Main.TrailIndexPage | Brake System | Low Fluid Warning Light

Low Fluid Warning Light

This component is part of Brake System.

Brake Warning, Door A-Jar Light

When you open the door on a W113-type car, the red brake warning light in the central instrument cluster comes on. This is to signify that the electrical brake warning system is working OK. So why is the red brake light associated with opening the door?

Legislation demanded brake fluid low and door ajar warnings. There was only one light position remaining on the instrument cluster, so Mercedes-Benz decided to use the same light for both functions.

Mercedes-Benz cars are designed so that when the key is switched to "on", the wiring section No 54 in electrical system is in use. Section No 30 is straight from 12V+ on battery. A primary rule is that section No 30 and No 54 are never to be connected, as this bypasses the ignition key and empties the battery.

The 113 courtesy light below the dash is connected to No 30. The light is illuminated when the door is opened. The brake fluid warning is connected to 54 (Light is on when car is running and level is low). Now, the M-B designers wanted to find a way to check that the warning light bulb was OK. And, they did that by using the courtesy light door switch. This also resulted in a "door ajar" warning function.

This design carries with it an unfortunate effect: If the brake fluid level is low, and the door is opened, 30 and 54 will be connected. This means the ignition will go on, and the fuel feed pump will start up. Fuel can be squirted around if one is performing work on the fuel system in the car.

A Simple Fix: Install a diode in the circuit between the fluid level sender and this red light bulb in the instrument. An Si diode, 1A with the (+) side facing the bulb. Can be easily hidden in the wiring harness of the car.

Another problem that sometimes occurs with this light coming on and the petrol pump going if the drivers door is opened is caused through a very fine pin hole developing in the float with-in the Brake reservoir that is used to indicate that there is low brake fluid. The indications are the same as low fluid with light and also the petrol pump going on when the drivers door is opened. It is difficult to see that the float has sunk to the bottom of the brake fluid without removing the reservoir cap. Replacing the float fixes the problem.
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