STUD 6B9273 - Caterpillar

Use this procedure to troubleshoot the electrical system if a problem is suspected with the power to the engines Electronic Control Module (ECM). Use this procedure if one of the following diagnostic codes is active or easily repeated.
Table 1
Diagnostic Codes Table
J1939 Code and Description Conditions which Generate this Code System Response
168-3
Battery Potential / Power Input #1 : Voltage Above Normal Electrical System Voltage : Voltage Above Normal The code is logged.
168-4
Battery Potential / Power Input #1 : Voltage Below Normal Electrical System Voltage : Voltage Below Normal The code is logged. The engine ECM requires the keyswitch to be in the ON position in order to maintain communications with Caterpillar Electronic Technician (ET).
Illustration 1 g03386700
Schematic for the electrical power supply (typical example).
Illustration 2 g03321350
Location of the J1/P1 and the J2/P2 ECM connectors (typical engine view).
(1) J1/P1 ECM connector
(2) J2/P2 ECM connector
Illustration 3 g03386797
P1 ECM connector
(P1-5) Digital return
(P1-43) Starter enable relay LH +
(P1-51) Starter enable relay LH -
(P1-52) +Battery
(P1-53) +Battery
(P1-55) +Battery
(P1-57) +Battery
(P1-61) −Battery
(P1-63) −Battery
(P1-65) −Battery
(P1-69) −Battery
(P1-70) Keyswitch
Illustration 4 g03386735
Engine interface connector
(Terminal 9) Digital Return
(Terminal 16) E-Stop 1
(Terminal 17) E-Stop 2 / Momentary stop
(Terminal 25) +Battery
(Terminal 27) Keyswitch
(Terminal 34) -Battery
(Terminal 35) -Battery
Illustration 5 g03323077
Terminal locations for the ECM relay and the connector (typical example)
(3) ECM relay
(Terminal 1) +Battery
(Terminal 2) Relay ground
(Terminal 3) Electrical power to the ECM
(Terminal 5) Relay control
Table 2
1. Inspect the Electrical Connectors and the Wiring
A. Turn the main disconnect switch to the OFF position.
B. Thoroughly inspect the ECM connectors. Also, thoroughly inspect the other connectors for the electrical power circuit. Refer to Troubleshooting, "Electrical Connectors - Inspect" for the proper procedure for inspecting the connectors and the wiring.
Electrical connectors
Result: The connectors and wiring appear to be OK.
Proceed to Test Step 2.
Result: There is a problem in the connectors and/or wiring.
Repair: The connectors or wiring and/or replace the connectors or wiring. Ensure that all of the seals are properly in place and ensure that the connectors are coupled.
Verify that the repair eliminated the problem.
2. Check the Voltage to the ECM
A. Disconnect the ECM connector J1/P1.
B. Restore the electrical power to the ECM.
C. Measure the voltage at the ECM. Measure the voltage between P1-52 and P1-61. Measure the voltage between P1-53 and P1-63.
ECM voltage
Result: The voltage is 24 3 VDC.
The ECM is receiving the correct voltage. The problem may be intermittent. If the problem is intermittent, refer to Troubleshooting, "Electrical Connectors - Inspect".
Result: The voltage is less than 24 3 VDC.
The wiring or components between the battery and the ECM may be preventing the voltage from reaching the ECM.
Proceed to Test Step 3.
3. Check the Charging Circuit
A. Turn the main disconnect switch to the OFF position.
B. Check the charging circuit.
Charging circuit
Result: The charging system is OK.
Proceed to Test Step 4.
Result: The charging system is Not OK.
There is a problem with the charging system. Make the necessary repairs. Refer to Special Instruction, REHS0354, "Charging System Troubleshooting".
Verify that the repair eliminated the problem.
4. Load Test the Battery
A. Load-test the battery. Refer to Systems Operation and Testing and Adjusting, "Battery - Test"
Load test
Result: The battery