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1982 Mercedes Benz 240D
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156 Posts
Discussion Starter #1
The only key I received when I purchased my car was the valet key but I dont think this would cause the issue im to describe.

When I lock my car by turning the key a full turn clockwise the doors lock, including the trunk and gas cap, and the alarm starts to sound. I then have to turn the key back CCW about 1/8 of a turn and the alarm stops but the vehicle will still start so I am assumeing that small bit disables the alarm.

How do I go about troubleshooting this?
 

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'91 C124 300CE, '06 W164 ML500, '00 BMW MCOUPE, '65 COBRA REPL.
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547 Posts
Not sure about the valet key deal, but your alarm probably like 99% of all other alarms, has issues.
The most common culprit are the door, trunk and hood sensors. For some reason, they go bad often.
The factory alarm is useless anyway. You can locate the unit under the passenger's carpet. Remove the floor mat (if any), then the carpet. The carpet is held in by 2 buttons. Just unsnap and remove it. Then you will see this:





You can disconnect it, or try and figure out what the problem is.
The horn, should be right in front of your brake booster or on the dr. side fender visible from the engine bay. Not sure how it is on your model.

Also the alarm has a connection to the back of your stereo. If the radio is removed the alarm will go off.
HTH
 

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1982 Mercedes Benz 240D
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156 Posts
Discussion Starter #3
First off thank you for your reply.

I didnt even think about it but I had to do a full rewiring job on the radio harness cause the previous owner spent 0 effort on the task. That makes sence now as to what the left over wires are for behind the radio.

That being said is there a way to jump these wires or something to send a false positive signal back to the alarm that the radio is still in the car?
 

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'91 C124 300CE, '06 W164 ML500, '00 BMW MCOUPE, '65 COBRA REPL.
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547 Posts
Not sure about the false signal, but I believe that the wire to the radio was a ground.

There is a brown with blue wire that tells the alarm system that your radio hasn't been stolen, I think this has to be grounded.

Try grounding it to any metal surface and see what you get.
Maybe others will chime in that have actually done it.
The PO of my car had an aftermarket stereo installed and not sure what he did, but my alarm is inoperative.
 

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1982 Mercedes Benz 240D
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156 Posts
Discussion Starter #5
Incase this ends up being something different is there a way to test the door modules you refered too?
 

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'91 C124 300CE, '06 W164 ML500, '00 BMW MCOUPE, '65 COBRA REPL.
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547 Posts
Sorry,
I have not gone that far into diagnosing mine yet.
Hope others chime in to help.
What I do know is that the switches are just switches. Open and close. With an volt tester you might be able to test them.
 

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1982 Mercedes Benz 240D
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156 Posts
Discussion Starter #7
That would make sence. if is just a simple series circuit then a break in one would signal the alarm. If that indeed is the case I could just use jumper wires to test.

Will have to wait for confirmation. Thanks again for your help. :)
 
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