1991 Mercedes-Benz 420SEL Powertrain Control Module Replacement Cost

The powertrain control module (PCM) is a small computer that monitors all engine control operations in real time. It uses a large network of sensors, solenoids, sending units, and other electronic components to both receive and send information which enables the vehicle to run at peak performance. Aside from engine performance, it also controls engine function be moderating fuel injection rates, valve timing, ignition timing, throttle body position, and many more features that were previously done mechanically. The PCM also allows for inputs from other systems on the vehicle, and may have the ability to control the transmission as well.

Battery power is supplied to the PCM, and the PCM is programmed to send that electricity at regulated voltage to engine sensors, other control units, other devices on the network, and every system in the vehicle that depends on information from the PCM to function properly. Information is simultaneously passed back into the PCM from all of these locations, allowing the PCM to monitor conditions, and be monitored for conditions at all times. Since many systems are being integrated at all times, the control modules of the vehicle will communicate via a common area communication (CAN-BUS) network. This allows for faster communication, less wires, and increased opportunity for advances in vehicle function.

When the PCM fails, being one of the more important computers on the vehicle, the PCM may make erroneous calculations, fail to connect, fail to operate, or fail to interpret data correctly. This leads to constant malfunctions in most systems in the vehicle because systems dependant on PCM data have improper signals from the PCM. This will trigger the check engine light, and the possibilities of on-board diagnostic (OBD) trouble codes stored are endless. However, in many situations, a computerized scanning tool, or a handheld scanner will fail to connect to the vehicle. The symptoms of the vehicle depend on the impacted systems, and the worst case scenario is a vehicle that does not start or stay running.