper wire is unnecessary as closing the circuit is what sets the alarm off rather than opening (removing jumper). In fact, opening the circuit after locking coincided with a further single slow blink of the indicators as if to show that, that part of the car is also now primed. Presumably when the jumper was left in, it still allowed locking but ignored the engine bay as if bonnet was already open. The faulty switch itself must therefore have been in some sort of unstable state where the alarm module can't decide if it's open or closed.
My guess is moisture or bad positioning allowing current to pass when the switch should be open, just enough to let the signal go above and below the threshold

[solcutter]

I'll have to try this I've just purchased a 2015 308 when I try to arm it the alarm goes off but not sure it's a sensor as I've also read there is some potential fault with corrupt bsi data from voltage changes like a jump start or similar