SPN 678 - FMI 4 (Diagnostic Code: 8 VDC Supply Voltage Below Normal)

SPN 687 FMI 4

SPN687 FMI4

8 Volt DC Supply: Voltage Below Normal (CDL Code: 41-4)

Figure 1.1 - SPN 678 FMI 4 (41-4)

Use this procedure to troubleshoot the electrical system if a problem is suspected with a sensor supply. 

8 VDC Supply: Voltage Above Normal (CDL Code: 41-3)

8 VDC Supply: Voltage Below Normal (CDL Code: 41-4)

→Comments: The Code is logged.

#Probable Causes

→Codes

→Sensors

→Sensor Supply Voltage

#Recommended Actions

Troubleshooting Test Steps:

1). Check for Codes

A. Connect CAT Electronic Technician (ET) to the Service Tool/ Diagnostic Port connector. Read More: How to Connecting Caterpillar ET Software to the ECM.

B. Turn the keyswitch to ON.

C. Determine if a Diagnostic Trouble Code is present (SPN 678 - FMI 4), refer to Figure 1.1 above.

Results: A -3 or -4 code is present. Proceed to Test Step 2.

Figure 1.2 - 8 VDC Supply Voltage Below Normal

2). Check for a Failed Sensor

A. Disconnect a sensor on the suspect sensor supply circuit (Coolant Level Sensor).

B. Monitor CAT ET while the sensor is disconnected to see if the Active Code changes to logged.

Results:

→The suspect sensor supply Active Code remains active after all sensors on the sensor supply circuit have been checked. Repair: A failed wiring harness has been detected. Repair or replace the wiring harness.

→The suspect sensor supply Active Code changes to logged when a sensor is unplugged (refer to Figure 1.3 below). Repair: Replace the failed sensor.

Figure 1.3 - No Active Codes (Coolant Level/ Low)

C. Connect the suspect sensor to the wiring harness.

D. Repeat steps A through C for each sensor on the suspect sensor supply. If the problem is not resolved, proceed to Test Step 3.

3). Check the Sensor Supply Voltage

A. Turn the keyswitch to OFF.

B. Unplug the suspect sensor from the wiring harness.
Results: A -4 code is activated on the suspect sensor supply. Repair: Remove the sensor. Check the sensor passage for a blockage or obstruction. Also, check for moisture frozen in the lines. If blockage is not found, replace the sensor.

Figure 1.4 - No Active Codes (Coolant Level/ OK)

C. Place a jumper wire between pin 1 and pin 2 on the suspect sensor wiring harness connector.
D. Connect to CAT ET.
E. Turn the keyswitch to ON.

F. Monitor the Diagnostic Codes using CAT ET.

Results: A -4 code is not activated on the suspect sensor supply. Repair: The problem is in the wiring from the ECM to the sensor connector. Repair the wiring or replace the wiring.

Figure 1.5 - Coolant Level Sensor

Read More:

Event Codes: High Intake Manifold Temperature (SPN 105 - FMI 15)

No Rail Fuel Pressure (Low Fuel Pressure) - Event Code: SPN 94 - FMI 17

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