This procedure tests that the correct voltage is being supplied to the Engine Control Module (ECM).
Use this procedure to troubleshoot the system when one of the following diagnostic codes is active or easily repeated:
- 0168-00 Electrical System Voltage high
- 0168-01 Electrical System Voltage low
- 0168-02 Electrical System Voltage erratic, intermittent, or incorrect
- 0168-03 Electrical System Voltage high
- 0168-04 Electrical System Voltage low
- 1834-02 Ignition Keyswitch loss of signal
The ECM receives electrical power (battery voltage) through the machine wiring. Unswitched battery+ voltage is supplied through P1: 7, 8, 15 and 16. The battery- is supplied through P1: 1, 2, 3, 9 and 10. The ECM receives the input from the keyswitch at P1:40 when the keyswitch is in the ON position or in the START position. When the ECM detects battery voltage at this input, the ECM will power up. When battery voltage is removed from this input, the ECM will power down.
The cause of an intermittent power supply to the ECM can occur on either the positive side or the negative side of the battery circuit. The connections for the unswitched battery+ are routed through a dedicated protection device (circuit breaker) on the machine.
The Machine Security System can interrupt electrical power to the keyswitch. This device can be the cause of intermittent power to the ECM. This device may also shut down the engine.
Battery power to the diagnostic connector is available and the battery power to the data link connector is independent of the keyswitch. Therefore, you will be able to power up the electronic service tool but you may not be able to communicate with the engine ECM. The engine ECM requires the keyswitch to be in the ON position in order to maintain communications. The ECM may power down a short time after connecting the electronic service tool if the keyswitch is in the OFF position. This effect is normal.
For intermittent problems such as intermittent shutdowns that could be caused by the wiring in the machine, temporarily bypassing the wiring in the machine may be an effective means of determining the root cause. If the symptoms disappear with the bypass wiring, the wiring in the machine is the cause of the problem. A means of bypassing the wiring in the machine is explained in this test procedure.
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| Illustration 1 | g01727874 |
Schematic for the ignition keyswitch and battery supply circuit | |
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| Illustration 2 | g01727876 |
Pin locations for the ignition keyswitch and battery supply circuit on the P1 connector (1) Battery ground (GND) (2) Battery ground (GND) (3) Battery ground (GND) (7) Battery+ (8) Battery+ (9) Battery ground (GND) (10) Battery ground (GND) (15) Battery+ (16) Battery+ (40) Ignition key switch | |
Test Step 1. Check for Active Diagnostic Codes or Logged Diagnostic Codes
- Connect the electronic service tool to the diagnostic connector.
- Turn the keyswitch to the ON position.
- Monitor the active diagnostic code screen on the electronic service tool. Check and record any active diagnostic codes or logged diagnostic codes.
Note: Wait at least 30 seconds in order for the diagnostic codes to become active.
Expected Result:
One of the following diagnostic codes is active or logged:
- 168-0 Electrical System Voltage high
- 168-1 Electrical System Voltage low
- 168-2 Electrical System Voltage erratic, intermittent, or incorrect
- 168-3 Electrical System Voltage high
- 168-4 Electrical System Voltage low
- 1834-2 Ignition Keyswitch loss of signal
Note: Diagnostic code 1834-2 can be generated by rapidly cycling the keyswitch. If diagnostic code 1834-2 is logged but not active, keyswitch cycling may be the cause.
Results:
- OK - Diagnostic codes 168-0, 168-1, 168-3 or 168-4 are active or logged.
Repair: Refer to Troubleshooting, "Battery".
STOP
- OK - Diagnostic code 168-2 or diagnostic code 1834-02 is active or logged. Proceed to Test Step 2.
- Not OK - No diagnostic code is active.
Repair: The fault is no longer present. If the problem is intermittent, refer to Troubleshooting, "Electrical Connectors - Inspect".
STOP
Test Step 2. Bypass the Machine Wiring
- Turn the keyswitch to the OFF position.
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Batteries give off flammable fumes which can explode. To avoid injury or death, do not strike a match, cause a spark, or smoke in the vicinity of a battery. |
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| NOTICE |
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Do not connect the bypass harness to the battery until all of the in-line fuses have been removed from the Battery+ line. If the fuses are not removed before connection to the battery, a spark may result. |
Note: This bypass harness is only for test applications. This bypass harness must be removed before the machine is returned to service. The bypass harness can be used to identify interruptions in battery power to the ECM or to the keyswitch circuit.
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| Illustration 3 | g01727878 |
Schematic for the bypass application harness | |
- Disconnect P1 from the ECM connector.
- Connect a bypass harness to the ECM.
- Remove the fuse from the Battery+ wire of the bypass harness and connect the Battery+ and the Battery- wires directly to the battery terminals.
- Install the in-line fuse in the Battery+ wire of the bypass harness.
Note: This bypass directly connects the circuit for the keyswitch to the ECM. The ECM will remain powered until the connection to the unswitched battery+ line is disconnected. Remove the fuses from the in-line fuse holder to power down the ECM. Do not connect the bypass to the battery terminals or do not remove the bypass from the battery terminals without first removing the in-line fuse.
- Connect the electronic service tool to the diagnostic connector and verify that communication can be established.
Note: Remove the bypass harness and restore all wiring to the original condition after testing.
Expected Result:
Installing the bypass eliminates the fault.
Note: The status of the "Ignition Keyswitch" will always indicate ON while the bypass harness is installed.
Results:
- OK - The symptoms disappear when the bypass harness is installed. Also, the symptoms return when the bypass harness is removed. The fault is in the machine wiring that supplies power to the ECM. Refer to the troubleshooting guide for the machine in order to investigate the fault.
STOP - Not OK -
Repair: Perform the following repair:
- Make sure that the latest flash file for the application is installed in the ECM. Refer to Troubleshooting, "Flash Programming".
- Contact the technical communicator at the Caterpillar dealer.
Note: This consultation can greatly reduce the repair time.
- If the technical communicator recommends the use of a test ECM, install a test ECM. Refer to Troubleshooting, "Replacing the ECM".
- Use the electronic service tool to recheck the system for active diagnostic codes.
- If the fault is resolved with the test ECM, reconnect the suspect ECM.
- If the fault returns with the suspect ECM, replace the ECM.
- Use the electronic service tool in order to clear all logged diagnostic codes and then verify that the repair eliminates the fault.
STOP