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P0377Timing Reference High Resolution Signal "B" Too Few Pulses

The engine computer is receiving fewer pulses than expected from the secondary (Signal B) high-resolution timing reference signal.

Definition

The PCM counts the high-resolution timing pulses on the Signal "B" channel between low-resolution reference marks and sets P0377 when pulses are missing from the train. Missing pulses are typically caused by damaged trigger wheel teeth, a weak or failing sensor, excessive sensor air gap, or an intermittent connection. Because the PCM momentarily loses precise crankshaft position, symptoms include rough running, hesitation, misfire-like stumbles, stalling, and hard starting, with the MIL on and possible default timing operation.

Common causes

  • Damaged or missing teeth on the reluctor/trigger wheel
  • Weak or failing position sensor with low output amplitude
  • Excessive air gap between sensor and trigger wheel
  • Intermittent open in the signal wiring or connector
  • Metallic debris on the sensor tip masking teeth
  • Faulty PCM input (rare)

Common misdiagnoses

  • Fuel system parts replaced for stumble/stall complaints that were signal dropouts
  • Sensor replaced repeatedly when the trigger wheel itself was damaged
  • Ignition coils or plugs replaced for misfire-like symptoms
  • PCM replaced without scoping the pulse train

Troubleshooting steps

  1. 1.Scope the pulse train

    Capture Signal B on a lab scope during cranking and at 2000 RPM; look for gaps where a pulse should occur and note whether the dropout repeats at the same crank position.

  2. 2.Check sensor output amplitude

    For magnetic sensors verify AC output meets spec while cranking (commonly 0.5V AC minimum); low amplitude points to a weak sensor or wide air gap.

  3. 3.Inspect the trigger wheel

    A dropout that repeats at the same position each revolution indicates a damaged, filled, or missing tooth rather than an electrical fault.

  4. 4.Measure the air gap

    Where adjustable, set the sensor-to-reluctor gap to the service manual spec (typically 0.020-0.050 in) and check the wheel for runout.

  5. 5.Wiggle-test wiring and connectors

    Flex the harness and connector while watching the scope; a random dropout reproduced by movement indicates an intermittent open circuit.

  6. 6.Clean the sensor tip

    Remove the sensor and clear any accumulated metallic debris from the magnetic tip before condemning parts.

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