P0642-SENSOR REFERENCE VOLTAGE 1 CIRCUIT LOW
For a complete wiring diagram Refer to Section 8W
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.
| Possible Causes |
| SPEED SENSOR DTCS PRESENT |
| (T72) SENSOR SUPPLY VOLTAGE SHORT TO GROUND |
| INTERNAL SHORT IN THE ELECTROHYDRAULIC CONTROL UNIT ASSEMBLY |
| TRANSMISSION CONTROL MODULE |
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - NAG1 - STANDARD PROCEDURE)
Diagnostic Test
1. CHECK FOR SPEED SENSOR DTCSWith the scan tool, check for other transmission DTCs.
Are there any speed sensor DTCs present?
Yes
- Refer to the AUTOMATIC TRANSMISSION NAG1 - ELECTRICAL DIAGNOSTICS category and perform the appropriate symptom.
No
- Go To 2
2. CHECK FOR (T72) SENSOR SUPPLY VOLTAGE SHORT TO GROUND
Turn the ignition off to the lock position.
Disconnect the Electrohydraulic Control Unit Assembly harness connector.
Disconnect the TCM C2 harness connectors.
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. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - NAG1 - STANDARD PROCEDURE)
No
- Go To 3
3. CHECK FOR A SHORT IN THE ELECTOHYDRAULIC CONTROL UNIT
Reconnect the Electrohydraulic Control Unit Assembly harness connector.
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. Refer to 21- AUTOMATIC TRANSMISSION-AUTOMATIC TRANSMISSION NAG1/SERVICE INFORMATION for proper service procedures.
- Perform NAG1 TRANSMISSION VERIFICATION TEST. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - NAG1 - STANDARD PROCEDURE)
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 the TCM per the Service Information. Refer to 8 - ELECTRICAL/ELECTRONIC CONTROL MODULES/TRANSMISSION CONTROL MODULE for the appropriate service procedures.
- Perform NAG1 TRANSMISSION VERIFICATION TEST - VER 1. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - NAG1 - STANDARD PROCEDURE)