P0791
P0791 - Transfer Speed Sensor Circuit
For a complete wiring diagram, refer to.
Theory of Operation
The transmission control system uses three speed sensors, one to measure input rpm, one to measure transfer speed output rpm of the main transmission assembly, and a third to measure the final transmission output rpm. These inputs are essential for proper transmission operation. Therefore, the integrity of this data is verified through system checks.
• When Monitored: The transmission gear ratio is monitored continuously while the transmission is in gear.
• Set Condition: If there is an excessive change in the transfer rpm in any gear.
Always perform the pre-diagnostic troubleshooting procedure before proceeding. Refer to => Non-DTC Based Diagnostics. Pre-Diagnostic Troubleshooting Procedure
Test Step 1 - Determine if the DTC is Current
• Properly support the vehicle.
• Be sure to keep hands and feet clear of rotating wheels.
• Start the engine in park.
• Raise the drive wheels off of the ground.
• Firmly apply the brakes and place the transmission selector in drive.
• Release the brakes and allow the drive wheels to spin freely.
• The drive wheels must be turning at this point.
• With the scan tool, read the transfer rpm
Is the transfer rpm less than 100?
Yes
• Go to Test Step 2.
No
• Go to Test Step 8.
Test Step 2 - Check the PCM and Wiring with the Transmission Simulator
The starter solenoid relay must be removed from the TIPM. Failure to do so can result in personal injury or death.
Removal of the starter solenoid relay from the TIPM will prevent the vehicle from being started in gear.
• Turn the ignition OFF to the lock position.
• Remove the starter solenoid relay from the TIPM.
• Install the transmission simulator, Miller tool #8333 and the electronic transmission adapter kit.
• Ignition ON, engine not running.
• With the transmission simulator, set the "input/output speed" switch to "ON" and the rotary switch to the "3000/1250" position.
• With the scan tool , read the input and transfer rpm.
Does the input rpm read 3000 and the transfer rpm read 1250 (within 50 rpm)?
Yes
• Replace the transfer speed sensor.
• Perform the Transmission Verification Test. Refer to => Non-DTC Based Diagnostics. Transmission Verification Test
No
• Go to Test Step 3.
Test Step 3 - Check the Transfer Speed Signal Circuit For An Open
Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool #8815 to perform diagnosis.
• Turn the ignition OFF to the lock position.
• Disconnect the PCM C4 harness connector.
• Disconnect the transfer speed sensor harness connector.
• Measure the resistance of the circuit between the appropriate terminal of Miller tool #8815 and the harness connector.
Is the resistance more than 5.0 ohms?
Yes
• Repair the circuit for an open.
• Perform the Transmission Verification Test. Refer to => Non-DTC Based Diagnostics. Transmission Verification Test
No
• Go to Test Step 4.
Test Step 4 - Check the Speed Sensor Ground Circuit For An Open
• Measure the resistance of the ground circuit between the appropriate terminal of Miller tool #8815 and the harness connector.
Is the resistance more than 5.0 ohms?
Yes
• Repair the circuit for an open.
• Perform the Transmission Verification Test. Refer to => Non-DTC Based Diagnostics. Transmission Verification Test
No
• Go to Test Step 5.
Test Step 5 - Check the Transfer Speed Sensor Signal Circuit For A Short To Ground
• Measure the resistance between ground and the signal circuit.
Is the resistance less than 5.0 ohms?
Yes
• Repair the circuit for a short to ground.
• Perform the Transmission Verification Test. Refer to => Non-DTC Based Diagnostics. Transmission Verification Test
No
• Go to Test Step 6.
Test Step 6 - Check the Transfer Speed Sensor Signal Circuit For A Short To Voltage
• Ignition ON, engine not running.
• With the scan tool in the TIPM, actuate the TCM output.
• Measure the voltage of the signal circuit.
Is the voltage more than 0.5 volt?
Yes
• Repair the circuit for a short to voltage.
• Perform the Transmission Verification Test. Refer to => Non-DTC Based Diagnostics. Transmission Verification Test
No
• Go to Test Step 7.
Test Step 7 - Check the Speed Sensor Ground Circuit For A Short To Voltage
• Continue to actuate the TCM output.
• Measure the voltage of the ground circuit.
Is the voltage more than 0.5 volts?
Yes
• Repair the circuit for a short to voltage.
• Perform the Transmission Verification Test. Refer to => Non-DTC Based Diagnostics. Transmission Verification Test
No
• Using the schematics as a guide, check the PCM terminals for corrosion, damage, or terminal push out. Pay particular attention to all power and ground circuits. Check for service bulletins for any possible causes that may apply. If no problems are found, replace and program the PCM. Refer to With the scan tool, perform Quick Learn.
• Perform the Transmission Verification Test. Refer to => Non-DTC Based Diagnostics. Transmission Verification Test
Test Step 8 - Intermittent Wiring and Connectors
• The conditions necessary to set the DTC are not present at this time.
• Using the schematics as a guide, inspect the wiring and connectors specific to this circuit.
• Wiggle the wiring and connectors while checking for shorted and open circuits.
• With the scan tool, check the DTC event data to help identify the conditions in which the DTC was set.
• Check for service bulletins for any possible causes that may apply.
Were there any problems found?
Yes
• Repair as necessary.
• Perform the Transmission Verification Test. Refer to => Non-DTC Based Diagnostics. Transmission Verification Test
No
• Test complete.