793F CMD Off-Highway Truck (Command for Hauling) Power Train Electronic Control System Caterpillar

Illustration 1	g02732776
Electrical system voltage connections

This diagnostic code is associated with the electrical system voltage that is supplied to the chassis electronic control module (ECM). When the ECM detects voltage that is erratic, intermittent, or incorrect, the ECM activates the diagnostic code FMI 02. The ECM activates this diagnostic code when the system voltage is not in the range of 18 VDC to 32 VDC for more than a few seconds.

The possible causes of this diagnostic code are listed below:

• A poor connection of the ground circuit

• A poor connection in the system voltage circuit

• A charging system component has failed.

• The ECM has failed. This is unlikely.

Before performing this procedure, verify the following:

• All circuit breakers and fuses in the charging circuit are closed.

• The supply circuit of the ECM system is closed.

If a problem with a circuit breaker or a fuse is found, correct the problem and verify that this diagnostic code is active before performing this procedure.

Test Step 1. CHECK THE SUPPLY CIRCUITS

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

2. Disconnect The machine harness connector "J1" from the ECM.

3. At the "J1" machine harness connector of the ECM, measure the resistance between the unswitched supply voltage (wire 177-RD(Red)) with the length of spare wire to each of the three positive contacts of the battery (wire 126-RD(Red)). The contacts for the three +battery connections are illustrated in the connection diagram at the beginning of this procedure.

4. At the "J1" machine harness connector of the ECM, measure the resistance between the switched supply voltage (wire 112-PU(Purple)) to the contacts of the keyswitch (wire 308-YL(Yellow)).

Expected Result:

Each resistance measurement is less than five ohms.

Results:

• OK - Each resistance measurement is less than five ohms. The supply circuit is correct. Reconnect the machine harness connector "J1" to the ECM before proceeding to the next test step. Proceed to Test Step 2.

• NOT OK - One or more of the resistance measurements is greater than five ohms. There is an open circuit or a poor connection in the supply circuit.

  Repair: The open or a possible poor connection is in the system voltage circuit of the machine harness of the ECM. Verify the following:

  • No open circuit breakers in the circuit

  • No fuses in the circuit
  Refer to the complete Electrical Schematic for this machine. Check all of the connectors and splice points that are in the supply circuit. Verify the following:

  • All connections are clean.

  • All connections are secure.

  • All connections are in good condition.
  Repair the machine harness or replace the machine harness.

  STOP

Test Step 2. CHECK THE GROUND CIRCUITS

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

2. Disconnect the machine harness connector "J1" from the ECM.

3. At the machine harness connector "J1" for the ECM, measure the resistance between each of the three negative battery contacts and frame ground. The contacts for the negative battery connections are illustrated in the connection diagram at the beginning of this procedure.

Expected Result:

Each resistance measurement is less than 5 ohms.

Results:

• OK - Each resistance measurement is less than 5 ohms. The ground circuit is correct. Proceed to Test Step 3.

• NOT OK - One or more of the resistance measurements is greater than five ohms. There is an open circuit or a poor connection in the ground circuit.

  Repair: The open or a possible poor connection is in the ground wire of the ECM. The wires are A253-BK(Black) contacts J1-13, 23, and 57 in the machine harness. Refer to the complete machine Electrical Schematic for this machine. Check the connection point between the frame ground for wires A253-BK(Black). Verify the following:

  • The ground connection is clean.

  • The ground connection is tight.

  • The ground connection is in good condition.
  Check all the connectors and splice points that are in the ground circuit. Verify the following:

  • All connections are clean.

  • All connections are secure.

  • All connections are in good condition.
  Repair the machine harness or replace the machine harness.

  STOP

Test Step 3. CHECK THE CURRENT OUTPUT OF THE ALTERNATOR

1. Before performing this test step, partially discharge the batteries. Perform one of the following in order to discharge the batteries:

   • Crank the engine for approximately 30 seconds without starting the engine.

   • Turn the lights and leave the lights on about 10 minutes with the engine off.

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

3. Connect one multimeter probe to the "B" terminal of the alternator and connect one multimeter probe to the case of the alternator. 403-GN(Green) is connected to the "B" terminal of the alternator and the case of the alternator is a frame ground.

4. Connect a clamp on 8T-0900 AC/DC Clamp-on Ammeter or an equivalent ammeter around the 403-GN(Green) output wire .

5. Turn the disconnect switch to the ON position.

6. Start the engine.

7. Run the engine at approximately half throttle.

8. Immediately check the output current of the alternator.

Expected Result:

The output current of the alternator at half throttle is approximately 40 - 50 amp. The full current output rating of the alternator is approximately 95 amp. The current measurement is stable.

Results:

• OK - The output current of the alternator at half throttle is approximately 40 - 50 amp. The output current is stable. Proceed to Test Step 4.

• NOT OK - The output current of the alternator at half throttle is not stable. The measurement is showing a large variation of current values. The current output of the alternator is not correct.

  Repair: Repair the alternator or replace the alternator. Refer to Troubleshooting, REHS0354 manual for detailed instructions on troubleshooting the charging system components.

  STOP

Test Step 4. CHECK THE VOLTAGE OUTPUT OF THE ALTERNATOR

The multimeter and the ammeter are still connected. The engine is running.

1. Run the engine for approximately ten minutes at half throttle.

2. Check the voltage at the alternator output terminal "B".

Expected Result:

The voltage measurement is 28 ±1 VDC. There is little variation or no variation in the voltage measurement.

Results:

• OK - The voltage measurement is 28 ±1 VDC. The voltage is stable. The voltage output of the alternator is correct. Proceed to Test Step 5.

• NOT OK - The voltage is not stable. There is a large variation in the measured voltage. The voltage output is not correct.

  Repair: Repair the alternator or replace the alternator. Refer to Troubleshooting, REHS0354 manual for detailed instructions on troubleshooting the charging system components.

  STOP

Test Step 5. CHECK SYSTEM VOLTAGE AT THE ECM

Use a calibrated digital multimeter for the measurements in this procedure.

1. Turn the key start switch and the battery disconnect switch to the OFF position.

2. Perform the following with a multimeter:

   1. Insert a 7X-1710 Multimeter Probe along the power contact of the "J1" machine harness connector for the ECM. The wires at the power contacts are J1-31, 38, and 39 (wire 126-RD(Red)).

   2. Use the other multimeter probe to make contact to the frame ground.

3. Turn the disconnect switch to the ON position.

4. Start the engine and run the engine at half throttle.

5. Measure the voltage between each unswitched +battery contacts of the ECM J1-31, 38, and 39 (wire 126-RD(Red)) and frame ground.

6. While you observe the voltage measurement from the multimeter, wiggle the machine harness connectors for the ECM. Pull on the wires and move the wires gently.

Expected Result:

All voltage measurements are in the range of 18 VDC to 32 VDC at all times during the manipulation of the harness and connectors.

Results:

• OK - The voltage measurements are in the range of 18 VDC to 32 VDC. The system voltage is correct at all times during the manipulation of the harness and connectors. Proceed to Test Step 6.

• NOT OK - The voltage measurements are unstable. The voltage measurements are showing a large amount of variation. The manipulation of the machine harness and connectors does not affect the measurement. Repeat these steps starting with Test Step 1.

• NOT OK - The measurement from the positive contact of the battery to the frame ground is stable. The measurement is in the range of 18 VDC to 32 VDC. However, the measurement between the +battery contact of the ECM and the negative battery contact of the ECM is not stable in the correct range at all times. Repeat these steps starting with Test Step 1.

Test Step 6. CHECK THE STATUS OF THE DIAGNOSTIC CODE

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

2. Reconnect any harness connectors that were disconnected and checked.

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

4. Operate the machine.

5. Determine if the CID 0168 FMI 02 diagnostic code is active.

Expected Result:

The CID CID 0168 FMI 02 diagnostic code is active.

Results:

• OK - The CID 0168 FMI 02 diagnostic code is active.

  Repair: Perform this procedure again. Verify the following:

  • The voltages of the alternator are correct.

  • The circuit connections are in good condition.

  • The circuit connections are tight.
  If the problem remains and no cause can be found, the ECM may have failed.

  It is unlikely that the ECM has failed. Exit this procedure and perform this procedure again. Prior to replacing an ECM, always contact your dealership's Technical Communicator for possible consultation with Caterpillar. This consultation may greatly reduce repair time. If the ECM requires replacement, refer to the Testing and Adjusting, "Electronic Control Module (ECM) - Replace" Section of this manual.

  STOP

• NOT OK - The CID 0168 FMI 02 diagnostic code is not active. The problem does not exist at this time.

  Repair: The initial diagnostic 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