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Table 1
Diagnostic Codes Table    
Code and Description    Conditions which Generate this Code    System Response    
791-5 Compression Brake Low/High Solenoid current below normal     The Electronic Control Module (ECM) detects an open circuit or a short circuit to the +Battery for the "low/high solenoid". The ECM can only detect this condition when the output driver is not activated.     The code is logged.    
791-6 Compression Brake Low/High Solenoid current above normal     The ECM detects a short to ground condition for the "low/high solenoid" during an attempt to activate the compression brake. The ECM can only detect this condition when the output driver is activated.    
792-5 Compression Brake Medium/High Solenoid current below normal     The ECM detects an open circuit or a short circuit to the +Battery for the "medium/high solenoid". The ECM can only detect this condition when the output driver is not activated.     The code is logged.    
792-6 Compression Brake Medium/High Solenoid current above normal     The ECM detects a short to ground condition for the "medium/high solenoid" during an attempt to activate the compression brake. The ECM can only detect this condition when the output driver is activated.    The operator has the ability to turn on the compression brake through a switch in the cab. If the engine retarder is installed on the engine, the configuration parameter for the engine retarder must be set to "Enabled" for this function to operate. The requested level of braking is sent over the Cat Data Link from the transmission ECM. The engine's ECM will activate the appropriate compression brake solenoids in order to meet the requested level of braking. "Compression Brake Solenoid Override"The "Compression Brake Low/High Override" and the "Compression Brake Med/High Override" are diagnostic overrides that are available on Caterpillar Electronic Technician (ET). Use the overrides to assist in troubleshooting problems in the circuit for the compression brake. The ECM will only detect shorts in the circuit for the compression brake when the overrides are active. The overrides must be activated and then deactivated before the ECM will detect an open circuit. The ECM will only detect opens in the circuit for the compression brake when the overrides are not active.Note: The engine speed must be zero before the overrides will function.
Illustration 1 g01199118
Location of the compression brake solenoid (typical example)
Illustration 2 g01395529
Schematic for the compression brake circuitTest Step 1. Inspect the Electrical Connectors and the Wiring
Electrical shock hazard. The electronic unit injector system uses 90-120 volts.
Remove electrical power from the engine's ECM. A strong electrical shock hazard is present if the electrical power is not removed from the engine ECM.
Thoroughly inspect ECM connector J2/P2 and the connector for the valve cover. Refer to Troubleshooting, "Electrical Connectors - Inspect" for details.
Illustration 3 g01141312
P2 ECM connector (P2-1) Compression brake solenoid low/high (P2-8) Compression brake solenoid med/high (P2-14) Digital return
Illustration 4 g01395532
Valve cover connector (10) Compression brake solenoid med/high (11) Compression brake solenoid low/high (12) Digital return
Perform a 45 N (10 lb) pull test on each of the wires in the ECM connector that are associated with the compression brake solenoids.
Check the allen head screw for each of the ECM connectors and the machine connectors for the proper torque. Refer to Troubleshooting, "Electrical Connectors - Inspect" for the correct torque values.
Check the harness and wiring for abrasion and for pinch points from the valve cover back to the ECM.Expected Result:All connectors, pins, and sockets are completely coupled and/or inserted and the harness and wiring are free of corrosion, of abrasion, or of pinch points.Results:
OK - The harness and connectors appear to be OK. Proceed to Test Step 2.
Not OK - There is a problem with the connectors and/or wiring.Repair: Repair the connectors or wiring and/or replace the connectors or wiring. Ensure that all of the seals for each of the connectors are properly in place and ensure that the connectors are completely coupled. Verify that the repair eliminates the problem.STOPTest Step 2. Check for Active Diagnostic Codes and/or Logged Diagnostic Codes
Connect Cat ET to the service tool connector.
Turn the keyswitch to the ON position.
Access the diagnostic overrides under the "Diagnostics" menu.
Start the "Compression Brake Low/High Override" on Cat ET.
Once Cat ET displays the status of the compression brake solenoid, stop the "Compression Brake Low/High Override".
Monitor the active diagnostic code screen on Cat ET. Check and record any active diagnostic codes.
Start the "Compression Brake Med/High Override" on Cat ET.
Once Cat ET displays the status of the compression brake solenoid, stop the "Compression Brake Med/High Override".
Monitor the active diagnostic code screen on Cat ET. Check and record any active diagnostic codes.Expected Result:No diagnostic codes are active or logged for the compression brake.Results:
OK - No diagnostic codes are active.Repair: The problem may be intermittent. If the problem is intermittent, refer to Troubleshooting, "Electrical Connectors - Inspect".STOP
Not OK - A 791-5 diagnostic code or a 792-5 diagnostic code is active at this time. If both diagnostic codes are active, the problem is most likely in the shared return wire. Inspect the wire and/or connectors for an open circuit. Proceed to Test Step 3.
Not OK - A 791-6 diagnostic code or a 792-6 diagnostic code is active at this time. Proceed to Test Step 6.Test Step 3. Create a Short Between the Wires of the Suspect Compression Brake Solenoid at the Valve Cover ConnectorNote: In order to troubleshoot a 791-5 diagnostic code, you must use the "Compression Brake Low/High Override" on Cat ET. In order to troubleshoot a 792-5 diagnostic code, you must use the "Compression Brake Med/High Override" on Cat ET.
Turn the keyswitch to the OFF position.
Disconnect the harness at the valve cover connector.