Hello.
I have an annoying alarm fault on my mercedes c class. It randomly keeps going off and the fault that keeps coming up is a glass breakage sensor has a malfunction there is an open circuit.

The alarm is wired through to the heating elements in the rear 3 Windows which I can prove by sitting in the car, arming the alarm and then disconnect one of the wires. It sets off the alarm as if the glass were broken.
Apparently it is possible that this circuit is at fault but I don't see how.

What I would like to know is, if I disconnect the two wires on each window - can I bridge the plug with a simple piece of wire to complete the circuit and eliminate the window circuit like my attached picture to see if it stops the alarm going off or will I need to do something else ?

Thank you for helping

 

The alarm puts a very low level current through the heating elements - low enough not to cause heating. It may be that a dirty connection is causing a seemingly open circuit because of the low current used. Try cleaning all the contacts/connectors.

I doubt, though I could be wrong, that the alarm is looking for a specific resistance in the heater elements - unlike a house alarm which puts a certain resistance in circuit to determine if a sensor has been bypassed - so I'd try bypassing each window in turn with a short. Be careful though, because if you turn a heated window on it will potentially blow a fuse, unless you can separate the alarm wiring from the heater wiring but I suspect that's not the case as its all integrated. It may be possible that a suitable value resistor might pacify the alarm whilst not shorting out the heater circuit.

Ultimately the only long-term safe solution is to replace the dodgy window.

 

The alarm puts a very low level current through the heating elements - low enough not to cause heating. It may be that a dirty connection is causing a seemingly open circuit because of the low current used. Try cleaning all the contacts/connectors.

I doubt, though I could be wrong, that the alarm is looking for a specific resistance in the heater elements - unlike a house alarm which puts a certain resistance in circuit to determine if a sensor has been bypassed - so I'd try bypassing each window in turn with a short. Be careful though, because if you turn a heated window on it will potentially blow a fuse, unless you can separate the alarm wiring from the heater wiring but I suspect that's not the case as its all integrated. It may be possible that a suitable value resistor might pacify the alarm whilst not shorting out the heater circuit.

Ultimately the only long-term safe solution is to replace the dodgy window.

Yes I will do that. Thank you.

Well, I have bypassed all three windows now. I won't use the heating button as yes it may well blow fuses at the moment. If it stops going off then at least the problem is in one of the three rear windows or associated connections Will have to do a process of elimination.

 

Chances are the BCM is actually involved with checking the integrity of the heating elements and does so after the fuse. It's a common thing with other Daimler electrical systems and I suspect the SAM module code is related. I just know the two would be related if it was a Freightliner truck. If you want some extra safety you should be able to pull the fuse and still have everything functioning.

 

Well,
I started off replacing the battery in the siren module under the wheel arch. Looks like sometimes it goes flat. (That's not too say the module is duff)

Removing the siren module for week whilst ordering and doing so , the lights just flashed so that was better than the siren going off!
So, replaced the siren now and taken these windows out of the equation and so far in 48hrs no alarm has gone off.

I'll give it a few more days .
I'm not really bothered about having the alarm to be honest but trying to disable it at the fuse/Sam I discovered it's coupled with the central locking system. So that was out.
I can't find any way to disable it apart from unplugging the siren which is half a job!!
Not sure why but removing these window circuits might be a fix though.......

 

Actually I was wrong about the fuses. Pulling fuses would always give a missing fuse or open circuit code on the truck SAMs on the circuits it actually checked. It's been a while and it was a rare thing... memory confusion.

Normally on the trucks if it detected a fault it would disable whatever was faulted. If it's detecting a problem with the window before you set the alarm then it should just ignore that window until you fix it.

 

If the TS bypasses the heater element with a jumper wire then switching on the defroster will send power to the short circuit. That may be a problem.

 

If the TS bypasses the heater element with a jumper wire then switching on the defroster will send power to the short circuit. That may be a problem.

Yes. I won't use the defroster at the moment. It can't be a permanent fix but so far 3 days on the alarm has not sounded. So then something is wrong in the windows I think

 

It can't be a permanent fix but so far 3 days on the alarm has not sounded. So then something is wrong in the windows I think

Assuming the connectors are clean and bright then there's one or more micro-breaks in the heater element in one or more windows. Most commonly this will be the tailgate window, but rear secreens in sedans are subject to shocks from slamming trunk lids down. By shorting out one at a time over a couple of days it should become apparent which - unless your on-board diagnostics tell you which window is throwing an error code, but i didn't get that impresson from the photo you posted!

 

Did a quick search of the code "b1a6313" out of curiosity and found this... https://forums.mbclub.co.uk/threads/alarm-going-off-intermittantly.192500/

Noticed the siren code and thought I would pass the link along. It seems the siren itself can cause the window codes (at least on this year / model)

Edit:
And then I found this https://www.диагностика.бг/wp-content/uploads/2019/08/MERCEDES-C-220-CDI-2011.pdf which shows the window code coming from the rear SAM. Without wiring diagrams or knowing the details of the perspective cars it's a guess on our end.

 

Did a quick search of the code "b1a6313" out of curiosity and found this... https://forums.mbclub.co.uk/threads/alarm-going-off-intermittantly.192500/

Noticed the siren code and thought I would pass the link along. It seems the siren itself can cause the window codes (at least on this year / model)

Edit:
And then I found this https://www.диагностика.бг/wp-content/uploads/2019/08/MERCEDES-C-220-CDI-2011.pdf which shows the window code coming from the rear SAM. Without wiring diagrams or knowing the details of the perspective cars it's a guess on our end.

So it's been a few weeks now and no alarm.
I guess then something is wrong with a window circuit somehow.
Is there anything I can use on my multi meter on the two tails of each window circuit to determine the problem window without having to connect up each window one by one over the next month.. and wait for it to trigger at 3am .....

 

If the windows open and close the way they should, then the motor portion can still be checked for resistance across the connections, and also resistance to the car-body-ground. It may also reveal something by checking the other side of the connector, back into the wire harness. But check for voltages first, before checking for resistance across or to body ground.
I don't see how the window circuit could be a problem source, except that perhaps a "bad actor" could reach in a window with a long pole and operate a window switch to open a window far enough to allow reaching in and unlocking a door.
I really doubt that any openable windows have heaters for defrosting.

 

If the windows open and close the way they should, then the motor portion can still be checked for resistance across the connections, and also resistance to the car-body-ground. It may also reveal something by checking the other side of the connector, back into the wire harness. But check for voltages first, before checking for resistance across or to body ground.
I don't see how the window circuit could be a problem source, except that perhaps a "bad actor" could reach in a window with a long pole and operate a window switch to open a window far enough to allow reaching in and unlocking a door.
I really doubt that any openable windows have heaters for defrosting.

No it's not opening windows . It's the rear quarters and the back boot window I have disconnected and bridged with a piece of wire. Since then , no alarm has gone off

 

So it's been a few weeks now and no alarm.
I guess then something is wrong with a window circuit somehow.
Is there anything I can use on my multi meter on the two tails of each window circuit to determine the problem window without having to connect up each window one by one over the next month.. and wait for it to trigger at 3am .....

If it's just an intermittent problem (which it sounds like) the only way you are going to use a meter to find it is to sit in your car all night and watch it like a hawk. You can measure the resistance of your elements, but once again being an intermittent problem your meter won't pick it up any issues unless you do it at the right time. It could also be a problem in the harness itself that doesn't show itself with the connector shorted simply because there is not enough resistance in the system. Have you rechecked your codes?

Have you tried reconnecting everything since you replaced your siren? I reread the entire thread to see if I missed anything and that is one detail that was never mentioned. A communication issue like your code in the initial post with the siren could cause the issue. If it's not getting the right data it may be assuming a cut wire and going off. If it's always happening at the same time I would be even more suspect of a communication issue over the heater elements. The elements should be pretty random unless something is hitting your car at the same time every night and wiggling the break in whatever it is.

I know it may look like a window issue and very well could be, but I have also seen people get repeatedly burned by concentrating on the wrong code. Spend the $20 and sign up for a month of Mitchell's for your car. It will help a lot and give you some factory insight instead of random strangers trying to troubleshoot a system they can't see, have no experience with, and don't know exactly how it is supposed to work.

 

If it is the window heater circuit, the symptoms can be rather confusing. The resistance is quite low in order to have enough power to provide heat. So during warm weather it should be possible to substitute a resistor to see if the problem is indeed the window heater traces, or some other portion of the system. Probably a 4.7 ohm resistor would be a reasonable value to try. No high power rating needed because no defrosting required.