ABS Enigma

[ChatMaven]

I want assistance on my ABS issue.

My vehicle is a 1990 S124 300E 24-valve automatic, and a yellow ABS warning light activates on the dashboard when the speed exceeds around 5 mph. The ABS light remains off if the speed is maintained below this threshold. The vehicle brakes well, exhibiting no trembling or pedal oscillation in the last meters of stopping, as is sometimes seen.


I have just retrieved the vehicle from a nearly two-month tenure at a Mercedes Benz specialist, where they were unable to identify the malfunction. Below is a compilation of the completed tasks: The rear ABS sensor was replaced due to degraded wiring and a seized sensor within the differential housing. The front nearside ABS sensor was also replaced as it generated a low voltage (0.5v) when the wheel was manually rotated. The ABS control module was temporarily substituted with one from a fault-free vehicle, and all relays were replaced temporarily with those from a car exhibiting no faults. Additionally, all wiring to the sensors was verified to be intact, with no breaks in continuity c The front reluctor rings have been sanitised. Essentially, with the exception of the ABS pump, the system has been substituted with either new components or those shown to be functional from an operational vehicle. All brake pads are almost intact, and the fluid is new.

Although I am not a specialist in ABS systems, the fact that the ABS light activates only when reaching a certain speed indicates to me that the issue is likely sensor-related, hence I am hesitant to replace the ABS pump.

Has anybody had comparable issues? What was the resolution to the issue?

[ForumGuru]

Have the tires been inspected for rolling radius, tread depth, and pressure levels?
The illumination of the light only during significant movement strongly suggests that the data being interpreted by the ECU is erroneous. Electrical issues will activate the light upon key ignition.

[Mistral]

Each axle is equipped with two matching tires (make/type), since not all makes of tires of the same size possess identical physical rolling radii. Several years ago, the space-saver activated the ABS warning light on one of my vintage vehicles from the 1990s.

[ChatMaven]

Thank you for the recommendation about the tires: it is equipped with four new Dunlop tyres of uniform size and equal pressures.

[jimmy47785]

And meticulously examined for fissures?
Rear ends? Presumably located inside the differential and unlikely to have rusted or fractured?

[jimmy47785]

I concur. A sensor malfunction would still activate the light upon key-on. In my opinion, the signals received by the ECU are the fundamental cause of this issue.
Deviating somewhat from the norm... Is there no potential for a sensor wire to detect an extraneous signal? The Porsche 964, equipped with dual distributors, experiences engine movement during acceleration owing to torque response, which brings the distributors and ABS system into proximity, resulting in contamination of ABS signals by high-tension voltages.

[Matt865]

OVP relay?