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Matt
Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 20583
Experience:  BEng hons Mech engineering, in auto industry 22 years
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MERCEDES C250 CLASSIC TD AUTO. The alarm keeps going off and

Customer Question

MERCEDES C250 CLASSIC TD AUTO. The alarm keeps going off and disturbing neighbours. I only have the ignition key which also locks the doors. (no conventional door keys given me only ignition) I have left it unlocked for now and it is quiet but as soon as I lock it with the ignition key fob (?immobiliser) I expect it will go off. I have heard the alarm circuit has some little batteries. The car is old 1998 and the user manual long gone!
Please help
Submitted: 2 years ago.
Category: Mercedes
Expert:  Matt replied 2 years ago.
Hello

whats likely to have happened here is the alarm battery insider the siren has failed

they tend to last about 10-12 years and are kept charged up by the main battery on teh car but as they age and cannot hold a charge anymore the alarm gets into a fault state and falsely thinks its being stolen

usually a new siren with a new battery solves the issue
Customer: replied 2 years ago.

whats likely to have happened here is the alarm battery insider the siren has failed


 


usually a new siren with a new battery solves the issue


 


OK so


1. how do I firstly locate the siren and battery to endeavor to remove it?


2.Having located it can I just disconnect the cables leading to the siren or will this lead to further problems such as being unable to start the car?


3. The car is old so less likely that anyone would bother to steal it. I dont want to spend too much money on it. Should I take it to a mercedes agent and get a star diagnostic run on it before proceeding?


4. If I find the old siren and put in a new rechargeable battery ( assuming it has not corroded the circuit board) would this be a low cost solution?

Expert:  Matt replied 2 years ago.
Hello John

the siren is tucked away in the wheel arch

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I think you can just unplug the siren with no alarm issues but don't expect the alarm to work / immobilse the car

although you should be able to lock it


and yes it should be possible to rebuild the siren with a new battery
Customer: replied 2 years ago.

Thanks for info so far.


This car is an estate.


1. I fired up the alarm and the location of the siren appears to be on passengers side (Left hand side) but in front of the front wheel tyre not behind it.


2. There also appears to be an armoured cable going down from rear of front left hand lamp cluster towards this location. Would this be the cable that fires up the alarm?


3. so to confirm is the access to the siren from the outside of the car by removing the plastic splash guard in front of the near side tyre?


 


Many Thanks


 

Expert:  Matt replied 2 years ago.
Hello John


Yes you access the alarm by removing the wheel and then the wheel arch liner

I'm not sure about the cable but quite possible as I can see how you'd want to protect it to revent a quick disconnect by a theif

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