Hello everyone.

Here are some of my thoughts (and some questions) on the built-in alarm.

As this is not another useless general thread of no meaning where everyone can post their opinions without actually knowing anything important - like what oil in engine, what rubber on wiper, how to polish chrome bumpers etc I don't expect answers but still I thought you might want to share my findings with me.

Right. The 124 300D 2.5 Turbo has a built-in alarm. There is a switch with 3 cables in the door right at where the key goes in that activates the alarm. When the key is turned clockwise the central locking locks the doors and the alarms gets armed. When the key is turned anticlockwise, the switch disarms the alarm and central locking unlocks the doors. When locked with a key, when one opens the door the alarm activates and sounds horn and flashes lights. So far so good. Also, this is an important feature - when the key is held for more than 3 seconds, all windows and roof will close.

Now here is the problem - an external alarm. I have an external alarm fitted in the car. It is by far better than the built-in alarm. Apart from the locking function to electric circutry it also allows me to remote lock and unlock the car which is an advantage.

The trouble stems from interaction between these 2 alarms. You can make the new alarm remotely un/lock the doors and arm the car. There are no arguments between those 2 systems because you don't lock the doors with a key - thus the built-in alarm does not arm itself. It is like the car had never been locked for the built-in alarm.

Problems start to arise when you want to use the very helpful feature of having your windows and roof closed when you remotely lock the car. Then the new alarm also operates the 3 wires that run to the door lock - when remotely locked it holds the wires connected for 12 seconds for all windows to close.
But with that the built-in alarm also gets armed.

When unlocking, it holds the wires also to give the built-in alarm feeling the car was unlocked with a key so it disarms.

The real problem is this : it doesn't work. Every now and then (sometimes yes and sometimes no) when remotely locked the built-in alarm goes off for no apparent reason. Also when unlocking and opening the door the alarm goes off. I have tried various ways to overcome this but at no avail.

The situation is like this - you can have an external alarm fitted and it will work perfectly. However, when you want it to also close windows the built-in alarm will go off.

How am I to overcome this? I tried to switch the built-in alarm off. I thought unplugging the plug underneath the passenger floormat (right to the red airbag plug) will help. I unplugged it but the built-in alarm goes off still. So, doing this does not turn the alarm off.

Is there a way to completely switch the alarm off?

Also I am afraid that turning it off will render the window-closing feature unoperational.

I thought that simulating the turning of the key with the external alarm would work - it does the same as a real key does - closes 2 wires to let the system know the car has been locked. But it does not. I am afraid the problem may lay in that there is a time gap counted in miliseconds between physical locking with a key and when the central locking starts working.

It means that when I lock the car with a key then I turn on the switch at the key in the door a few miliseconds earlier than the central locking unit gets a signal to lock the doors. This is the way it is supposed to work.

When I lock the doors with remote (and try to close any windows open), exactly the opposite happens - the remote unit on alarm gets activated by the central locking unit first and THEN it closes the wires. It is only a matter of a fraction of a second difference but I think this is what causes the alarm go off every now and then. Sometimes it locks and closes perfectly, maybe depending on what singnal was faster that moment.

Anyhow, my question is following - does someone know how to turn the built-in alarm completely off? As if it wasn't there.

Would there be a small gadget on market that could delay electric signals for a few miliseconds?

I would have given the car to a MB shop to have it solved but they have no idea also. They said to bring the car in and they would try. So they would keep fiddling with it the same way I did, trial and error. I don't feel like paying several hours of work on something with no clear result.