725, 730, 735, 740 and 740 EJECTOR Articulated Trucks Power Train Electronic Control System Caterpillar

Illustration 1	g00759642
Schematic of the Pressure Switch (Service Brake)

This diagnostic code is recorded when the ECM reads the voltage of the service brake (wire 720-PU) as being above normal for seven hours continuously during machine operation.

The possible causes of this diagnostic code are listed:

• There is an open in wire (J764-BR). The open is between the switch and the return contact (J1-12).

• The switch has failed.

• Wire 720-PU is open between the switch and the ECM (J1-46).

• The ECM has failed. This is unlikely.

The results of this diagnostic code are listed:

• The indicator for the service brake on the display is NEVER ON.

• When the service brake is being applied, the downshift points do not occur at a RPM that is higher than normal. The elevated shift function does not operate.

Test Step 1. CHECK THE SWITCH.

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

2. Ensure that the diagnostic code is active.

3. Disconnect the switch from the machine harness.

4. Install a jumper wire between the signal wire (720-PU) and the return wire (J764-BR) at the pressure switch.

Expected Result:

The diagnostic code changes from being active to being NOT active as the jumper is installed and then removed.

Results:

• OK - The diagnostic code is active. Then, the diagnostic code is NOT active as the jumper is installed and the jumper is removed. The pressure switch is failed.

  Repair: Repeat this test "CHECK THE SWITCH" in order to verify that the switch has failed. Replace the service brake pressure switch.

  Stop.

• NOT OK - The diagnostic code remains active. The diagnostic code does NOT change. The machine harness has failed or the ECM has failed. Proceed to test step 2.

Test Step 2. CHECK FOR AN OPEN CIRCUIT.

1. Remove the jumper wire of the previous test "CHECK THE SWITCH".

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

3. Disconnect the J1 and J2 machine harness connectors from the ECM.

4. At the pressure switch, install a jumper wire from the signal wire (720-PU) to frame ground.

5. At the pressure switch, install a jumper wire from the return wire (J764-BR) to frame ground.

6. At the J1 machine harness connector for the ECM, measure the resistance from contact J1-46 (wire 720-PU) to frame ground.

7. At the J1 machine harness connector for the ECM, measure the resistance from contact J1-12 (wire J764-BR) to frame ground.

Expected Result:

Each resistance measurement should measure less than 5000 ohms.

Results:

• OK - Each resistance measurement is less than 5000 ohms. Proceed to test step 3.

• NOT OK - One or both resistance measurements is NOT less than 5000 ohms. There is an open in the machine harness. The open is in the wire (s) with the high resistance measurement.

  Repair: Repair the machine harness or replace the machine harness. Verify that the diagnostic code is no longer active.

  Stop.

Test Step 3. CHECK FOR A SHORT TO THE +BATTERY CIRCUIT.

1. The key start switch and the disconnect switch remain in the OFF position.

2. Remove all jumper wires that were previously installed.

3. The J1 and J2 machine harness connectors remain disconnected from the ECM.

4. At the J1 machine harness connector for the ECM, measure the resistance from the signal contact J1-46 (wire 720-PU) to all possible sources of positive voltage. The possible sources of positive voltage are contacts: 725 and 730 machines J1-11, J1-58, J2-1, J2-15, J2-16, J2-17, J2-23, J2-25, J1-39, J2-40, J2-44, J2-45, J2-47, J2-51, J2-57, J2-58, J2-60, J2-62, J2-63, J2-64, J2-65, J2-68 and J2-70. 735 machines J1-11, J1-58, J2-1, J2-14, J2-15, J2-16, J2-17, J2-23, J2-25, J1-39, J2-40, J2-44, J2-45, J2-47, J2-51, J2-57, J2-58, J2-60, J2-61, J2-62, J2-63, J2-64, J2-65, J2-68 and J2-70. 740 machines J1-11, J1-58, J2-1, J2-14, J2-15, J2-16, J2-17, J2-23, J2-25, J1-39, J2-40, J2-44, J2-45, J2-47, J2-55, J2-57, J2-58, J2-60, J2-68 and J2-70.

Expected Result:

The resistance for each measurement should be greater than 5000 ohms.

Results:

• OK - Each resistance measurement is greater than 5000 ohms. The harness is correct. The pressure switch has failed.

  Repair: Replace the pressure switch and verify that the new pressure switch corrects the problem. If the diagnostic code remains exit this procedure. It is unlikely that the ECM has failed. Perform this diagnostic code procedure again. If the cause of the diagnostic code is not found, replace the ECM. See Testing and Adjusting, "Electronic Control Module (ECM) - Replace".

  Stop.

• NOT OK - Each resistance measurement is NOT greater than 5000 ohms. The machine harness has failed.

  Repair: There is a short in the machine harness. The short is between the signal wire (720-PU) and the wire with the low resistance measurement. Repair the machine harness or replace the machine harness.

  Stop.