Transmission Control Module (TCM) - NAG1

P0642-SENSOR REFERENCE VOLTAGE 1 CIRCUIT LOW





For a complete wiring diagram, refer to the Wiring Information.

Theory of Operation

The Sensor Supply Voltage circuit supplies a 6-volt power supply to the two input speed sensors. The Sensor Supply Voltage circuit is constantly monitored for correct voltage between 4.8 to 7.2 volts. If the voltage does not stay within the 4.8 to 7.2 volt range, the appropriate DTC will set.

- When Monitored:
Continuously with the ignition on and no undervoltage condition exist.

- Set Condition:
When the monitored sensor voltage is not within specified limits drops below 4.8 volts.





Always perform the NAG1 Pre-Diagnostic Troubleshooting procedure before proceeding. NAG1 Pre-Diagnostic Troubleshooting Procedure

1. CHECK FOR SPEED SENSOR DTCS
1. With the scan tool, check for other transmission DTCs.

Are there any speed sensor DTCs present?

Yes

- Perform NAG1 TRANSMISSION VERIFICATION TEST. NAG1 Transmission Verification Test

No

- Go To 2

2. CHECK FOR (T72) SENSOR SUPPLY VOLTAGE SHORT TO GROUND




1. Turn the ignition off to the lock position.
2. Disconnect the Electrohydraulic Control Unit Assembly harness connector.
3. Disconnect the TCM C2 harness connectors.
4. Measure the resistance between ground and the (T72) Sensor Supply Voltage circuit in the TCM harness connector.

Is the resistance below 5.0 ohms?

Yes

- Repair the (T72) Sensor Supply Voltage circuit for a short to ground
- Perform NAG1 TRANSMISSION VERIFICATION TEST. NAG1 Transmission Verification Test

No

- Go To 3

3. CHECK FOR A SHORT IN THE ELECTROHYDRAULIC CONTROL UNIT




1. Reconnect the Electrohydraulic Control Unit Assembly harness connector.
2. Measure the resistance between ground and the (T72) Sensor Supply Voltage circuit in the TCM harness connector.

Is the resistance below 5.0 ohms?

Yes

- Repair the Electrohydraulic Control Unit Assembly for a short to ground.
- Perform NAG1 TRANSMISSION VERIFICATION TEST. NAG1 Transmission Verification Test

No

- Using the schematics as a guide, check the Transmission Control Module (TCM) terminals for corrosion, damage, or terminal push out. Pay particular attention to all power and ground circuits. If no problems are found, replace and program the TCM.
- Perform NAG1 TRANSMISSION VERIFICATION TEST. NAG1 Transmission Verification Test