SPN 108 - FMI 3 (Diagnostic Code: Atmospheric/ Barometric Pressure Voltage High/ On-Highway Truck)

SPN 108 FMI 3

SPN108 FMI3 - Atmospheric/ Barometric Pressure High 

Figure 1.1 - SPN 108 FMI 3 (Murphy PowerView)

PID 0108 FMI 03

#Conditions Which Generate This Code:

The Engine Control Module (ECM) detects the following conditions:

→The signal voltage from the Atmospheric/ Barometric Pressure Sensor is greater than 4.95 VDC for more than 30 seconds.

→The ECM has been powered for at least two seconds.

#System Response:

The ECM will log the Diagnostic Code and the ECM will trigger a snapshot. The Check Engine lamp will illuminate while this Diagnostic Code is active. The ECM will default to 100 kPa (15 psi) for Atmospheric/ Barometric pressure.

#Possible Performance Effect:

The Engine power will be derated by 10 percent.

#Troubleshooting:

The Engine Control Module (ECM) continuously creates a Pull-up voltage on the signal wire for each sensor. The ECM uses this Pull-up voltage in order to detect a problem in the signal circuit. When the ECM detects voltage that is above a threshold on the signal wire, the ECM activates a high voltage -3 Diagnostic Code. When the ECM detects voltage that is below a threshold on the signal wire, the ECM activates a low voltage -4 Diagnostic Code. Read More: Pull-up Voltage and How the ECM Uses it to Determine Diagnostic Condition.

The most likely cause of a code is a problem with an electrical connector or wiring. The least likely cause of a code is the ECM. 

#108-3 Diagnostic Code and High Crankcase Pressure

Some Atmospheric/ Barometric Pressure Sensors that are located in the Engine block actually sense Crankcase pressure rather than Atmospheric pressure. With this type of installation, Crankcase pressure of approximately 14 kPa (2 psi) can cause the 108-3 Diagnostic Code to be generated.

Before you proceed with this test, determine whether the sensor is sensing Crankcase pressure or Atmospheric pressure.

If the sensor is sensing Crankcase pressure, connect Caterpillar Electronic Technician (ET) to the ECM and clear the Diagnostic Code. Do not start the Engine. Maintain electrical power to the ECM. Wait at least 30 seconds for generation of the Diagnostic Code. Slowly wiggle the wiring for the sensor in an attempt to generate an intermittent code. If the 108-3 code is generated, continue with the functional test/ troubleshooting. If the 108-3 code is not generated, there is either a problem with high Crankcase pressure or the code is intermittent, due to a loose connector or damaged wiring.

Figure 1.2 - Logged Diagnostic Codes (108-3)

Test Step 1. Determine if the Code is Logged or Active

A. Connect CAT ET to the Service Tool connector. Read More: How to Connecting Caterpillar ET Software to the ECM.

B. Restore electrical power to the Engine ECM.

C. Monitor the Active Diagnostic Code and the Logged Diagnostic Codes on CAT ET. Wait at least 30 seconds in order for Diagnostic Codes to become active. Identify the Diagnostic Code.

Test Step 2. Check the Integrity of the Connections at the Connectors

A. Restore the electrical power to the ECM. Do not start the Engine.

B. Check for an Intermittent Problem at the Connector for the Suspect Sensor and ECM Connector:

→Carefully install the spoons (multimeter probes) into the terminal locations for the sensor supply and the sensor return at the appropriate ECM connector (P2/J2 - 14).

→While you observe the voltage reading on the multimeter, wiggle the wires and pull on the wires at the connector for the suspect sensor. The voltage reading will not vary more than 0.5 volts for a solid electrical connection. Read More: Wiggle Test on Caterpillar ET Software.

→Remove the spoons (multimeter probes) from the connector.

C. Remove the electrical power from the ECM.

Figure 1.3 - Barometric Pressure Voltage High (108-3)

Test Step 3. Check the Supply Voltage at the Sensor Connector

Measure the voltage between terminal 1 (sensor supply) and terminal 2 (sensor return) at the harness connector for the sensor that relates to the code (P203/ Atmospheric Pressure Sensor).

Expected Result: The voltage is between 4.5 VDC and 5.5 VDC.

Test Step 4. Check for Battery Voltage on the Signal Wire

Measure the voltage between the signal terminal and the return terminal on the harness connector for the sensor that relates to the Diagnostic Code (P203/ Atmospheric Pressure Sensor).

Expected Result: The voltage measurement is less than the +Battery voltage.

Test Step 5. Check the Signal Wire for an Open Circuit

A. Disconnect the sensor that relates to the Diagnostic Code.

B. Connect a jumper wire between the signal terminal and the return terminal on the harness connector for the sensor that relates to the Diagnostic Code. This will replace the sensor with a short circuit.

Figure 1.4 - Jumper Wire (P203/ Atmospheric Pressure Sensor)

C. Look for a -4 Diagnostic Code (108-4/ SPN 108 - FMI 4).

Figure 1.5 - Barometric Pressure Voltage Low (108-4)

D. Replace the Atmospheric Pressure Sensor, with CAT P/N: 276-8548.

#Additional Notes:

A plugged Crankcase breather will cause excessive Crankcase pressure. Leakage of the crankshaft seal can occur. Sealing of the piston rings can be lost. This will enable Crankcase blowby to further increase the Crankcase pressure.

If the piston rings do not seal, blowby will increase the Crankcase pressure and the problem will become worse. Performance will deteriorate. The Engine will have poor combustion. Deposits will build up on the pistons, on the valves, and in the cylinder heads. Downward pumping of the pistons against the increased Crankcase pressure will further reduce the performance.

Sticking of the pistons will also increase the Crankcase pressure. If the problem is not resolved, piston seizure will result. If a problem with a piston is suspected, do not operate the Engine. Inspect the cylinder liner for scoring and for scuffing. Replace the piston rings and the cylinder liner, if necessary.

To eliminate Crankcase pressure as the cause of the 108-3 Diagnostic Code, inspect the Crankcase breather. If the breather is plugged, clean the breather or replace the breather. Make sure that the air passages for the breather's system are clear.

To verify that the plugged breather was the problem, operate the Engine at a high load for at least 30 seconds. If the code does not recur, return the Engine to service.

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