SHIELD AS 6W9599 - Caterpillar

Diagnostic Trouble Code (DTC) Root Causes
When the shift lever is in the REVERSE position, a signal is sent to the Transmission/Chassis Electronic Control Module (ECM). The Transmission/Chassis ECM switches the backup alarm relay coil to frame ground. When the backup alarm relay is energized, +battery is applied to the backup alarm.If the backup alarm is always off, check these items:
The fuse for the relay is open.
The backup alarm has failed.
The machine harness has failed.
The machine harness is shorted to frame ground.
The backup alarm relay has failed.
The ECM has failed.Electrical Information and Schematic
Illustration 1 g03397146
Schematic of the backup alarmNote: The diagram above is simplified schematic of the relay connections for the Backup Alarm. The schematic is electrically correct. However, not all of the possible harness connectors are shown. For the complete schematic information reference one of the documents below (depending on your machine configuration).Refer to the latest revision of the Electrical Schematic, UENR3204, "725C (T4) Articulated Truck Electrical System" for machines with serial number prefix (S/N: TFB).Refer to the latest revision of the Electrical Schematic, UENR3205, "725C (LRC) Articulated Truck Electrical System" for machines with serial number prefix (S/N: LFB).Troubleshooting Procedure
Note: Prior to beginning this procedure, inspect the harness connectors that are involved in the circuit. Poor connections can often be the cause of a problem in an electrical circuit. Verify that all connections in the circuit are clean, secure and in good condition. If a problem with a connection is found, correct the problem and verify that this diagnostic code is active before performing this procedure.Note: The voltage to operate the Backup Alarm is supplied from location G5 on the fuse panel (7.5A). If the Backup Alarm is not functioning, the technician must confirm that continuity exists across the fuse.
Table 1
Troubleshooting Test Steps    Values    Results    
1. Check The Relay Circuit
A. Turn the key start switch and the battery disconnect switch to the OFF position.
B. Remove the backup alarm relay.
C. Turn the battery disconnect switch and the key start switch to the ON position.    
The backup alarm is OFF.     OK: The backup alarm is OFF.
Proceed to Test Step 2.
NOT OK: The backup alarm is ON.
Proceed to Test Step 4.    
2. Check The Relay Contacts
A. Turn key start switch and disconnect switch OFF.
B. Disconnect harness connection at relay.
C. Use the schematic to identify the wires for the coil contacts of the relay.
C. Check the resistance between coil contacts of the backup alarm relay.    
The resistance is less than 500 Ω.     OK: The resistance measurement is less than 500 Ω.
Proceed to Test Step 3.
NOT OK: The resistance is greater than 5K Ω.
Repair: The coil resistance is higher then the tolerance. The relay coil for the Backup Alarm has failed, replace the relay.
STOP    
3. Check The Return Circuit Of The Relay For A Short-To-Ground
A. At the connector of the backup alarm relay measure the resistance between contact 2 (wire 331-OR) J2-5 and frame ground.    
The measured resistance is greater than 5 Ω.