ECM/PCM Power Relay Control Circuit High
P0687 indicates the PCM has detected abnormally high voltage in the circuit that powers the PCM or monitors its power supply voltage. This overvoltage condition can potentially damage the PCM and other controllers if not addressed promptly, even though the vehicle may still start and run.
What You Might Notice
- Check Engine Light illuminated
- Vehicle may start normally despite code storage
- Intermittent stalling or loss of power
- Erratic engine behavior or drivability issues
- Potential PCM shutdown in some applications
Most Common Causes
- 1
Faulty PCM Power Relay
A stuck or failing relay that supplies battery voltage to the PCM can cause continuous high voltage delivery. The relay may be stuck in the closed position, allowing unregulated voltage to reach the PCM.
- 2
Voltage Regulator Failure
A malfunctioning alternator voltage regulator can cause the charging system to overcharge, pushing excessive voltage through the PCM power circuit. This is common when the regulator loses its ability to control output voltage.
- 3
Wiring Issues or Corrosion
Corroded, damaged, or improperly spliced wiring in the PCM power supply circuit can create high resistance or short conditions that cause voltage spikes. Poor ground connections can also contribute to voltage regulation problems.
- 4
Alternator Output Problem
An alternator producing excessive voltage output without proper regulator control can send overvoltage through the entire electrical system including the PCM power feed.
How to Diagnose It
- 1
Check Faulty PCM Power Relay — A stuck or failing relay that supplies battery voltage to the PCM can cause continuous high voltage delivery. The relay may be stuck in the closed position, allowing unregulated voltage to reach the PCM.
- 2
Check Voltage Regulator Failure — A malfunctioning alternator voltage regulator can cause the charging system to overcharge, pushing excessive voltage through the PCM power circuit. This is common when the regulator loses its ability to control output voltage.
- 3
Check Wiring Issues or Corrosion — Corroded, damaged, or improperly spliced wiring in the PCM power supply circuit can create high resistance or short conditions that cause voltage spikes. Poor ground connections can also contribute to voltage regulation problems.
- 4
Check Alternator Output Problem — An alternator producing excessive voltage output without proper regulator control can send overvoltage through the entire electrical system including the PCM power feed.
How to Fix It
- 1
Test and Replace PCM Power Relay
Use a multimeter to check the relay's operation and measure voltage output. If voltage exceeds specification (typically around 13.5-14.5V), replace the relay. Verify proper relay seating in its socket.
- 2
Inspect and Repair PCM Power Circuit Wiring
Visually inspect the entire PCM power supply wiring harness for corrosion, damage, or loose connections. Clean corroded terminals and repair any damaged insulation. Verify all ground connections are clean and tight.
- 3
Test Alternator Output Voltage
With the engine running, measure alternator output voltage at the battery terminals and at the PCM power input. Voltage should stay between 13.5-14.5V. If higher, the voltage regulator needs replacement.
- 4
Replace PCM if Damaged
If voltage testing shows the PCM or its power supply circuit is damaged beyond repair, the PCM itself may need replacement. This is typically a last resort after confirming all external power sources are functioning properly.
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