Code
P0789
HUMMER
P — Powertrain
Shift/Timing Solenoid Intermittent
AI status
Completed
Completed
100%
Causes
- Damaged or corroded connector or terminal
- Broken, chafed or shorted wiring harness to solenoid
- Faulty shift/timing solenoid (internal intermittent)
- Poor ground or low battery/charging voltage during operation
- Contaminated or sticking valve in valve body
- Intermittent TCM/PCM output driver or internal fault
Symptoms
- Intermittent or harsh/erratic shifting
- Delayed or missed gear engagement
- Transmission enters limp/backup mode under some conditions
- MIL (Check Engine/Transmission) illuminated intermittently
- Occasional gear hunting or slipping feeling
What to check
- Read freeze frame and loop data to confirm conditions when code set
- Perform battery/charging system voltage check (engine running and cranking)
- Visually inspect solenoid connector, pins and harness for corrosion, damage or water intrusion
- Wiggle test wiring and connectors while monitoring live data or status LED to check intermittency
- Scan live PIDs for solenoid commanded vs actual/status (TCM data)
- Measure solenoid coil resistance with meter and compare to spec, check for open/short
Signal parameters
- Coil resistance: typically tens of ohms (varies by model) — compare to OEM spec
- Supply voltage: battery/ignition 12V nominal; should be present at rest or when commanded depending on circuit
- Control signal: often PWM duty-cycle when commanded — duty varies with gear/pressure demand
- PWM frequency: commonly tens to a few hundred Hz (varies by design) — confirm with OEM spec or oscilloscope
- Ground continuity to PCM/TCM when commanded
Diagnostic algorithm
- Retrieve codes and freeze frame; note vehicle speed, engine RPM, shift state and battery voltage when fault occurred.
- Inspect solenoid connector and wiring for corrosion, looseness, pin damage or water ingress. Repair or clean as needed.
- Check battery and charging voltage; ensure stable 12–14.5V during tests.
- With harness connected, back-probe solenoid connector. Command solenoid ON/OFF with scan tool and observe voltage/ground and any PWM waveform with a multimeter/oscilloscope. Look for intermittent loss of signal when wiggle-testing harness.
- Measure coil resistance at the connector and compare to OEM spec. Disconnect connector when measuring to avoid parallel circuits. An open or wildly fluctuating reading indicates a bad coil.
- If wiring and connectors are good but waveform is intermittent, swap or bench-test the solenoid by applying known good 12V/ground and observe mechanical response and continuity. Replace solenoid if intermittent under direct power.
- If solenoid and harness test good, check PCM/TCM driver output for intermittent faults or perform wiggle test at PCM connector. Consider module software updates or replacing module if output driver is confirmed faulty.
- Inspect transmission fluid and valve body for contamination or sticking valves if electrical checks pass; service valve body/replace solenoid body as required.
- After repairs, clear codes, perform relearn procedures if required, and road test to confirm issue is resolved.
Likely causes
- Corroded connector pins or poor pin contact at solenoid
- Wiring damage or intermittent short/ground due to harness chafe
- Faulty solenoid that works sometimes and fails other times
- Contaminated transmission fluid/valve body causing intermittent valve movement
Fault status
Status
P0789 — Shift/Timing Solenoid Intermittent. The transmission control module detected an intermittent or unstable signal from the shift/timing solenoid circuit. May cause rough or erratic shifting; requires electrical and hydraulic checks.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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Repair manuals
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138
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