Troubleshooting the ABS and ASR warning lights on a 1994 Mercedes-Benz can be a process of elimination, especially for DIY enthusiasts. Recently, while working on a similar issue, I managed to locate and temporarily repair a fault that restored some functionality, and I wanted to share the process. Although a proper diagnostic scan tool for a 1994 Mercedes would certainly streamline this process, sometimes a hands-on approach can lead to discovering simple fixes.
The first step was locating the rear speed sensor end connector. On the wagon model, this is found under a black plastic cover near the rear right passenger door. After removing three 10mm bolts, the plastic cover can be detached, revealing a circular black knob. Turning this knob counterclockwise and pulling it down exposes four sets of wires with labeled part number end connectors. Among these, I identified:
- A: 124 543 57 26 L A Top
- A: 124 543 58 26 R A Bottom – This particular connector had a broken wire.
- D: 124 540 22 17 R Wheel Speed Sensor
- E: 124 540 21 17 L Wheel Speed Sensor
The broken wire on connector A: 124 543 58 26 R A Bottom was the obvious culprit. After stripping both ends of the broken wire, I used blue connectors (14-16 gauge) to reconnect them. This was a temporary fix, but immediately after connecting the two broken wires, I noticed the Diagnostic Self Test began working again. Additionally, the Transmission fluid meter in the instrument cluster panel, which had been inactive, also started functioning. According to online searches, this specific wire seems to be related to an electrical cable, and while this repair is temporary, it indicates progress.
While working on the rear, I was able to easily locate the left wheel speed sensor on the differential, part number E: 124 540 21 17 L Wheel Speed Sensor. Surprisingly, locating the right speed sensor, D: 124 540 22 17 R Wheel Speed Sensor, proved more challenging and remains unresolved at this point.
I attempted to measure the resistance on both identified wheel speed sensors. However, the confined space and limited movement under the wagon made it difficult to get a reliable reading. Determining the correct ohms setting for accurate resistance measurement in this situation is still a question.
Following the wire repair, a test drive was conducted. Unfortunately, the ABS/ASR warning lights remained illuminated, indicating the issue is not fully resolved. However, the changes observed with the diagnostic self-test and transmission fluid meter suggest that repairing the broken wire was a step in the right direction toward resolving the ABS/ASR problem.
Further investigation using the self-diagnostic test module (the 16-pin square box near the battery) revealed diagnostic code flashes 4 and 5. Interestingly, a three banana plugs code reader did not register any codes.
- Code 4 flashes: Indicates a fault in the Air injection system, specifically the hot film mass air flow sensor with a hot wire.
- Code 5 flashes: Points to a fault in the Exhaust gas recirculation system.
The next steps to further diagnose and repair the ABS/ASR system include addressing these diagnostic codes, as well as physically inspecting and cleaning the wheel speed sensors. One immediate task is to remove a stripped bolt on the right passenger side to gain better access to the right wheel speed sensor area.
While this DIY approach has yielded some progress, for a more comprehensive diagnosis of a 1994 Mercedes-Benz, especially concerning ABS/ASR and engine management systems, utilizing a compatible 1994 Mercedes Scan Tool would likely provide more detailed fault codes and system insights, potentially speeding up the repair process and offering a more accurate understanding of the underlying issues. Any feedback or suggestions based on these findings would be greatly appreciated.
