False Alarms

andyxstewart

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This is my first post on this forum so apologies if I haven't done everything quite right.

I have a 2009 Mercedes C200 estate, W204 I believe, which has developed a false alarm issue.

I've read lots of posts here and other forums regarding this issue, most of them pointing the finger of blame at the siren.

I know it's not the anti-tow system, I've had a false alarm even when that was disabled before leaving the car.

My local Mercedes dealer said they had never replaced a siren on one of these models however they do suffer from water ingress. Their advice was to check the connector for evidence of corrosion. I did and it was a clean as a whistle, I removed it anyway to eliminate it and to shut it up.

For those of you that are trying this, the siren is behind the wheel lining of the passenger side (sometimes called the left hand side or here in the UK it's referred to as the nearside) front wheel. The lining is in two halves, it's behind the half closest to the passenger door not the one closest to the bumper.

Removing the siren hasn't helped I'm still getting false alarms, no siren but flashing lights.

It looks like the problem may well be down to door/tailgate /bonnet switches or movement detector within the cab. Does anyone know where these are located?

Are any of these cheap fault code readers any use? My local dealer wants £96+vat just to plug my car into a diagnostic machine.

Any help appreciated.
 
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andyxstewart

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Update

Just thought I'd post progress update.

Any time the alarm has gone off now is when I've forgotten to disable the motion sensor before locking the car. I know it's hard to be sure with an intermittent fault like this but I'm leaning towards the motion sensor inside the car. Does anyone know where this is?
 

Chee

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In my W211 its in the rear light dome. Once you lock the car it flips open.
 
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andyxstewart

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Thanks

Thanks for replying Chee.

I've checked through the window when locking the car, nothing flips down.
 
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andyxstewart

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I thought I'd give a quick update on this issue whilst I'm logged in looking for something else. Never fixed this issue, OBD didn't find any problems and garage couldn't find anything either. Ended up removing the siren just to keep it quiet, alarm still goes off regularly for no apparent reason.
 

LostKiwi

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Find the motion detectors and give them a hoover out. Sometimes dust accumulations can affect the motion sensors.
 

television

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We have had cars where a spider has lived to a good old age in the car.
 
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andyxstewart

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Just another quick update. Turns out the false alarms were caused by the Window Loop sensor. Basically it's the rear windscreen and the two side windows, remember this is an estate car, anti break loop. Seemingly, the loop in the heated rear screen should be less that 20 ohms, mine was 45 ohms. I'm not replacing the rear window for that.
I've disconnected the wires on the rear screen and inserted an 18 ohm resistor between the wires. Will see if that cures the problem, haven't checked the two side windows yet.

Hopefully that may help someone else.
 

television

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Good luck and I hope that this will help.
 

bridgeman

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Just another quick update. Turns out the false alarms were caused by the Window Loop sensor. Basically it's the rear windscreen and the two side windows, remember this is an estate car, anti break loop. Seemingly, the loop in the heated rear screen should be less that 20 ohms, mine was 45 ohms. I'm not replacing the rear window for that.
I've disconnected the wires on the rear screen and inserted an 18 ohm resistor between the wires. Will see if that cures the problem, haven't checked the two side windows yet.

Hopefully that may help someone else.
Hi ive had the same fault on my c180 k sport , Mercedes have done a temp repair but told me I need a new rear screen ?? if fault returns I will try your fix how did you fix resistor was I length of cable with a resistor in it pluged into the two heated screen wires ???? thanks bridgeman
 
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