Central locking system

[EE80Liftback]

You could have used the signal on the door switch right away. This actually is a "ground signal" when the door is open.

The ceiling light bulb is connected to 12V on one side the other side goes to the door switch. If the door is closed the metal part of the door switch doesn't connect the wire to the chassis, hence there is no circuit and the bulb does not light. If you close the door the switch connects the wire to the chassis and the bulb lights.

If you measure the voltage on the wire once the door is open you measure 12V, however, what you measure is a current flow trough the bulb. Since your voltage meter consumes hardly any current the bulb doesn't light.

If you do not believe me try one of these:

1) Connect one wire of your multimeter to ground and the other one to the wire in between bulb and door switch. If you now set your multimeter to measure resistance (ohms) you should have a small reading when the door is closed (current can flow from wire to chassis) and a large reading or some indication of infinity (usually O.L.) if the door is open.

2) Instead of using a meter you could use another bulb. If you connect one side to ground and the other to the wire you'll see the bulb you connected and the bulb on the ceiling both shining at half their usual intensity if the door is open and the ceiling light on and connected bulb off when the door is closed. This shows that the lights are in series once the door is open (proving you measured the 12V you originally read from the meter trough the ceiling bulb) and the wire is connected to the chassis (hence, no voltage so no light on the connected bulb) if the door is closed.