Mercedes-Benz W124 ASD warning light on: fault codes and common faults
Mercedes-Benz W124 ASD light on: how do you read the fault codes?
The diagnostic socket is in the engine bay next to the battery. With the engine running, connect pins 1 and 5 of the socket for about a second and count the flashes of the ASD light. Codes are cleared by holding the same pins together for about 15 seconds. The light should come on when the ignition is switched on; if it stays on continuously, there is a fault in the system.
What do the Mercedes-Benz W124 ASD fault codes mean?
1 = no fault. 2 = ASD control unit. 3 = brake light switch or the ASD switching function. 4 = front left, 5 = front right and 6 = rear wheel speed sensor. 7 = all three sensors. 8 = solenoid valve, brake light switch or brake light circuit.
What are the most common causes of the Mercedes-Benz W124 ASD fault light?
The most common are a faulty brake light switch (it has a separate circuit for the ASD, so the brake lights can work even though the ASD side is broken) and oxidised solder joints in the overvoltage protection relay near the battery. The relay is repaired by re-soldering the joints or replacing it. The light can be on even though the ABS brakes work normally.
What else can be behind the ASD light in a Mercedes-Benz W124?
A wheel speed sensor may be covered in brake dust or its wire may be broken; the resistance of the rear wheel sensor should be 600–1600 Ω and the tightening torque is 8 Nm (front sensors 22 Nm). A leaking pressure accumulator or a break in the solenoid valve circuit can also turn the light on.