1990 Mercedes C320 P0720 & P0600: ABS Light On, Transmission Stuck in First Gear, Speedometer Failure
I own a 1990 Mercedes-Benz C320. The MIL (Engine Light) is constantly on, followed by the ABS light turning on. A few seconds later, the dashboard displays warnings for ABS, ESP, and BAS systems. When I shift from PARK to DRIVE, the transition feels abrupt. As soon as I start driving, the transmission refuses to shift out of first gear, and the speedometer shows no reading. While revving the engine in first gear, the ABS light eventually turns off, after which the transmission shifts properly and the speedometer begins working. However, while driving at around 45 mph, the ABS light reactivates, the speedometer drops to zero, and the vehicle hesitates until I release the accelerator. When stopping at a signal or traffic light with the ABS light still on, the car remains in whatever gear it was in. To restart, I must shift into PARK, turn off the engine, then restart — only after this does the transmission return to first gear. During acceleration, the ABS light eventually turns off and the transmission shifts normally. I performed an OBD-II scan and received diagnostic codes P0720 and P0600. The P0720 code indicates a missing or faulty output speed signal from the rear axle — I need to confirm whether this refers to the output shaft speed sensor inside the transmission or the wheel speed sensor. Is the output speed sensor located in the transmission? Or is it part of the wheel speed sensing system? Can this issue be resolved by me as a DIY repair? Any clarification on these codes and how to fix them would be greatly appreciated. Thank you!
The P0720 code indicates no speed signal is being received from the rear axle, which typically points to a faulty output shaft speed sensor located inside the transmission. In some cases, this issue may require replacing the automatic transmission conductor plate — where the sensor is mounted. The plate is situated on top of the valve body within the transmission. To properly diagnose and verify the signal, use an OBD-II scan tool capable of monitoring transmission data and live speed sensor readings while driving. The P0600 code indicates a missing or faulty CAN bus message from the Active Body Control (ABC) module, which may result in system shutdown due to instability warnings.