1996 W202 Alarm - how do you stop it?

[rooted]

Hi, I have a 1996 W202 C180 Estate. I'm trying to disconnect the battery so I can replace a failed alternator. However, when I take off the battery earth (I mean the main 12V battery in the boot), after about five seconds the alarm starts sounding. Replace the earth and it stops immediately. If I disconnect the alarm, it continues sounding - whether the main battery earth is connected or not.

The alarm part number is Q3 50 06364, which from looking at other threads was a dealer fitted version. Is there any way I can stop the alarm going off, so I can disconnect the main battery and carry on with replacing the alternator?

I don't need to retain the alarm so I'd be happy to ditch it if that solves the problem. An auto electrician I spoke to said the alarm will continue to sound until its back-up battery runs out. Is that correct? How long would that take? The alarm is pretty loud and I'm not endearing myself to the neighbours!

Any advice would be much appreciated

 

[Dec]

Disconnecting the car battery will trigger the alarm, the alarm sees this as tampering with the power supply.
Try unplugging the siren.
Back up batteries could last maybe several hours, unknown really.

Dec

 

[rooted]

Is there a quick way to deactivate the siren? The auto electrician suggested putting the alarm unit (unplugged of course) in a bucket of water! Presumably so it would short out and fail or the sound would at least be muffled. Seems a bit extreme, but desperation is beginning to set in!

 

[Dec]

I forgot about the backup batteries, could you put it in the fridge… inside a plastic bag, don’t damage it just in case the car wont start without it.

Dec
edit; the brains of the alarm are probably inside the siren if there are more than 2 wires going into the siren.

 

[rooted]

Thanks - good point! I suppose I could disconnect the alarm/siren unit, cover it with blankets to muffle the sound temporarily and see if the car starts without it.

If the car starts then I guess I can go ahead and do something terminal (no pun intended) to the alarm to shut it up!

 

[Dec]

Don’t shut the boot while the battery is disconnected or make sure you can open the boot with a key blade when you disconnect the battery.

Dec

 

[grober]

Simplest thing is to unplug the alarm wrap it in some sound deadening material and stick it in a compartment/box cupboard /shed where the sound is deadened. There is usually a key switch at the rear of the alarm to allow you to de-activate it. --- if you have the key. The alarm is just that an alarm with a simple circuit board and backup battery- most of the components can readily be replaced other than the circuit board if damaged by a corroding backup battery as the alarm can be stripped down.

MB Security System...help please - Page 4 - Mercedes-Benz Owners' Forums

 

[rooted]

Thanks. I'll leave the boot (tailgate) open while the battery's disconnected. I don't have the key for the alarm unfortunately. Some more experimenting tomorrow I think... after the neighbours have finished their lie-ins.

 

[rooted]

A postscript, should anyone be interested. Disconnected the alarm (it will activate, whether the key is in the ignition or not), wrapped it in many, many towels and blankets to mute its banshee wailing and checked the engine started without the alarm in circuit. It did. It was then a simple task to replace the alternator (the original intention). Out of interest, the alarm's backup battery kept it sounding for 18 hours after removal from the car. Quite impressed with that.