75ZT Community

Is there a way of testing this is working without smashing any glass or a trip to a local T4 (Testbook version 4.Computer Diagnostic System) Dealer
Didn't realise mine has it fitted until today

Yes. Lock the car with a window open. Wait for 30 seconds and then lean in and wave your arm around. If working the alarm will trigger.

This doesn't set it off.
So looks like its been disabled or not working. Everything looks connected up at the sensor end

Yes it seems that way. Do you have a multimeter? Check the supply and ground first. I'm not sure if the supply is only switched on when the alarm is armed. If these are OK, moniotr the output. From memory this sits at battery volts until
triggered by movement, and then is pulled to ground.

Which version is it? Is it the square one in the centre, or the rectangular one at the side? I've read that some of the later rectangular ones do give up the ghost.

Its the square one in the centre.
The kids have been left in the car and it locked when they were younger and I first got it.
I was paying for Petrol etc without the alarm going off and they didn't always stay in their car seats.
I remember the dealer saying something about the alarm going off all the time and them having to replace the battery.
Its an ex Mg Rover group car and was bought from a main dealer when 18 months old
So my money is on them disabling it on the options before me taking delivery.

MN190 wrote:The kids have been left in the car and it locked when they were younger and I first got it.
I was paying for Petrol etc without the alarm going off and they didn't always stay in their car seats.

The system to cover the interior won't be activated as long as there's any movement detected inside the car for a short period of time after locking it. So you'd have to sit very quiet for about 10 to 30 seconds (don't remember the exact value) after locking the car with the remote, then you can trigger the alarm.

Leaving a window open won't work unless it is a manual window. If an electric window is left open, the BCU (Body Control Unit) disables the volumetric sensor, but it won't for a manual window and warns in the handbook not to leave a manual window open and lock with the fob.

To test, sit inside the car, lock it with the fob button, wait 20 seconds and wave your arms about. The alarm should sound and the hazards flash. If you don't hear an alarm, but can hear a relay clicking, then the alarm horn has died.

I have recently retrofitted the square type (BMW part) sensor to my car, I'll do a How-To at some point.

Are you sure it knows the window is open? There are no sensors on the window motors, which is why there's no one touch up except on the drivers.

It's possible the BCU relies on the stall detection to decide this, but I doubt it.

Here's a quote from the Technical Brochure (1998): "The GM6 will not activate volumetric sensing if the vehicle is locked with any of the electric windows, or the sunroof open. It is not possible for the GM6 to recognise that a manual window left open and this, therefore, will not inhibit the activation of volumetric sensing".

Also my handbook, RCL 0306ENG 3rd edition (1999) says: "NOTE: Interior space protection cannot be activated if a door, window or sunroof is open, or if the starter switch is turned on".

Well, this could be wrong - the info is from early publications and it could be that the inhibition feature with widows open didn't actually make it into production. I'll have to check my car now to test this!

I'd agree about the stall detection, because if it used this then every time you lock the car with the fob the windows would all be driven (I think!) and this doesn't happen.

Interesting. The workshop manual says the same. It's the only car I know that does this, even more modern cars that know exactly where the windows are don't do this. Maybe Thatcham rules meant this isn't allowed any more.

You could use stall detection without activating every time. You would drive the windows upwards and monitor for stall current when the driver operates the switch upwards. Once you detect stall you would remember 'glass is up' in the BCU memory. The only issue would be if the window was almost, but not quite closed. The system would still turn of the interior monitoring.

Finally, where did you get hold of the technical brochure? They were quite rare as far as I know.

I got an original (unopened!) copy of eBay for a fiver. Money well spent I thought.

The same info is in the 'Body electrics' document on page 52, which I think can be found on this site somewhere. There is overlap between the technical brochure and the body electrics document, although the former is much larger and contains info on most aspects of the car.

Body Electrics doc.

xpdfdocuments/engineandtechnical/r75_body_electrics.pdf