Volumetric ultrasonic alarm triggering

[rl11]

I have a 2015 Diesel 2.0 DS5 (DS). Recently the car alarm has regularly started going off whenever it is locked - via key fob or the one remaining working keyless handle. If I press the internal alarm button, so the internal alarm is disabled (red light on button), the alarm does not go off. Therefore it appears to be a fault with the volumetric/ultrasonic sensors? Are there only the two sensors in the front pillars? I have tried blasting them with a high speed air fan, to see if that cleared any dust or insects etc - didn't work.
1. Is it likely the internal sensors are at fault? If so, how easy to replace and where can I get them?
2. I have seen mention of an alarm battery maybe not holding charge? If so, where is it, how easy to replace and where can I get one?
3. I have seen mention of the actual car battery being at fault. Not sure how old mine is but I have no electronic issues other than this one but FYI it doesn't hold enough charge for ECO stop/start to work unless I take the car on a long run. After a long run it works for a week or so.
4. Is it possible to permanently disable the internal sensors? I live in an area where break-ins are very rare.
Thanks

[xantia_v6]

It is typical for the ultrasonic sensors to fail with age, particularly the one that is acting as an ultrasonic transmitter.

Personally I would just unplug the sensors and forget about them.

[Rp0thejester]

My C8 used to do that due to the pull down 'keep an eye on the kids' mirror if I didn't put it back up. All I can suggest is you keep those sensors clear of anything nearby.

[LeRoy KinCaid]

I remember my 406 Coupé started doing this and I saw somewhere that a light spray of hairspray onto each sensor eye would help, didn't believe it but tried anyway and amazingly it seemed to work, apparently the sensors become too sensitive or something with age and the light film of hairspray mitigates this, the sensors still worked after the hairspray as the alarm would still trigger when the dash cam would fall from the windscreen in hot weather.

[andyandjulie]

I'd heard that one too LeeRoy, good comment.

[rl11]

I tried charging the car battery - was at 60% but now 100% - didn't work.
Tried the hairspray option on the front two sensors - didn't work.
Found a 3rd sensor on the back pillar near the boot on the passenger side. Fine spider web on this, so thought that was it. Blasted clean with high power air blower. Alarm still went off. Tried hairspray on that sensor - didn't work.
Covered each one in turn with black electrical tape - still going off with all three covered!
Only way to stop it is to press the button that disables the internal sensors.
Presumably there will be some kind of error code being recorded? Does anyone know of a reader that I can buy that will tell me the exact problem? - or will it just be a generic error that says volumetric sensor was triggered?
I also looked at unplugging the sensors but the wires disappear into the pillars. I could cut them but as covering with tape didn't work, I'm not sure that cutting the wires will work either.

[PaulC5]

This topic might help you locate the alarm parts viewtopic.php?t=70642

On our 2007 C5 the alarm module was in the left front pillar at the glove box level but from the link the one in your car will not be there. I had to remove the glove box to get to it and even then it was not easy to remove. I ran it for a while with the sensors disconnected and also for a while with the siren disconnected (located in the rear left wing) but if I disconnected the module the indicators flashed continually when the engine was running. I was doing this to see if the alarm was causing the main battery to go flat, not due to any alarm faults.

[rl11]

Is the battery inside the alarm module ECU (9665582380)? I can only find used ones on eBay, so they are going to be oldish and may not be much better than mine?

[PaulC5]

On our C5 the batteries were in the alarm siren but I could not open it to see them. The module was just a plastic box with a circuit board inside and looks like ones found with a search using the part number you have posted. If it is replaced I think you have to programme it to say what type of seat coverings it has and possibly another couple of things so if you did get a used one it might need programming. However, if your problem is a faulty sensor then replacement of the module might not help so you might be best to initially find where it is and unplug the sensors.

[Rp0thejester]

I don't see how putting tape over the sensors will benefit you. If there volumetric sensors then it'll keep going off. Anything can set them off, a mirror, fluffy dice, a child's toy on the back of a seat or hung up somewhere. Iirc not even a bsi reset can stop it. Either disconnect completely or find what's setting it off.

[rl11]

@Rp0thejester - So a volumetric sensor will trigger, if something moves in the car, even if the sensors are covered over with tape? I was covering them up expecting that this would eliminate all possibilities that movement was taking place, as they couldn't detect anything past the tape. Would covering with metal stop them detecting? e.g. a coin - or do they just not work like that?

[wheeler]

@Rp0thejester - So a volumetric sensor will trigger, if something moves in the car, even if the sensors are covered over with tape? I was covering them up expecting that this would eliminate all possibilities that movement was taking place, as they couldn't detect anything past the tape. Would covering with metal stop them detecting? e.g. a coin - or do they just not work like that?

I would say that is correct, covering the interior sensors should stop it detecting movement however if the sensors are faulty it may still trigger the alarm.
I often use this method where i want to let the BSI go to sleep to do for example a current draw test but i need to leave a door or window open, temporary covering the sensors with tape stops the alarm triggering if you need to move around inside the car without triggering the alarm.

Not sure where the alarm ECU is located on this model bu the normally have a smaller brown plug this is the sensors.

[rl11]

Went out to look for any more volumetric sensors (found 3 so far) and to do a bit more testing, to see if it consistently triggers after 45 secs (found manual says it sets after 45 secs) and it's not triggering today! Was going off every time I locked it yesterday (after a short while). Not driven anywhere yet so whether something has settled down overnight?

[xantia_v6]

My understanding of the main failure mode is that the transmitter (one of the sensors) starts to rattle because the peizo crystal becomes loose in its mounting or perhaps starts to fracture. The ultrasonic rattle is a bit random and the computer detects ghost objects flying around in the car.

[Rp0thejester]

My sensors are fitted above the rear view mirror, having the optional child mirror down would set the sensors off as it was six inches in front of the sensors. Volumetric as I understand it aren't motion sensors, if they were every gust of wind would set them off. There used to sense when something/someone is their when they shouldn't be, hence leaving the convertible top down but still locking the car.