797 and 797B Off-Highway Truck Power Train Electronic Control System Caterpillar

Illustration 1	g00877820
Schematic of the Sensor Power Supply

The sensor supply is a power supply that is internal to the ECM. The sensor supply provides +8 DCV in order to operate the sensors.

This diagnostic code will be recorded when the transmission ECM reads the voltage for the circuit of the sensor supply as being below normal.

The results of this diagnostic code are listed:

• All of the associated sensors will also log diagnostic codes.

The possible causes of the diagnostic code are listed:

• +8 DCV (wire 799-CF4-WH(White)) is shorted to ground between the sensor and the ECM.

• The ECM has failed. This is unlikely.

Test Step 1. CHECK THE SENSOR.

1. Ensure that the CID 0041 FMI 04 diagnostic code is present. The diagnostic code should be active.

2. Observe the status of the diagnostic code. The diagnostic code is active or the diagnostic code is NOT active.

3. Disconnect the machine harness from the sensor.

Expected Result:

The diagnostic code is NO longer active.

Results:

• OK - The diagnostic code is NO longer active. The sensor has failed.

  Repair: Replace the sensor.

  STOP

• NOT OK - The diagnostic code remains active. The sensor is NOT causing the diagnostic code. Proceed to Test Step 2.

Test Step 2. CHECK FOR A SHORT TO GROUND.

1. The sensor remains disconnected from the machine harness.

2. Turn the disconnect switch and the key start switch to the OFF position.

3. Disconnect the machine harness from the ECM.

4. At the ECM, measure the resistance between the contact of the sensor supply voltage and frame ground. See the schematic for more detailed information about the voltage of the sensor power supply for the sensor that is being tested.

Expected Result:

The resistance that is measured is greater than 5000 ohms.

Results:

• OK - The resistance that is measured is greater than 5000 ohms. The harness circuit is correct. The ECM may have failed.

  Repair: It is unlikely that the ECM has failed. Exit this diagnostic code procedure and perform this diagnostic code procedure again. Replace the ECM if the cause of the diagnostic code is NOT found. See Testing and Adjusting, "Electronic Control Module (ECM) - Replace".

  STOP

• NOT OK - The resistance that is measured is less than 5000 ohms. The machine harness has failed.

  Repair: There is a short between frame ground and the contact for the sensor supply voltage in the machine harness. Repair the machine harness or replace the machine harness.

  STOP