1991 Volkswagen Jetta 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.

In most situations, driving a vehicle with a failed PCM is impossible as the systems needed to run the vehicle are not being instructed any longer. The PCM is a vital computer, and with this component sending erroneous signals, the vehicle will most likely never start, or the engine will randomly shut off. If a vehicle is responding very strangely to driver inputs, the vehicle should be towed to a repair facility.

The engine control module, or powertrain control module, is known to fail frequently in small groups of vehicles, however, failure in most vehicles is not extremely common. Most vehicles will be crushed with the original PCM in place. That is not to say it cannot happen, in fact, as early as 75,000 miles, up to 125,000 miles is the most common range for PCM replacement for several popular vehicles.

When the PCM is suspected of failure, the particular specifications of that unit must be referenced so the technician knows what the correct readings should be. The PCM will be tested by comparing actual readings to specified readings, following any specific diagnostic steps for that PCM, and confirming diagnosis after each pin has been tested for all necessary parameters. Since there are so many possible failure symptoms, it is not possible to speculate which pin to check, but reference materials will guide the technician through the process of locating the pins/wires that correspond to the issue they are chasing.

Replacing the PCM is normally easy since diagnostics will require the PCM be accessed for diagnostic testing prior to replacement. The technician may need component location reference material to find where the manufacturer installed the PCM. In most vehicles, the PCM is in the engine bay, but a few manufacturers place the PCM inside the vehicle. In some vehicles, the PCM is even located in the trunk. Once the PCM has been located, any obstructions to testing and replacement must be removed, and testing begins. If the PCM fails diagnostic testing, the PCM will be unplugged, replaced, and reconnected to the vehicle prior to final assembly and testing.

Electrical diagnostics in general can be problematic, and there are many variables to account for. When electrical problems arise, diagnostics may take several hours to complete, or just a few minutes. This will significantly impact the cost of repairs. Choosing a facility with proper equipment and training for their staff may significantly reduce diagnostic time needed.

The battery connects directly to the engine, meaning it is grounded to the same point as many engine sensors. If the battery is incorrectly installed in a vehicle, such as installing or jump-starting the battery backwards, the PCM can be destroyed, along with some sensors and wiring. In many occasions, the PCM connector, harness, or pins inside the connector socket fail, and cannot make a proper connection to the PCM. Except for a broken or missing pin, these items can be replaced, and the vehicle retested.

Replacing the PCM on most vehicles is fairly easy, but accessing the control module usually requires searching behind panels if the component cannot be identified through a vehicle repair database. In many cases, the PCM looks much like a few other computers that are mounted in the vehicle, and removing a few panels to find the wrong module will quickly become frustrating. If proper experience, and reference materials are available, the PCM can be replaced, but many will need to be calibrated by the manufacturer. If this process is understood, and the PCM is adapted to the vehicle, this is a money saver, however, if diagnostic procedures are not fully understood, this should be trusted to a professional.