Adding Motion Sensor to BMW Alarm

Here is what I did – it is fairly straight forward but please make modifications at your own risk.

The sensor I used came from Sound Conceptions – “www.autotoys.com”. The sensor is listed as “sensor: single zone perimeter sensor (radar)” and is $24.95 as of today (3/30/98). You can find it in the storefront – security section. It has three wires to hook it up – battery, ground and trigger. The wires are just long enough to reach where I mounted the sensor. If you want to experiment with different mounting locations, you may want to extend the wires.

For safety, unplug the BMW alarm harness at both ends before making any connections. I soldered and taped all connections.

Battery

connect this to the fused battery wire in the alarm wiring harness this is a yellow wire with an inline fuse. Connect to the alarm side of the fuse, thus the fuse protects the new wiring.

Ground

connect this to the ground wire in the harness – brown wire.

Trigger

this will get connected to the hood switch sensor wire (white with red stripe) in the wiring harness. To avoid potential interference between the new and old sensor – I isolated them with two diodes. The diodes are 1N4001 which you can get at Radio Shack. Cut the existing sensor wire and splice in a diode with the cathode (banded end) toward the sensor – this sensor is active low. On the alarm side of this diode, add another with the banded side toward the new sensor – connect the new sensor trigger wire to the cathode of this diode. See the crude diagram below:

I mounted the sensor in the console just forward of the gear shift. Remove the gear shift boot and the foam insert. If you put just the hook portion of some stick on velcro tape on the back of the sensor, you can stick it to the carpeting under the console. You will want to play with the sensitivity adjustment on the sensor. I have mine currently set about 3/4 of the way to fully sensitive.

Performance is good but a little inconsistent. If someone sits in your car, the alarm will definitely go off. An arm reaching in will set off the alarm if the arm is moved around. Repositioning the sensor might help – the directions say the higher it is in the car the better.

Discuss this article and other Safety/Security upgrades in the

///MZ3.Net discussion forum.