For my own use, and to fully understand the circuits involved, I took Garry's excellent analysis and slightly amended it. When I was finished, I thought someone else might appreciate a slightly different version of the same analysis. Here it is:
DOOR AJAR LIGHT, BRAKE FLUID LIGHT & FUEL PUMP ACTIVATION
When you open the door on a W113 model car, the red brake warning light comes on. This light was originally installed to indicate a low fluid fault in the brake system. So why is this red brake light associated with opening the door? It is because continuing safety legislation demanded both low brake fluid and door ajar warnings. As there was only one light position left on the kombi instrument, it was decided to use the same light for both functions.
All Mercedes cars are designed so that wiring section No. 30 runs straight from 12V+ on battery and is always energized. On the other hand, it is only when the ignition key is switched to "on", that wiring section No. 54 in the electrical system is energized. A primary rule is that sections Nos. 30 and No 54 are never to be connected, since this bypasses the ignition key and empties the battery.
The W113 is designed so that the courtesy light below the dash is connected to No. 30 and is lit as door is opened. The brake fluid warning is connected to No. 54 (light is on when car is running and fluid level is low). The M-B designers wanted a way to check that the brake fluid warning light lamp was OK and they did this by connecting it to the courtesy light door switch that also allowed it to serve a "door ajar" warning function. This design carries with it an unfortunate side effect: If the brake fluid level is low, and the door is opened, 30 and 54 will be connected through the common lamp and 34 will be activated the same as if the ignition key is turned to “on,” and the electric fuel feed pump will start up. Fuel could be squirted around if one is performing work on the fuel system in the car.
Now, some remedies: a) A wise thing to do is to disconnect the battery before starting any maintenance work on the fuel system. b) Check that the float does not have a pin hole in it and has sunk to the bottom of the brake fluid canister thus giving a false low fluid warning that might result in unintended electrical connections if the door is opened. c) If you wish to modify the electrical systems, fit a diode in the circuit between the fluid level sender and the red light lamp in the kombi instrument. A Si diode, 1A, with the (+) side facing the lamp will do fine, and it may be easily hidden in the wiring harness of the car.
