545RFE Automatic Transmission

P1794-SPEED SENSOR GROUND ERROR





For a complete wiring diagram refer to Diagrams/Electrical. Electrical Diagrams


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 trouble code is set (DTCs P0729 and P0731-36).


2) An excessive change in input or output speeds indicating signal intermittent which will result in the DTCs P0716 and/or P0721 being 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 the 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 to 2. This DTC can take up to five minutes of problem identification before illuminating the MIL.







Always perform the 45RFE/545RFE Pre-Diagnostic Troubleshooting Procedure before proceeding.



Diagnostic Test


1. CHECK TO SEE IF DTC P1794 IS CURRENT

Engine Running, Shift lever in park.

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. PCM AND WIRING

Turn the ignition off to the lock position.

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 personal injury or death.

Install the Transmission Simulator, Miller tool 8333.

With the Transmission Simulator, set the Input/Output Speed selector switch to the "3000/1000" position. Turn the Input/Output Speed switch to "ON".

Ignition on, engine not running.

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





Turn the ignition off to the lock position.

Disconnect the PCM C4 harness connector and connect Miller tool #8815.

Disconnect the Transmission Simulator, Miller tool #8333.


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 Miller tool #8815 to perform diagnosis.

Measure the resistance of the (T13) Speed Sensor Ground circuit between both Input and Output Speed Sensor harness connectors and the appropriate terminal of Miller tool #8815 to the.

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 45RFE/545RFE TRANSMISSION VERIFICATION TEST.

No

- Go To 4



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





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 45RFE/545RFE TRANSMISSION VERIFICATION TEST.

No

- Go To 5



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





Ignition on, engine not running.

With the scan tool under TIPM, actuate the Transmission.

Measure the voltage of the (T13) Speed Sensor Ground circuit.

Is the voltage above 0.5 volt?

Yes

- Repair the (T13) Speed Sensor Ground circuit for a short to voltage.
- Perform 45RFE/545RFE 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 the PCM. With the scan tool, perform QUICK LEARN.
- Perform 45RFE/545RFE TRANSMISSION VERIFICATION TEST. 45RFE/545RFE Transmission Verification Test



6. INTERMITTENT WIRING AND CONNECTORS

The conditions necessary to set this 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.

Where there any problems found?

Yes

- Repair as necessary.
- Perform 45RFE/545RFE TRANSMISSION VERIFICATION TEST. 45RFE/545RFE Transmission Verification Test

No

- Test Complete.