Technical Description of OBD-II Trouble Code
The circuit controlling the ECM/PCM power relay is experiencing high voltage.
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to all vehicles manufactured from 1996 onwards (VW, BMW, Chrysler, Acura, Audi, Isuzu, Jeep, GM, etc.).
While this code is generic, the specific causes may vary slightly between different brands and their respective engines.
When a P0687 code is stored, it typically indicates that the powertrain control module (PCM) or one of the other controllers has detected a high voltage condition in the circuit that powers up the PCM or the circuit that monitors the PCM power supply voltage.
Most OBD-II equipped vehicles use a contact relay to supply battery voltage to the PCM. However, some manufacturers still use a fused circuit, although this design is becoming less common. Power relays usually have five terminals, with a constant supply of battery voltage at the primary input terminal. When the ignition switch is turned to the ON position, the secondary input terminal receives battery voltage through a fused circuit. Terminal number five is a signal wire for the controller area network (CAN), while the fourth terminal supplies an output voltage signal for the PCM. These are the three primary circuits related to the conditions of a P0687 code being stored. The ground terminal is typically grounded to an engine or chassis ground, although some systems have a ground provided by a relay controller.
Normally, when the ignition switch is turned to the ON position, a small coil inside the relay is energized, resulting in a light ticking noise. This noise indicates that the contacts inside the relay are closed. When the contacts close, they complete a circuit that provides battery voltage to the PCM output terminal and the CAN bus signal terminal. With the contacts closed, the PCM should be energized and the service engine soon (SES) lamp should be illuminated in the key-on-engine-off (KOEO) position. In the key-on-engine-running (KOER) position, the service engine soon lamp should be off (if no codes are stored). The PCM should receive battery voltage whenever the ignition is in the ON position.
Symptoms
The P0687 code usually does not cause a no-start condition. However, in some cases, the PCM may be disabled when this code is stored. Even if the vehicle starts and there are no drivability issues, excessive voltage may damage the PCM or other controllers. It is important to address this code promptly.
Causes
Possible causes for this code to be set include:
Diagnostic and Repair Procedures
A good starting point is to check for technical service bulletins (TSB) specific to your vehicle. There may be a known issue with a known fix provided by the manufacturer, which can save time and money during diagnosis.
Begin the diagnosis with a visual inspection of the system’s wiring harnesses, connectors, and components. Test the alternator to ensure it is not overcharging. Additionally, check the battery and battery cable ends for looseness and excessive corrosion.
To accurately diagnose a P0687 code, you will need a suitable scanner (or code reader), a digital volt/ohmmeter (DVOM), and a wiring schematic. Sources like All Data can provide wiring schematics, connector pinout diagrams, connector face views, component locations, and ground locations. Retrieve all stored trouble codes using the scanner or code reader and make a note of them for future reference.
Use All Data or the vehicle service manual (or owner’s manual) to locate the PCM power relay. Lightly touch it, hold it in your fingers, or listen to it with a stethoscope as the ignition switch is turned to the ON position. If you hear an audible click when the ignition switch is turned on, begin testing with the PCM output terminal.
With the engine running, check for excessive battery voltage at the primary input terminal of the PCM power relay. If there is excessive voltage at the primary terminal, suspect that the alternator is overcharging. Test the voltage at the secondary input terminal. If excessive voltage is present, suspect a shorted circuit that is connected to voltage. Pay close attention to the electrical portion of the ignition switch.
Check for voltage on the ground terminal of the PCM power relay in the KOER position. If there is voltage where a ground should be, check for a shorted ground circuit that is connected to voltage.
If the alternator is charging properly, begin testing the system circuits for a short to voltage. Before testing resistance with the DVOM, make sure to disconnect all related controllers from the harness. Repair or replace any shorted circuits as necessary.
