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Circuit/System Testing

ECM Park Switch Circuit Malfunction (4T65-E)

  1. Ignition OFF, disconnect the harness connector at the AT Inline 20-Way Connector.
  2. Ignition ON, install a 3A fused jumper wire between the signal circuit terminal W and ground. Verify the scan tool ECM PNP switch parameter displays Park/Neutral.
    • If not the specified value, test the signal circuit terminal W for an open/high resistance. If the circuit tests normal, replace the ECM.
  3. If all circuits test normal, test or replace the internal mode switch (IMS).

Starter Relay Circuit Malfunction

  1. Ignition OFF, disconnect the starter relay.
  2. Test for less than 1 ohm of resistance between the relay coil ground circuit terminal 85 and ground.
    • If greater than the specified range, test the ground circuit for an open/high resistance.
  3. Ignition ON, verify a test lamp illuminates between the relay switch B+ circuit terminal 30 and ground.
    • If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
  4. Ensure the parking brake is applied and the transmission is in PARK. Momentarily install a 40A fused jumper wire between the relay switch B+ circuit terminal 30 and the relay controlled output circuit terminal 87. The starter solenoid should engage and the engine should begin cranking.
    • If the solenoid does not engage, test the relay controlled output circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the starter motor.
  5. Connect a test lamp between the relay coil control circuit terminal 86 and the relay coil ground circuit terminal 85.
  6. With transmission range selector in park or neutral, cycle the ignition key between the OFF and CRANK position. The test lamp should turn ON and OFF when changing between the commanded states.
    • If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the ECM.
    • If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.
  7. If all circuits test normal, test or replace the starter relay.
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