Powertrain Control Module (PCM) - 66RFE

P1794-SPEED SENSOR GROUND ERROR

Special Tools:






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

Theory of Operation

The transmission system uses two speed sensors, one to measure input rpm and one to measure output rpm. These inputs are essential for proper transmission operation. Therefore, the integrity of this data is verified through the following checks:

1) When in gear, if the gear ratio does not compare to a known gear ratio, the corresponding in-gear DTCs P0729 and P0731-36 is set.

2) An excessive change in input or output speeds indicating an intermittent signal which results in DTCs P0716 and/or P0721 set.

3) If the common speed sensor ground circuit is lost, both sensor inputs will read the signal from the input speed sensor at idle in neutral. Since the input speed sensor reads 60 teeth from the input clutch hub and the output speed sensor reads 30 teeth from the park gear, the result is an apparent speed ratio of 1:2 and may cause DTC P1794 to set when at a stop.

- When Monitored:
The gear ratio is monitored continuously while the transmission is in gear.

- Set Condition:
After a controller reset in neutral and a ratio of input to output, of 1:2. This DTC can take up to five minutes of problem identification before illuminating the MIL.





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

1. CHECK IF THE DTC IS CURRENT
1. Engine Running, Shift lever in park.
2. With the scan tool, read the Transmission Output and Input Speed Sensor states.

Is the Output Speed Sensor reading twice the Input Speed Sensor reading?

Yes

- Go To 2

No

- Go To 6

2. CHECK THE PCM AND WIRING
1. Turn the ignition OFF to the LOCK position.
2. Remove the Ignition Switch Feed fuse from the TIPM.

CAUTION: Removal of the Ignition Switch Feed fuse from the TIPM will prevent the vehicle from being started in gear.

WARNING: The Ignition Switch Feed fuse must be removed from the TIPM. Failure to do so can result in possible serious or fatal injury.

3. Install the (8333A).
4. With the Transmission Simulator, set the Input/Output Speed selector switch to the "3000/1000" position. Turn the Input/Output Speed switch to "ON".
5. Ignition ON, Engine Not Running.
6. With the scan tool, monitor the Input and Output Speed Sensor state.

Does the Input speed read 3000 rpm and the Output speed read 1000 rpm, within 50 rpm?

Yes

- Go To 6

No

- Go To 3

3. (T13) SPEED SENSOR GROUND CIRCUIT OPEN




1. Turn the ignition OFF to the LOCK position.
2. Disconnect the PCM C4 harness connector and connect the (8815A).
3. Disconnect the (8333A).

CAUTION: 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 the (8815A) to perform diagnosis.

4. Measure the resistance of the (T13) Speed Sensor Ground circuit between both Input and Output Speed Sensor harness connectors and the appropriate terminal of the (8815A).

Is the resistance above 5.0 ohms on any of the above measurements?

Yes

- Repair the (T13) Speed Sensor Ground circuit for an open.
- Perform the 66RFE TRANSMISSION VERIFICATION TEST. 66RFE Transmission Verification Test

No

- Go To 4

4. (T13) SPEED SENSOR GROUND CIRCUIT SHORTED TO GROUND




1. Measure the resistance between ground and the (T13) Speed Sensor Ground circuit.

Is the resistance below 5.0 ohms?

Yes

- Repair the (T13) Speed Sensor Ground circuit for a short to ground.
- Perform the 66RFE TRANSMISSION VERIFICATION TEST. 66RFE Transmission Verification Test

No

- Go To 5

5. (T13) SPEED SENSOR GROUND CIRCUIT SHORTED TO VOLTAGE




1. Ignition ON, Engine Not Running.
2. With the scan tool under TIPM, actuate the Transmission.
3. Measure the voltage of the (T13) Speed Sensor Ground circuit.

Is the voltage above 0.5 volts?

Yes

- Repair the (T13) Speed Sensor Ground circuit for a short to voltage.
- Perform the 66RFE TRANSMISSION VERIFICATION TEST. 66RFE Transmission Verification Test.

No

- Using the schematics as a guide, check the Powertrain Control Module (PCM) 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 PCM. With the scan tool, perform a Quick Learn.
- Perform the 66RFE TRANSMISSION VERIFICATION TEST. 66RFE Transmission Verification Test

6. CHECK THE WIRING AND CONNECTORS
1. The conditions necessary to set this DTC are not present at this time.
2. Using the schematics as a guide, inspect the wiring and connectors specific to this circuit.
3. Wiggle the wires while checking for shorted and open circuits.
4. With the scan tool, check the Event Data to help identify the conditions in which the DTC set.

Were any problems found?

Yes

- Repair as necessary.
- Perform the 66RFE TRANSMISSION VERIFICATION TEST. 66RFE Transmission Verification Test.

No

- Test Complete.