PIPE 0878509 - 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 any of the diagnostic codes in Table 1 is active or easily repeated.
Table 1
Diagnostic Codes Table
CDL Code J1939 Code Code Description
(code descriptions may vary) Comments
168-2 168-2 Electrical System Voltage : Erratic, Intermittent, or Incorrect The engine is running. The battery voltage falls below 6 VDC and the voltage returns above 9 VDC for three voltage readings in the last 7 seconds.
168-3 168-3 Electrical System Voltage : Voltage Above Normal The ECM detects voltage that is above the acceptable value.
168-4 168-4 Electrical System Voltage : Voltage Below Normal The ECM detects voltage that is below the acceptable value.
1834-2 158-2 Ignition Key Switch loss of signal The ECM detects an erratic signal from the keyswitch because the keyswitch was cycled at least three times in the last second The engine ECM requires the keyswitch to be in the ON position in order to maintain communications with Caterpillar Electronic Technician (ET).
Illustration 1 g02060084
Schematic for the electrical power supply circuit
Table 2
Troubleshooting Test Steps Values Results
1. Determine the Diagnostic Code
A. Establish communication between the Cat ET and the ECM . Refer to Troubleshooting, "Electronic Service Tools", if necessary.
B. Start the engine. Run the engine until the engine is at normal operating temperature.
C. Observe the "Active Diagnostic" screen on Cat ET. Wait at least 30 seconds so that any codes may become active.
D. Look for these codes:
1. 168-2
2. 168-3
3. 168-4
4. 1834-2 or 158-2
Diagnostic code
Result: A 168-2, 158-2 or a 1834-2 diagnostic code is active.
Proceed to Test Step 2.
Result: A 168-3 diagnostic code is active.
Proceed to Test Step 3.
Result: A 168-4 diagnostic code is active.
Proceed to Test Step 4.
2. Inspect the Electrical Connectors and the Wiring For the Charging Circuit
A. Turn the keyswitch to the OFF position.
B. Check the harness connections at the battery and at the alternator.
C. Check the harness between the battery and the alternator.
Electrical connectors
Result: The harness and connectors appear to be OK.
There is a problem with the charging system. Refer to Special Instruction, REHS0354, "Charging System Troubleshooting".
Result: The connectors and/or wiring are not OK.
Repair the wiring and/or the connectors. Replace parts, if necessary. Ensure that all of the seals are properly in place and ensure that the connectors are coupled. Verify that the repair eliminates the problem.
3. Check the Charging Circuit
A. Check the charging circuit. Refer to Special Instruction, REHS0354, "Charging System Troubleshooting".
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".
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 voltage is low or the battery did not pass the load test.
Recharge or replace the faulty batteries. Verify that the repair eliminates the problem.
Result: The batteries pass the load test. The measured voltage is between