Code
P0629
HUMMER
P — Powertrain
Fuel Pump Relay Control Circuit High Voltage
Views:
UK: 17
EN: 32
RU: 22
AI status
Completed
Completed
100%
Causes
- Open or high-resistance wiring between PCM and fuel pump relay coil
- Short to battery voltage (B+) on the relay control circuit
- Faulty fuel pump relay (stuck or internal coil short)
- Poor or corroded grounds or power supply to relay/PCM
- Aftermarket wiring or components incorrectly installed
- Faulty PCM or internal driver transistor (less common)
Symptoms
- No fuel pump operation or intermittent pump operation
- Engine cranks but does not start or stalls
- Fuel pump fuse or relay may appear hot or show visual damage
- MIL (check engine light) illuminated
- Possible hard starting or poor drivability
What to check
- Scan for P0629 and related codes; record freeze frame and test readiness
- Visual inspection of fuel pump relay, fuse, connector, and wiring for damage/corrosion
- Backprobe relay control pin and measure voltage with key ON and while cranking
- Check continuity and resistance between PCM control pin and relay coil pin
- Measure relay coil supply (battery) and ground paths for proper voltages
- Swap with known-good identical relay (if available) to rule out relay failure
Signal parameters
- Control circuit expected: near battery voltage (~12–14.5 V) when relay is unenergized
- When PCM commands relay ON, control circuit is typically pulled low (near 0–1 V)
- High-voltage fault occurs when control line reads significantly above expected ON voltage (for example several volts when it should be near 0 V)
- Relay coil resistance (typical range depends on relay, commonly ~50–200 ohms) — compare to specification
Diagnostic algorithm
- Verify code and capture freeze frame. Note operating conditions when code set (key on, cranking, running).
- Perform a thorough visual inspection of relay, fuse, connectors, and wiring for damage, corrosion, or aftermarket splices.
- With ignition ON (engine off), backprobe the fuel pump relay control terminal. Observe voltage: it should be near battery when inactive and drop near 0 V when PCM commands ON. If it remains high when commanded ON, proceed.
- Attempt to activate fuel pump via scan tool or crank engine while monitoring the control circuit. Confirm whether the circuit ever goes low. If it goes low and pump runs, fault may be intermittent wiring/connector.
- Remove relay and measure coil resistance. Replace relay if out of specification or suspected faulty. Re-test.
- Check continuity between PCM control pin and relay socket. Repair any open/high resistance wiring or corroded connectors.
- Check for short to B+ on control circuit (voltage present when PCM is expected to pull line low). Isolate wiring sections to locate short and repair.
- Verify power feed and ground for relay coil and fuel pump (fuses, fusible links, grounds). Clean/repair grounds and connectors as needed.
- If wiring, relay, and power/ground are good, test PCM driver output. Replace PCM only after exhausting wiring and component checks and following manufacturer-specific PCM test procedures.
- Clear codes and road test to confirm repair. Monitor for reoccurrence and related codes.
Likely causes
- Disconnected / damaged connector at fuel pump relay or PCM
- Wiring chafed and shorted to B+ under vehicle or near relay
- Relay coil failed shorted to B+ or internally open
- Corroded ground or blown fuse in pump relay supply circuit
- PCM driver fault if wiring and relay test good
Fault status
Status
Fuel Pump Relay Control Circuit High Voltage — PCM detected an abnormally high voltage on the fuel pump relay control line. This may prevent proper relay operation and can cause no-start or stalling conditions. Check wiring, relay, fuses, grounds and PCM driver.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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