CX28 Transmission For Combat and Tactical Vehicles Transmission ; Transfer Case Electronic Control System Caterpillar

Illustration 1	g01238171
Schematic of the transmission oil temperature sensor

This diagnostic trouble code is recorded when the ECU reads the voltage of the temperature sensor as being below normal. The voltage of the signal wire is below normal and/or shorted to ground.

The possible causes of this diagnostic trouble code are listed here:

• The signal circuit in the machine harness is shorted to ground.

• The sensor has failed.

• The ECU has failed. The ECU failing is unlikely.

Test Step 1. CHECK THE SENSOR.

1. Turn the disconnect switch and the key start switch to the ON position.

2. Observe the status of the diagnostic trouble code. Then disconnect the machine harness from the sensor.

Expected Result:

If the sensor is not the cause of the diagnostic trouble code, the diagnostic trouble code remains active. If the sensor has failed, the diagnostic trouble code is NO longer active.

Results:

• OK - The diagnostic trouble code is active. The sensor is not causing the diagnostic trouble code. The machine harness or the ECU may have failed. Proceed to test step 2.

• NOT OK - This diagnostic trouble code is NO longer active. The machine harness is correct and the ECU is operating properly. The sensor has failed.

  Repair: Replace the sensor.

  STOP

Test Step 2. CHECK THE SIGNAL CIRCUIT FOR A SHORT TO GROUND.

1. The sensor remains disconnected from the machine harness.

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

3. Disconnect the machine harness from the ECU.

4. At the machine harness connector for the ECU, measure the resistance between the signal contact and frame ground.

Expected Result:

The resistance is greater than 5.0 ohms.

Results:

• OK - The resistance is greater than 5.0 ohms. The harness circuit resistance is correct. Proceed to test step 3.

• NOT OK - Resistance is less than 5.0 ohms. The machine harness has failed.

  Repair: There is a short between frame ground and the signal circuit in the machine harness. Repair the machine harness or replace the machine harness.

  STOP

Test Step 3. CHECK IF THE DIAGNOSTIC TROUBLE CODE REMAINS.

1. Inspect the harness connectors. Clean the contacts of the harness connectors and check the wires for damage to the insulation that is caused by excessive heat, battery acid, or chafing.

2. Perform a 45 N (10 lb) pull test on each of the wires that are associated with the circuit.

3. Reconnect all harness connectors. Make sure that the harness connectors are fully seated and that the clips for each connector are fastened.

4. Turn the disconnect switch and the key start switch to the ON position.

5. Operate the vehicle.

6. Determine if the SPN 177 FMI 4 is active.

Expected Result:

The diagnostic trouble code is no longer active.

Results:

• YES - The SPN 177 FMI 4 is not active. The diagnostic trouble code does not exist at this time. The initial diagnostic trouble code was probably caused by a poor electrical connection or a short at one of the harness connectors that was disconnected and reconnected. Resume normal machine operation.STOP

• NO - The SPN 177 FMI 4 is active. The diagnostic trouble code has not been corrected.

  Repair: Check the circuit again. If the Caterpillar Electronic Technician (ET) is available, use the Cat ET service tool in order to perform a "Wiggle Test" on the machine wiring harness. The "Wiggle Test" can detect shorts and opens in the machine wiring harness that are momentary or intermittent. It is unlikely that the ECU has failed. Exit this procedure and perform this procedure again. If the cause of the diagnostic trouble code is not found, replace the ECU. Prior to replacing an ECM, always contact your dealership for possible consultation with Caterpillar. This consultation may greatly reduce repair time. See Testing and Adjusting, "Electronic Control Unit - Replace".

  STOP