BAND ASSEM. 4M8766 - Caterpillar

Table 1
Diagnostic Trouble Codes for the Start Relay
J1939 Code Code Description Comments
677-3 Starter Motor Relay : Voltage Above Normal The Electronic Control Module (ECM) detects a high voltage condition (open circuit) in the start relay control circuit.
677-4 Starter Motor Relay : Voltage Below Normal The Electronic Control Module (ECM) detects a low voltage condition (short circuit) in the start relay control circuit.
Follow the troubleshooting procedure to identify the root cause of the fault.
Illustration 1 g06087955
Schematic for the start relay
Illustration 2 g03836694Note: When investigating a 677-4 code, disconnect the wires from the starting motor and insulate the wire terminals.Complete the procedure in the order in which the steps are listed.
Table 2
Troubleshooting Test Steps Values Results
1. Inspect Electrical Connectors and Wiring
A. Inspect the terminals on the start relay. Refer to Troubleshooting, "Electrical Connector - Inspect" for details.
B. Perform a 45 N (10 lb) pull test on each of the wires in the ECM connectors that are associated with the start relay.
C. Check the harness for abrasion and pinch points from the start relay back to the ECMs.
Loose connection or damaged wire
Result: There is a fault in a connector or the wiring.
Repair: Repair any faulty connectors or replace the wiring harness. Ensure that all the seals are properly in place and ensure that the connectors are correctly coupled.
Use the electronic service tool to clear all logged diagnostic codes and verify that the repair eliminates the fault.
Result: All connectors, pins, and sockets are correctly coupled and/or inserted. The harness is free of corrosion, abrasion, and pinch points. The fuses are not blown.
Proceed to Test Step 2.
2. Check for Active Diagnostic Codes
A. Connect the electronic service tool to the diagnostic connector.
B. Turn the keyswitch to the ON position.
Note: Do not start the engine.
C. Use the electronic service tool to check for active diagnostic codes.
D. Turn the keyswitch to the OFF position.
Diagnostic codes
Result: Diagnostic code 677-3 is active or recently logged.
Proceed to Test Step 3.
Result: Diagnostic code 677-4 is active or recently logged.
Proceed to Test Step 6.
3. Check the Supply Voltage at the Start Relay Connector
A. Remove the start relay.
B. Turn the keyswitch to the START position.
C. Measure the voltage between Test Point 1 on the relay socket and a suitable ground.
D. Turn the keyswitch to the OFF position.
E. Install the start relay.
At least 11 VDC
Result: The voltage is not within the expected range.
Repair: Repair or replace the supply wiring to the start relay.
Use the electronic service tool to clear all logged diagnostic codes and verify that the repair eliminates the fault.
Result: The voltage is within the expected range.
Proceed to Test Step 4.
4. Check the Return Wire from the Start Relay to the Engine ECM
A. Turn the keyswitch to the OFF position.
B. Remove the start relay.
C. Disconnect the P2 connector from the engine ECM.
D. Measure the resistance between Test Point 2 on the start relay socket and P2:92.
E. Connect the P2 connector to the engine ECM.
F. Install the start relay.
Less than 2.0 Ohms
Result: The measured resistance is more than 2.0 Ohms.
Repair: Repair or replace the return