Volumetric ultrasonic alarm triggering

[myglaren]

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.

That is exactly how they worked on my Civic, the previous Mazda 6 and the current one.
I only knew the Civic was alarmed when I left the windows open and the alarm triggered a bit later from a breeze.
Same with the previous 6. The current one I had left a rear window open and reached in for my camera from the rear seat and set it off.

A neighbour had the same thing with her BMW/Audi?whatever SUV.

Can't remember how it was on the C5s but the windows were very often open.

[wheeler]

My understanding is that one is a little speaker & the other is a little microphone, The speaker sends out a little (inaudible) sound wave & the microphone constantly monitors it, if the sound wave is disturbed that it triggers the alarm. If you cover the speaker & microphone then it wont detect any changes?
As i said i often use this method to be able to move around inside the car when the alarm is armed to stop setting the alarm off & it works for me.

[rl11]

I've found three that all look the same - like a teeny speaker in a plastic surround. Possibly these are all the "speakers" and there is a "microphone" somewhere else that needs covering - for test purposes

[xantia_v6]

There will be one transmitter and one or more receivers. The receiver hears the sound from the transmitter as well as echoes from hard surfaces (windows). The ECU is looking for changes in the pattern of echoes due to movement of an object or something obscuring a reflective surface.

[rl11]

Wife just back from shopping; didn't go off at shops, going off at home again. I guess there is no way of knowing if it's the ECU or transmitter or receiver that's faulty? Or would an automotive electrician be able to figure out?

[xantia_v6]

The computer records a log of events that triggered the alarm, but it probably won't help much in this case.

The cause is most probably the transmitting sensor, but it is probably best to replace them as a set, if not too expensive.

[wheeler]

Wife just back from shopping; didn't go off at shops, going off at home again. I guess there is no way of knowing if it's the ECU or transmitter or receiver that's faulty? Or would an automotive electrician be able to figure out?

With something so intermittent i would imagine they would just try unplugging the sensors & see if the fault goes away.
I would try to locate the alarm ECU, will probably look something like this, the brown connector is the sensor connection.
If it the fault goes away either replace or leave unplugged if you are not bothered about the alarm.
Looks like the sensors are a full harness & i bet they are quite expensive.

[rl11]

Put up with disabling internal sensors. Then it became I could only lock with key or alarm would go off. Then had problems starting!
Finally went to Citroën dealer and they said my battery was knackered and the battery management unit was faulty. Both now replaced and I no longer have a problem with car alarm. So looks like it wasn't a problem with the alarm eco or alarm rechargeable battery after all.