Troubleshooting
784C, 785C, 789C, 793C and 793D Off-Highway Truck/Tractors Power Train Electronic Control System.
Backup Alarm Always On
System Operation Description:
The ECM applies power to the relay whenever the ECM receives a signal from the shift lever switch.
If the backup alarm is always ON, check these items.
* The harness circuit is shorted to the +battery circuit.
* The ECM has failed.
* The relay has failed.
Test Step 1. CHECK THE RELAY.
1. Turn the disconnect switch and the key start switch to the ON position. DO NOT start the engine. Ensure that the diagnostic code for the relay is active.
2. Disconnect the relay from the machine harness connector.
3. Observe the status of the diagnostic code. The diagnostic code is active or the diagnostic code is NOT active.
Expected Result:
The diagnostic code is NOT active when the relay is disconnected.
Results:
* YES - The circuit is correct. The relay has failed.
Repair: Repeat this test "CHECK THE RELAY" in order to verify that the relay has failed. Replace the relay.
Stop.
* NO - The diagnostic code remains active. Proceed to test step 2.
Test Step 2. CHECK THE SIGNAL CIRCUIT OF THE RELAY FOR A SHORT.
1. Turn the key start switch and the disconnect switch to the OFF position.
2. The machine harness connector remains disconnected from the relay.
3. Disconnect the machine harness connectors J1 and J2 from the ECM.
4. At the J1 and J2 machine harness connectors for the ECM, measure the resistance from the signal contact of the relay J2-37 (wire 321-BR) to all possible sources of positive voltage.
Expected Result:
Each resistance should be greater than 5000 ohms.
Results:
* YES - Each resistance should be greater than 5000 ohms. The machine harness is correct. The ECM has failed.
Repair: It is unlikely that the ECM has failed. Exit this procedure and perform this diagnostic code again. Recheck the status of the diagnostic code. The diagnostic code is active or the diagnostic code is NOT active. Replace the ECM if the cause is not found. See Testing and Adjusting, "Electronic Control Module (ECM) - Replace".
Stop.
* NO - There is a short in the machine harness. The short is between the signal contact of the relay and the circuit with the low resistance measurement.
Repair: Repair the machine harness or replace the machine harness.
Stop.
