P1794
P1794 - Speed Sensor Ground Error
For a complete wiring diagram, refer to.
Theory of Operation
The input, transfer and output speed sensors use a common ground circuit. The loss of this common ground results in the input signal being sensed for both. After a reset in neutral, and after observing a specific ratio, the speed check fault counter will increment. Because the speed sensors and the thermistor share the same ground circuit, this DTC may indicate a loss of the common speed sensor ground. In some cases this fault will cause a gear ratio error DTC to be set.
• When Monitored: Every 7ms after a controller reset with transmission in neutral.
• Set Condition: After a PCM reset in neutral and Input and output sensor ratio equals 2.50 to 1.0 ± 50.0 rpm.
Always perform the pre-diagnostic troubleshooting procedure before proceeding. Refer to => Non-DTC Based Diagnostics. Pre-Diagnostic Troubleshooting Procedure
Test Step 1 - PCM and Wiring
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.
• Check connectors - Clean/repair as necessary.
• 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, monitor the Input, transfer and output speed sensor readings.
• Due to that the transmission simulator only has two speed sensor connectors, the output and transfer speed sensors need to share the same connector. You will be required to perform this test twice to validate both speed sensors wiring.
Does the input speed read 3000 rpm and the output speed read 1250 rpm, ± 50 rpm?
Yes
• Go to Test Step 2.
No
• Go to Test Step 3.
Test Step 2 - Speed Sensor Ground Circuit 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 Input, transfer and output speed sensor harness connectors.
• Disconnect the PCM C4 harness connector and install Miller tool #8815.
• Measure the resistance of the speed sensor ground circuits between the appropriate terminals of Miller tool #8815 and the Input, transfer, and output speed sensor harness connectors.
Is the resistance more than 5.0 ohms on either circuit?
Yes
• Repair the speed sensor ground circuits for an open.
• Perform the Transmission Verification Test. Refer to => Non-DTC Based Diagnostics. Transmission Verification Test
No
• Using the schematics as a guide, check the powertrain control module terminals for corrosion, damage, or terminal push out. Pay particular attention to all power and ground circuits. Check for tune-ups or 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 3 - 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 wires 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.
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.