Hello fellow w124 lovers, Is this accurate to you? how often do you guys do a health check on ASR ABS system?
Maintaining the ABS (Anti-lock Braking System) and ASR (Acceleration Slip Regulation/Traction Control) on a W124 Mercedes-Benz (1985–1995) involves
addressing specific, often aging, electronic and hydraulic components that can cause the system to enter "limp mode" (limited to ~40 mph). Common issues include, but are not limited to, the OVP relay, brake light switch, wheel speed sensors, and the Electronic Throttle Actuator (ETA).
Key Maintenance & Troubleshooting Steps
- Overvoltage Protection (OVP) Relay: The most common cause of ABS/ASR lights and poor idle. Located behind the battery, it often develops cracked solder joints on the circuit board.
- Brake Light Switch (S9/1): A frequent failure point located under the dashboard/steering column. A faulty switch will cause ASR/BAS lights to illuminate.
- Electronic Throttle Actuator (ETA/E-Gas): On later models (M104 engines), the wiring harness for the throttle body degrades, causing ASR failure and limp mode.
- Wheel Speed Sensors: There are three sensors (one on each front wheel, one on the rear differential). These should be inspected for dirt, metallic shavings, or damaged wiring.
- Hydraulic Unit & Fluid: Ensure brake fluid is changed regularly (hygroscopic) and never allow mineral oil into the system. The ABS/ASR hydraulic unit, pump, and accumulator may require inspection if the brakes feel spongy or the pump fails to activate.
- Fuses & Relays: Check the fuses, particularly the one on top of the OVP relay, and clean the ABS relay (A 001 542 70 19).
- Tire Size: Ensure all four tires are of the correct, matched size. Discrepancies in tire diameter can confuse the wheel speed sensors and trigger the ASR system.
Symptoms of System Failure
- ABS/ASR Light On: The light may come on immediately or while driving.
- Limp Mode: The car has severely reduced acceleration, often restricted to around 40 mph or 55 km/h.
- Erratic Idling: Often associated with OVP or E-Gas failures.
Diagnostics
For ASR issues on later W124 models (e.g., 1994 E320), it is necessary to scan for diagnostic trouble codes, specifically using the 38-pin or 16-pin connector to pinpoint whether the issue is with the ABS module, throttle actuator, or a sensor.
