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P0370Timing Reference High Resolution Signal "A" Malfunction

The high-resolution engine timing reference signal used for precise spark and misfire detection is faulty or missing.

Definition

Some ignition systems — notably GM designs such as the LT1's optical distributor — supply the PCM with a high-resolution pulse train (for example 360 pulses per revolution) in addition to the basic low-resolution reference, giving the PCM fine-grained crankshaft position for spark timing and misfire detection. P0370 sets when this high-resolution signal "A" is missing, erratic, or does not agree with the low-resolution reference. Symptoms can include stalling, surging, hesitation, hard starting or a no-start, and a MIL, since the PCM may lose precise timing control when the signal fails.

Common causes

  • Failed optical sensor or pickup in the distributor (e.g., GM Opti-Spark), often from moisture or oil contamination
  • Damaged wiring or connector in the high-resolution signal circuit
  • Failed crankshaft/camshaft high-resolution sensor on non-distributor systems
  • Contaminated or damaged trigger disc/reluctor
  • Electrical interference from faulty secondary ignition components
  • PCM input circuit fault (least common)

Common misdiagnoses

  • Ignition coil replaced for what is a timing-signal fault
  • PCM replaced before the distributor or sensor signal is scoped
  • Fuel pump or filter replaced for stalling caused by signal dropout
  • Spark plugs and wires replaced without addressing the signal source

Troubleshooting steps

  1. 1.Identify the signal source for the application

    Determine from service data whether the high-resolution signal comes from an optical distributor (common on GM LT1) or a crank/cam sensor.

  2. 2.Check for moisture and oil intrusion

    On optical distributors, pull the cap and look for water, oil from a leaking front seal, or corrosion — the classic Opti-Spark failure mode.

  3. 3.Scope the high-resolution circuit

    While cranking, verify the expected fast pulse train (e.g., 360 pulses/rev) alongside the low-resolution signal; missing or ragged pulses confirm the fault.

  4. 4.Inspect wiring and connectors

    Check the harness between the sensor/distributor and PCM for chafe, and verify terminal condition and tension at both connectors.

  5. 5.Verify sensor power and ground

    Confirm the supply voltage and a clean ground at the sensor connector before condemning the sensor or distributor.

  6. 6.Replace the failed component and retest

    After replacing the distributor or sensor, verify stable timing signals at idle and under snap throttle with no dropouts.

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