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P0378Timing Reference High Resolution Signal "B" Intermittent/Erratic Pulses

The secondary (Signal B) high-resolution timing reference signal is cutting in and out or arriving erratically at the engine computer.

Definition

The PCM expects a continuous, evenly spaced high-resolution pulse train on the Signal "B" timing channel and sets P0378 when the signal drops out intermittently or arrives with erratic spacing. This is most often an intermittent wiring or connector fault, a sensor that fails when hot, or a trigger wheel with excessive runout. Symptoms come and go — intermittent stalling, surging, hesitation, and hard hot-restarts — which makes the fault easy to miss on a single test drive; the MIL illuminates and the PCM may intermittently revert to default timing.

Common causes

  • Intermittent open or short in the sensor harness or connector
  • Position sensor that fails intermittently when hot (thermal failure)
  • Loose, backed-out, or corroded connector terminals
  • Trigger wheel runout or a loose reluctor
  • Chafed wiring making intermittent contact with ground
  • Poor PCM ground connections

Common misdiagnoses

  • Fuel pump replaced for intermittent stalling
  • Ignition module or coil replaced for random cutouts
  • Sensor replaced when a connector terminal had lost tension
  • PCM replaced without heat-testing the sensor

Troubleshooting steps

  1. 1.Reproduce the fault with a scope connected

    Drive or idle the vehicle with a lab scope on Signal B and a min/max recording, watching for dropouts that coincide with the driveability symptom.

  2. 2.Wiggle-test the harness

    With the engine running, flex the harness at every clip and connector between sensor and PCM; a stumble or scope dropout during movement localizes the break.

  3. 3.Check terminal tension

    Disconnect the sensor and PCM connectors and verify each terminal grips a test pin firmly; repair any spread or corroded terminals.

  4. 4.Heat-test the sensor

    Warm the sensor with a heat gun to roughly operating temperature while monitoring the signal, since many position sensors only fail hot.

  5. 5.Check trigger wheel runout

    Rotate the engine by hand with a dial indicator on the reluctor; excessive wobble changes the air gap each revolution and produces erratic amplitude.

  6. 6.Verify grounds and shielding

    Load-test the sensor ground and PCM grounds for under 0.1V drop and confirm the cable shield is intact and terminated correctly.

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