Troubleshooting
784C, 785C, 789C, 793C and 793D Off-Highway Truck/Tractors Power Train Electronic Control System.
Backup Alarm Always Off
System Operation Description:
The ECM applies power to the backup alarm whenever the ECM receives a reverse signal from the position sensor for the transmission/chassis ECM.
One of the following conditions can cause the backup alarm to remain OFF:
* The backup alarm has failed.
* The harness circuit is open.
* The harness circuit is shorted to ground.
* The ECM has failed.
* The backup alarm relay has failed.
Note: The following test steps will cause related diagnostic codes to be logged.
Test Step 1. CHECK THE BACKUP ALARM.
1. Turn the key start switch and the disconnect switch to the OFF position.
2. Disconnect the backup alarm from the machine harness.
3. Use jumpers to connect the respective terminals of the backup alarm to the +battery terminal and ground.
Expected Result:
The backup alarm sounds when power is applied.
Results:
* YES - The backup alarm sounds when power is applied. The backup alarm operates correctly. Proceed to test step 2.
* NO - The backup alarm does not sound when power is applied.
Repair: The backup alarm has failed. Replace the backup alarm. Verify that the new backup alarm corrects the problem.
Stop.
Test Step 2. CHECK THE ALARM CIRCUIT FROM THE RELAY FOR AN OPEN.
1. The disconnect switch and the key start switch remain in the OFF position.
2. Disconnect the machine harness connector from the relay.
3. The machine harness connector from the backup alarm remains disconnected.
4. Measure the resistance from contact 1 (wire 612-GY) of the backup alarm connector. Measure the resistance to contact 87 of the harness connector for the relay.
Expected Result:
The resistance that is measured is less than 5 ohms.
Results:
* YES - The machine harness is correct. Proceed to test step 3.
* NO - The resistance that is measured is greater than 5000 ohms. The harness for the backup alarm has failed.
Repair: Repair the harness or replace the harness.
Stop.
Test Step 3. 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. The machine harness connector for the relay remains disconnected.
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 4.
Test Step 4. CHECK THE SIGNAL CIRCUIT FOR A SHORT TO GROUND
1. The machine harness remains disconnected from the backup alarm.
2. Turn the key start switch to the OFF position.
3. Disconnect the machine harness connector J1 and connector J2 from the ECM.
4. At machine harness connector J2, measure the resistance from the J2-37 (backup alarm signal) to all possible sources of ground.
Expected Result:
Each resistance should be greater than 5000 ohms.
Results:
* YES - Each resistance measurement is greater than 5000 ohms. The resistance is within an acceptable range. The machine harness is correct. Proceed to test step 5.
* NOT OK - The resistance that is measured is less than 5 ohms for one or more of the measurements.
Repair: There is a short in the machine harness. The short is between wire 321-BR (backup alarm signal) and the circuit with the low resistance measurement. Repair the machine harness or replace the machine harness.
Stop.
Test Step 5. CHECK THE SIGNAL CIRCUIT FOR AN OPEN
1. The key start switch and the disconnect switch remain in the OFF position.
2. Reconnect the relay to the harness connector.
3. The machine harness connector J2 remains disconnected from the ECM.
4. At machine harness connector J2, measure the resistance from J2-37 (backup alarm signal) to J2-3 (return).
Expected Result:
The resistance should measure approximately 360 ohms.
Results:
* OK - The resistance measures approximately 360 ohms. The machine harness is correct. Proceed to test step 6.
* NOT OK - The resistance that is measured is greater than 5000 ohms.
Repair: There is an open circuit in the machine harness. The open is in wire 321-BR (backup alarm signal) or in wire E790-PK (return) of the machine harness. Repair the machine harness or replace the machine harness.
Stop.
Test Step 6. CHECK IF THE PROBLEM REMAINS.
1. Inspect the harness connectors and clean the contacts of the harness connectors.
2. Reconnect all harness connectors.
3. Start the engine.
4. Operate the shift lever.
Expected Result:
The backup alarm works while the shift lever is in REVERSE.
Results:
* YES - The backup alarm works while the shift lever is in REVERSE. The diagnostic code is NOT present. The problem does not exist at this time. The initial problem 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 backup alarm does not work while the shift lever is in REVERSE.
Repair: The problem has not been corrected. It is unlikely that the ECM has failed. Exit this procedure and perform this 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.
