The alarm has started to go off on my car for no reason. It'll do it any time after I park the car and walk away. At present the only solution is to leave the car unlocked. How do I diagnose this problem?

I'm thinking of just disconnecting the alarm altogether. How can I do that without affecting any other function on the car?

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95 E320 Cabriolet, 159K

Disconnecting will cause the starter NOT to work..unless you bypass the starter interlock relay.

There are sensors on the doors, hood, trunk & inst cluster that could be causing the problem. The hood sensor is a common problem.

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Just my luck - the W129, W140, and W202 have a way to read the ATA diagnostic faults. They omitted this feature on my W124!

Unfortunately, I can't cause this hiccup on command. I have to wait until I'm asleep and have it wake me (and my neighbors) at 4am! That, or inspect every switch that is involved in the system.

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95 E320 Cabriolet, 159K

* Kestas, how about just disconnecting the alarm horn until you locate the problem? That won't keep the head lights from flashing or prevent draining the battery, but it might let everyone sleep.

Happened to my 89 300E
Just unhook the wire at alarm control unit, located underneath the carpet (Front passenger). Don't know if it apply to your car.

This was the solution the prior owner used on my 124.

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Thanks Robert. But I'm ahead of you. I disconnected the horn yesterday. My car is parked closer to the neighbors' bedrooms than my bedroom, so it's high priority I don't disturb my neighbors.

George, the alarm is easier to disconnect on your model. Like M.B.Doc said, the starter interlock relay must be jumpered if the alarm is disconnected on my model. I'll have to get the schematics out and find which wire needs to be jumpered.

The ATA diagnostic faults on other models actually point to which switch (or circuit) activated the alarm. On my model I have to guess.

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95 E320 Cabriolet, 159K

Terminal 1 and 3 have to be jumpered at K-38 in order to disconnect ATA
and still have starter function..

Thanks, Arthur. This saves me a lot of homework.

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95 E320 Cabriolet, 159K

Does it still do this, if you lock the door manually, and then close the door?

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09 Jetta TDI
1985 300D

I can central lock my car from the passenger door with the key, without putting the alarm on.

Why can't you ??????????

I thought one of the episodes was after locking via the passenger door. I'll have to check this out and verify.

Edit: On my car the anti-theft alarm is automatically armed 15 seconds after locking either door or trunk. There's a blinking indicator lamp in the dash that verifies my car is armed.

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95 E320 Cabriolet, 159K

does your model have a vacuum contorl unit with the PSE pump in it like the later w210 models ?

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Whether you think you can or cannot, Either way you're right!. by Henry Ford.

This I don't know. Sometimes, but not always, I hear a whirring noise coming from the right rear seat for about ten seconds when I hop in the car. Would this be a clue?

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95 E320 Cabriolet, 159K

yes,
if your battery is under the rear seat right side, chances are the PSE pump will be there too.
here's a thread explaining what happens when the part starts acting up. see if you can disconnect that and then see if the alarm goes bizzark .
besides this do a search on "pse pump" and you'll find more threads like the ones below.

http://www.peachparts.com/shopforum/showthread.php?t=177591&highlight=pse+pump

http://www.peachparts.com/shopforum/showthread.php?t=186706&highlight=pse+pump

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