The engine control module (ECM) controls engine cranking based on a power mode input and the status of the park/neutral position (PNP) switch or clutch pedal position sensor. With the transmission in Park/Neutral, voltage at the ECM PNP switch signal circuit is low. This indicates to the ECM that conditions are acceptable for cranking. When a power mode crank request is seen, the ECM applies voltage to the starter relay control circuit. This energizes the coil side of the relay, which pulls the switch side of the relay closed, applying voltage to the starter terminal S X2 and engaging the starter solenoid.

Schematic Reference

Starting and Charging Schematics

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, observe the scan tool ECM PNP Switch parameter while shifting the transmission through each range. The reading should change between In Gear and Park/Neutral. (automatic only)

⇒	If not the specified values, refer to PNP Switch Malfunction.

2. Ignition ON, command the Starter Relay ON and OFF with a scan tool, or turn the ignition switch to the START position. The starter solenoid should engage and the engine should crank.

⇒	If the solenoid does not engage, refer to STRTR Relay Malfunction.

⇒	If the solenoid engages but the engine does not crank, refer to Starter Solenoid Clicks, Engine Does Not Crank.

PNP Switch Malfunction (M30 4L60E, M70 4L70E)

1. Inspect the range selector lever cable for proper adjustment. Refer to Range Selector Lever Cable Adjustment.
2. Inspect the park/neutral position switch for proper adjustment. Refer to Park/Neutral Position Switch Adjustment.
3. Ignition OFF. Disconnect the harness connector from the park/neutral switch. Refer to Park/Neutral Position Switch Replacement.
4. Ignition ON. Probe the ground circuit terminal 7 with a test lamp that is connected to B+. Verify that the test lamp illuminates.

⇒	If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

5. Verify that the scan tool PNP Switch parameter is In Gear.

⇒	If not the specified value, test the signal circuit terminal 3 for a short to ground. If the circuit tests normal, replace the ECM.

6. Install a 3 A fused jumper wire between the ground circuit terminal 7 and the signal circuit terminal 3. Verify that the scan tool PNP Switch parameter is Park/Neutral.

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

7. If all circuits test normal, test or replace the park/neutral position switch.

PNP Switch Malfunction (MYC 6L80E, MYD 6L90E)

1. Inspect the range selector lever cable for proper adjustment. Refer to Range Selector Lever Cable Adjustment.
2. Ignition OFF. Disconnect the 16-way harness connector from the transmission.
3. Ignition ON. Measure voltage on the park/neutral signal circuit terminal 3 with a DMM. Verify that there is greater than 10 V on the park/neutral signal circuit.

⇒	If less than the specified range, test the circuit for a short to ground, or an open/high resistance. If the circuit tests normal, replace the ECM.

4. Ignition OFF, reconnect the 16-way harness connector to the transmission.
5. Remove the automatic transmission fluid pan. Refer to Automatic Transmission Fluid, Fluid Pan and/or Filter Replacement.
6. Disconnect the manual shift shaft position switch connector at the control solenoid valve assembly. Refer to Manual Shift Shaft Position Switch Replacement.
7. Ignition ON, verify that the scan tool PNP Switch parameter displays In Gear.

⇒	If not the specified value, replace the ECM.

8. Connect a 3 A fused jumper wire between the park/neutral signal terminal F at the control solenoid valve assembly and ground. The scan tool PNP Switch parameter should display Park/Neutral when the terminal is connected to ground.

⇒	If not the specified value, replace the ECM.

9. If all circuits test normal, test or replace the manual shift shaft position switch.

PNP Switch Malfunction (MW7 Allison)

1. Inspect the range selector lever cable for proper adjustment. Refer to Range Selector Lever Cable Adjustment.
2. Ignition OFF. Disconnect the engine chassis harness to transmission internal harness connector X175.
3. Verify that the scan tool PNP Switch parameter is In Gear.

⇒	If not the specified value, test the signal circuit terminal R for a short to ground. If the circuit tests normal, replace the ECM.

4. Install a 3A fused jumper wire between the signal circuit terminal R and ground. Verify that the scan tool PNP Switch parameter is Park/Neutral.

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

5. If all circuits test normal, test or replace the transmission internal mode switch.

STRTR Relay Malfunction

1. Ignition OFF, disconnect the STRTR relay.
2. Ignition ON, transmission in Park/Neutral. Verify that a test lamp illuminates between the relay coil ground circuit terminal and B+.

⇒	If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

3. Probe the STRTR Relay switch B+ circuit terminal with a test lamp that is connected to ground. Verify that the test lamp illuminates.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. Inspect the 175 A MEGA fuse for an open. If the circuit tests normal and the 175 A MEGA fuse is open, test the relay controlled output circuit for a short to ground.

4. Connect a test lamp between the relay coil ground circuit terminal and the relay coil control circuit terminal.
5. Ignition ON, transmission in Park/Neutral. Command the Starter Relay ON and OFF with a scan tool, or turn the ignition switch between the START and RUN positions. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the relay coil control circuit for a short to voltage. If the circuit tests normal, replace the ECM.

⇒

If the test lamp is always OFF, test the relay coil control circuit for a short to ground or and open/high resistance. Inspect the STRTR fuse for an open. If the STRTR fuse is open, test the relay controlled output circuit for a short to ground, or replace the starter solenoid (LMM) or starter. If the circuit and fuse test normal, replace the ECM.

6. Connect the STRTR relay.
7. Install a test lamp between the starter ring terminal S X2 and ground.

Important: The engine may begin to crank.

8. Ignition ON, transmission in Park, command the Starter Relay ON with a scan tool, or turn the ignition switch to the START position. The test lamp should illuminate.

⇒	If the test lamp does not illuminate, test the crank voltage circuit for an open/high resistance. If the circuit tests normal, test or replace the STRTR relay.

9. If all circuits test normal, test or replace the starter solenoid (LMM 6.6L Diesel), or the starter (all other engines).

PNP Switch Test (M30 4L60E, M70 4L70E)

1. Inspect the range selector lever cable for proper adjustment. Refer to Range Selector Lever Cable Adjustment.
2. Inspect the park/neutral position switch for proper adjustment. Refer to Park/Neutral Position Switch Adjustment.
3. Ignition OFF. Disconnect the harness connector from the park/neutral switch. Refer to Park/Neutral Position Switch Replacement.
4. With the PNP switch in the following positions, test for infinite resistance between the ground terminal 7 and the signal terminal 3.

•

R

•

OD

•

D

•

2

•

1

⇒	If not the specified value, replace the PNP switch.

5. With the PNP switch in the following positions, test for less than 2 Ω between the ground terminal 7 and the signal terminal 3.

•

P

•

N

⇒	If greater than the specified range, replace the PNP switch.

Relay Test

1. Ignition OFF, disconnect the STRTR relay.
2. Test for 60-180 Ω between terminals 85 and 86.

⇒	If not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 86

•

30 and 87

•

30 and 85

•

85 and 87

⇒	If not the specified value, replace the relay.

4. Install a 20 A fused jumper wire between relay terminal 85 and 12 V. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 Ω between terminals 30 and 87.

⇒	If greater than specified range, replace the relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.