P1794-SPEED SENSOR GROUND ERROR





For a complete wiring diagram Refer to Section 8W

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 P0731–36).

2) An excessive change in input or output speeds indicating signal intermittent which may result in the DTCs P0715 and/or P0720 to 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.


Possible Causes
  
(T13) SPEED SENSOR GROUND CIRCUIT OPEN
  
(T13) SPEED SENSOR GROUND CIRCUIT SHORT TO GROUND
  
(T13) SPEED SENSOR GROUND CIRCUIT SHORT TO VOLTAGE
  
TRANSMISSION CONTROL MODULE
  

Always perform the 45RFE/545RFE Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 45RFE/545RFE - STANDARD PROCEDURE)



Diagnostic Test

1.CHECK TO SEE IF DTC P1794 IS CURRENT




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

Yes
  • Go To 2


No
  • Go To 6

2.TCM AND WIRING



CAUTION: Removal of the Starter Relay will prevent the vehicle from being started in gear.


WARNING: The Starter Relay must be removed from the PDC. Failure to do so can result in personal injury or death.


NOTE: Failure to remove the Starter Relay can cause a Transmission - No Response condition.







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








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

Yes


No
  • Go To 4

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





Is the resistance below 5.0 ohms?

Yes


No
  • Go To 5

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






Is the voltage above 0.5 volt?

Yes


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. With the scan tool, perform QUICK LEARN.
  • Perform 45RFE/545RFE TRANSMISSION VERIFICATION TEST - VER 1. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 45RFE/545RFE - STANDARD PROCEDURE)

6.INTERMITTENT WIRING AND CONNECTORS






Where there any problems found?

Yes


No
  • Test Complete.