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:
- 168-0 Electrical System Voltage high
- 168-1 Electrical System Voltage low
- 168-2 Electrical System Voltage erratic, intermittent, or incorrect
- 1834-2 Ignition Keyswitch loss of signal
Engines with an EMCP3 control panel
The engine ECM receives electrical power (battery voltage) through the wiring in the cabinet for the control panel. Switched battery+ voltage is supplied via a 20 Amp circuit breaker to P1: 7, 8, 15 and 16. The battery- is supplied to P1: 1, 2, 3, 9 and 10. Switched battery+ voltage is supplied to P1:40 via an emergency stop button. When the engine ECM detects battery voltage at this input, the engine ECM will power up. When the keyswitch is turned to the OFF position or the emergency stop button is pressed, battery voltage is removed from this input and the engine ECM will power down.
The cause of an intermittent power supply to the engine ECM can occur on either the positive side or on the negative side of the battery circuit.
Battery power to the diagnostic connector is available and the battery power to the data link connector is independent of the control panel. Therefore, it is possible to power up the electronic service tool but the electronic service tool 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 is normal.
For intermittent faults such as intermittent shutdowns that could be caused by the wiring on the genset, temporarily bypassing the wiring on the genset may be an effective means of determining the root cause. If the symptoms disappear with the bypass wiring, the wiring on the genset is the cause of the problem. A means of bypassing the wiring on the genset is explained in this test procedure.
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| Illustration 1 | g02076321 |
Schematic for the keyswitch and battery supply circuit on engines with an EMCP3 control panel | |
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| Illustration 2 | g02076356 |
Schematic for the keyswitch and battery supply circuit on engines with an MCS control panel | |
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| Illustration 3 | g01501393 |
Pin locations for the ignition keyswitch and battery supply circuit on the P1 connector (1) Battery- (2) Battery- (3) Battery- (7) Battery+ (8) Battery+ (9) Battery- (10) Battery- (15) Battery+ (16) Battery+ (40) Ignition keyswitch | |
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
- 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, this may be the cause.
Results:
- OK - Diagnostic code 168-2 or diagnostic code 1834-2 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 fault is intermittent, refer to Troubleshooting, "Electrical Connectors - Inspect".
STOP
Test Step 2. Bypass the Wiring on the Genset
- 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 genset is returned to service. The bypass harness can be used in order to determine if the cause of the intermittent fault is interruptions in battery power to the ECM or to the keyswitch circuit.
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| Illustration 4 | g01622857 |
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 fuse 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 either the wiring that supplies power to the engine ECM, the ECM for the control panel or the keyswitch.
STOP - Not OK -
Repair: Perform the following procedure:
- Make sure that the latest flash file for the application is installed in the ECM. Refer to Troubleshooting, "Flash Programming".
- Use the electronic service tool to check for active diagnostic codes.
- If a diagnostic code is still active, contact the Technical Communicator at a Caterpillar dealer.
Note: This consultation can greatly reduce the repair time.
- If a test ECM is recommended by the Technical Communicator, install a test ECM. Refer to Troubleshooting, "Replacing the ECM".
- Use the electronic service tool to verify that the fault is eliminated.
- If the fault is eliminated with the test ECM, reconnect the suspect ECM. Verify that the active diagnostic code returns.
- If the active diagnostic code returns with the suspect ECM, replace the ECM.
STOP