2009 C-Class alarm fault

[FurtiveFreddy]

Friend has a 2009 C-Class estate and about a month ago the alarm starts going off intermittently.

He takes it to a dealer who charges him an hour of labour to diagnose the problem. I've seen the report and it claims they "chased the wiring to the rear and found fault within the rear sc sensor. Replacement rear screen required £786.60"

I've done some research for him and I assume this is a glass break sensor they are referring to.
I haven't found many owners reporting that issue. Most seem to have problems with the battery in the siren unit.

So what I'm wondering is firstly would that fault show up in memory if you plug a STAR diagnostic in and if so is the dealer just trying to justify charging an hour's labour for finding the fault?

Also, would the quick and cheap fix be to just unplug the glass break sensor or would that lead to the alarm going off all the time?

Any other suggestions other than paying for a new rear screen (even if he does go somewhere cheaper!)?

[FurtiveFreddy]

OK fair enough, that makes sense.

What would you do in his situation, considering he doesn't really want to shell out for a new rear window and that's the only option the dealer has given him.

[FurtiveFreddy]

Zulu 10 said:

The descriptions on the web suggest that the glass break sensor is simply a thin filament embedded in the glass of the screen - it seems that there's one in each rear side screen and one in the tailgate screen - similar to, but obviously thinner than, a heated rear window element.

I'd be tempted to find out where the connections are made to the glass and simply bridge them with a small resistor (100 Ohms or so).

Hopefully that will restore the sensor module's confidence that the glass hasn't been broken - though obviously a real break in won't trigger the alarm either.

If you struggle to identify where the connections and the module are then PM the VIN and I'll look at Starfinder.

Thing about this further, it's possible that the technies at the dealer simply bridged the connections to confirm their diagnosis, but liability issues probably preclude them from offering this as a 'fix'.

Thanks, that's helpful. I tried PM but doesn't seem to be enabled. If you want to PM me I can reply to that with the VIN?

[FurtiveFreddy]

Just in case anyone finds this thread and has the same problem, Zulu 10 was very helpful and told me which colour wires to look for at the rear of the car in order to diagnose and attempt to fix the problem.

In the end, there was a very simple way to fool the alarm into thinking the rear glass break circuit was intact, by removing the left rear access panel in the boot (where the back of the light cluster is) and spotting a black/brown wire (quite obvious).

I then connected a 100 ohm resistor between this wire and earth to simulate the glass break wire resistance and a week has now gone by without any false alarms, so I think I can be pretty sure that's sorted it.

Needless to say, if the rear glass was to now be smashed that won't in itself trigger the alarm but my friend is OK about that considering he's saved £700 and feels the risk is quite low. I would have though the movement sensor inside would pick that up anyway.