When the ignition switch is placed in the START position, a discrete signal is supplied to the body control module (BCM) notifying it that the ignition is in the START position. The BCM then sends a message to the engine control module (ECM) that crank has been requested. The ECM then verifies that the transmission is in Park/Neutral. If it is, the ECM then supplies 12 volts to the control circuit of the crank relay. When this occurs, battery voltage is supplied through the switch of the crank relay to the starter solenoid.

Schematic Reference

Connector End View Reference

Component Connector End Views

Description and Operation

Starting System Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, verify that no crank relay, theft deterrent, or automatic transmission DTCs are set that would cause the ECM not to enable engine starting.

⇒	If DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

2. Ignition ON, observe the scan tool BCM Current Power Mode parameter. Place ignition switch in the crank position, the parameter should display Crank Request.

⇒	If not the specified value, refer to Power Mode Mismatch.

3. Transmission range selector in park , verify the scan tool ECM PNP Switch and TCM Park/Neutral parameters display Park/Neutral.

⇒	If the TCM parameter does not indicate Park/Neutral. Refer to Range Selector Displays Incorrect Range for the 4L60/4L65/4L70-E transmission. or Range Selector Displays Incorrect Range for the 4L80/4L85-E transmission.

⇒	If the ECM Park/Neutral parameter does not indicate Park/Neutral. Refer to ECM Park Switch Circuit Malfunction.

4. Transmission range selector in park or neutral, attempt to start the vehicle. The crank relay should click and the engine should begin cranking.

⇒	If the crank relay does not click or the engine does not crank, refer to Crank Relay Circuit Malfunction.

ECM Park Switch Circuit Malfunction (4L60/4L65/4L70-E or 4L80/4L85-E)

1. Ignition OFF, disconnect the harness connector at the PNP switch harness connector at the transmission.
2. Ignition ON, install a 3A fused jumper wire between the signal circuit terminal 3 and ground. Verify the scan tool ECM PNP switch parameter displays Park/Neutral.

⇒	If not the specified value, test the signal circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

3. If all circuits test normal, test or replace the Park/Neutral position switch.

Crank Relay Circuit Malfunction

1. Ignition OFF, disconnect the crank relay.
2. Ignition OFF, test for less than 5.0 Ω between the relay 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.
5. Momentarily install a 40A fused jumper wire between the B+ circuit terminal 30 and the control output circuit terminal 87.
6. The starter solenoid should engage and the engine should begin cranking.

⇒	If the solenoid does not engage, test the control output circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the starter motor.

7. Connect a test lamp between the coil control circuit terminal 86 and the coil ground circuit terminal 85.
8. With the transmission range selector in park or neutral, cycle the ignition 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.

9. If all circuits test normal, test or replace the crank relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.