Code
P0378
Generic
P — Powertrain
Timing Reference High Resolution Signal B Intermittent/Erratic Pulses
AI status
Completed
Completed
100%
Causes
- Faulty high-resolution timing sensor (Hall or VR type)
- Damaged, corroded or loose connector at the sensor or ECU
- Chafed, shorted or open wiring between sensor and ECU
- Damaged or missing teeth/targets on reluctor/trigger wheel
- Incorrect sensor air gap or misalignment
- Electrical interference or poor grounding
Symptoms
- Check Engine MIL illuminated (P0378 stored)
- Intermittent rough idle or surging
- Misfires under load or at idle
- Hard to start or intermittent no-start
- Reduced power or hesitation during acceleration
- Occasional stalling
What to check
- Read freeze frame and permanent data with a scan tool; note engine speed when fault set
- Check for related DTCs (cam/crank position, misfire codes)
- Visually inspect sensor, connector and wiring harness for damage or corrosion
- Perform a wiggle test on wiring/connectors while monitoring the signal
- Measure sensor supply voltage, reference and ground at the connector
- Use an oscilloscope to observe the signal waveform at idle and at higher RPM
Signal parameters
- Hall-effect (digital) sensor: square wave, typically 0–5 V (pull-up to 5 V) with frequency proportional to engine speed
- Variable reluctor (VR) sensor: AC sine-like waveform; amplitude increases with RPM (often 0.2–12 V p-p depending on system)
- Expected clean, regular pulse spacing and consistent amplitude at a given RPM; erratic amplitude, missing pulses or noise indicates a problem
- Signal frequency and pulse-count depend on engine design (pulses per rev vary by manufacturer)
Diagnostic algorithm
- Connect a full-function scan tool and read all stored codes and freeze-frame data. Note related cam/crank codes or misfires.
- Visually inspect the sensor B location, connector and harness for corrosion, damaged insulation, rubbing or compression.
- With ignition ON (engine off), verify sensor reference power and ground at the connector per vehicle specifications.
- Start engine and monitor the timing reference B signal with a digital oscilloscope or lab scope. Look for missing pulses, noise, voltage dropouts or inconsistent duty cycle.
- Perform a wiggle test on the wiring and connector while observing the waveform; this can reveal intermittent opens or shorts.
- Inspect the reluctor/trigger wheel and sensor mounting: check air gap, sensor alignment and for damaged/missing teeth or debris.
Likely causes
- Connector corrosion or poor pin contact at sensor
- Wiring damage (broken strand, chafed insulation, short to ground)
- Failed timing reference (cam/crank) sensor
- Damaged or displaced reluctor/target wheel
Fault status
Status
Powertrain control module detected intermittent or erratic pulses on Timing Reference High Resolution Signal B. This can cause misfires, rough running, hard starting or reduced engine performance.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
Repair manuals
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Code
P0378
GWM
P — Powertrain
- Unstable pulses of the B timer signal
AI status
Completed
Completed
100%
Causes
- Faulty high-resolution timing sensor (Hall or VR type)
- Damaged, corroded or loose connector at the sensor or ECU
- Chafed, shorted or open wiring between sensor and ECU
- Damaged or missing teeth/targets on reluctor/trigger wheel
- Incorrect sensor air gap or misalignment
- Electrical interference or poor grounding
Symptoms
- Check Engine MIL illuminated (P0378 stored)
- Intermittent rough idle or surging
- Misfires under load or at idle
- Hard to start or intermittent no-start
- Reduced power or hesitation during acceleration
- Occasional stalling
What to check
- Read freeze frame and permanent data with a scan tool; note engine speed when fault set
- Check for related DTCs (cam/crank position, misfire codes)
- Visually inspect sensor, connector and wiring harness for damage or corrosion
- Perform a wiggle test on wiring/connectors while monitoring the signal
- Measure sensor supply voltage, reference and ground at the connector
- Use an oscilloscope to observe the signal waveform at idle and at higher RPM
Signal parameters
- Hall-effect (digital) sensor: square wave, typically 0–5 V (pull-up to 5 V) with frequency proportional to engine speed
- Variable reluctor (VR) sensor: AC sine-like waveform; amplitude increases with RPM (often 0.2–12 V p-p depending on system)
- Expected clean, regular pulse spacing and consistent amplitude at a given RPM; erratic amplitude, missing pulses or noise indicates a problem
- Signal frequency and pulse-count depend on engine design (pulses per rev vary by manufacturer)
Diagnostic algorithm
- Connect a full-function scan tool and read all stored codes and freeze-frame data. Note related cam/crank codes or misfires.
- Visually inspect the sensor B location, connector and harness for corrosion, damaged insulation, rubbing or compression.
- With ignition ON (engine off), verify sensor reference power and ground at the connector per vehicle specifications.
- Start engine and monitor the timing reference B signal with a digital oscilloscope or lab scope. Look for missing pulses, noise, voltage dropouts or inconsistent duty cycle.
- Perform a wiggle test on the wiring and connector while observing the waveform; this can reveal intermittent opens or shorts.
- Inspect the reluctor/trigger wheel and sensor mounting: check air gap, sensor alignment and for damaged/missing teeth or debris.
Likely causes
- Connector corrosion or poor pin contact at sensor
- Wiring damage (broken strand, chafed insulation, short to ground)
- Failed timing reference (cam/crank) sensor
- Damaged or displaced reluctor/target wheel
Fault status
Status
Powertrain control module detected intermittent or erratic pulses on Timing Reference High Resolution Signal B. This can cause misfires, rough running, hard starting or reduced engine performance.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
Your experience will help others
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Was this AI description helpful?
Your feedback helps improve AI descriptions.
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Code
P0378
HUMMER
P — Powertrain
Timing Reference High Resolution Signal B Intermittent/Erratic Pulses
AI status
Completed
Completed
100%
Causes
- Faulty high-resolution timing sensor (Hall or VR type)
- Damaged, corroded or loose connector at the sensor or ECU
- Chafed, shorted or open wiring between sensor and ECU
- Damaged or missing teeth/targets on reluctor/trigger wheel
- Incorrect sensor air gap or misalignment
- Electrical interference or poor grounding
Symptoms
- Check Engine MIL illuminated (P0378 stored)
- Intermittent rough idle or surging
- Misfires under load or at idle
- Hard to start or intermittent no-start
- Reduced power or hesitation during acceleration
- Occasional stalling
What to check
- Read freeze frame and permanent data with a scan tool; note engine speed when fault set
- Check for related DTCs (cam/crank position, misfire codes)
- Visually inspect sensor, connector and wiring harness for damage or corrosion
- Perform a wiggle test on wiring/connectors while monitoring the signal
- Measure sensor supply voltage, reference and ground at the connector
- Use an oscilloscope to observe the signal waveform at idle and at higher RPM
Signal parameters
- Hall-effect (digital) sensor: square wave, typically 0–5 V (pull-up to 5 V) with frequency proportional to engine speed
- Variable reluctor (VR) sensor: AC sine-like waveform; amplitude increases with RPM (often 0.2–12 V p-p depending on system)
- Expected clean, regular pulse spacing and consistent amplitude at a given RPM; erratic amplitude, missing pulses or noise indicates a problem
- Signal frequency and pulse-count depend on engine design (pulses per rev vary by manufacturer)
Diagnostic algorithm
- Connect a full-function scan tool and read all stored codes and freeze-frame data. Note related cam/crank codes or misfires.
- Visually inspect the sensor B location, connector and harness for corrosion, damaged insulation, rubbing or compression.
- With ignition ON (engine off), verify sensor reference power and ground at the connector per vehicle specifications.
- Start engine and monitor the timing reference B signal with a digital oscilloscope or lab scope. Look for missing pulses, noise, voltage dropouts or inconsistent duty cycle.
- Perform a wiggle test on the wiring and connector while observing the waveform; this can reveal intermittent opens or shorts.
- Inspect the reluctor/trigger wheel and sensor mounting: check air gap, sensor alignment and for damaged/missing teeth or debris.
Likely causes
- Connector corrosion or poor pin contact at sensor
- Wiring damage (broken strand, chafed insulation, short to ground)
- Failed timing reference (cam/crank) sensor
- Damaged or displaced reluctor/target wheel
Fault status
Status
Powertrain control module detected intermittent or erratic pulses on Timing Reference High Resolution Signal B. This can cause misfires, rough running, hard starting or reduced engine performance.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
Repair manuals
Manual library for HUMMER
138
Browse 138 HUMMER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.
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HUMMER: 1993
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