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Fuel Delivery Pressure Sensor - Voltage Below Normal (Diagnostic Code: SPN 94 - FMI 4)

Fuel Pressure Short to Ground
Fuel Pressure Signal Short Low

Figure 1.1 - Fuel Press Signal Shrt Low/ 94-4 (CAT Messenger)

MID 036 CID 0094 FMI 04 
Conditions Which Generate This Code:
The Electronic Control Module (ECM) reads signal voltage that is below normal.

System Response:
The ECM will log the Diagnostic Code. The ECM flags Fuel Pressure as Invalid Data and a default value is used.

Possible Performance Effect:
There are no performance effects.

System Operation Description:
The troubleshooting procedures for the Diagnostic Codes of each Pressure Sensor are identical. The Electronic Control Module (ECM) supplies 5 ± 0.2 VDC to terminal A of each sensor. The sensor common connection is connected to terminal B of each sensor. The signal voltage from terminal C of each sensor is sent to the appropriate terminal on the ECM.

Pull-Up Voltage
The ECM continuously outputs a Pull-Up Voltage on the circuit for the sensor signal wire. The ECM uses this Pull-Up Voltage in order to detect an open in the signal circuit. When the ECM detects the presence of a voltage that is above a threshold on the signal circuit, the ECM will generate an open circuit diagnostic code (-03) for the sensor.

The presence of Pull-Up Voltage at the sensor connector indicates that the wires from the sensor connector to the ECM are not open. The absence of Pull-Up Voltage at the sensor connector indicates an open in the signal wire. If the voltage at the sensor connector is different from Pull-Up Voltage, the signal wire is shorted to another wire in the harness. Read More: PULL-UP Voltage and How the ECM Uses it to Determine Diagnostic Conditions.

#Troubleshooting:
Test Step 1. Inspect the Electrical Connectors and the Wiring
A. Turn the keyswitch to the OFF position.
B. Thoroughly inspect all connectors that are associated with the +5 V sensor supply for your engine.
C. Perform a 45 N (10 lb) pull test on each of the wires that are associated with the +5 V sensor supply.
D. Check the torque of the allen head screw on the ECM connectors and the customer connector.
E. Check the harness and wiring for abrasion and for pinch points from the sensors back to the ECM.
Expected Result: All connectors, pins and sockets are completely coupled and/or inserted and the harness and wiring are free of corrosion, of abrasion and of pinch points.

Test Step 2. Check the Supply Voltage at the Sensor Connector
A. Disconnect the Fuel Pressure Sensor J209/P209.
B. Turn the keyswitch to the ON position.
C. Measure the voltage between terminal A (+5 V sensor supply) and terminal B (sensor return) at the Fuel Pressure Sensor connectors.
Expected Result: The voltage is 5.0 ± 0.2 VDC.
Figure 1.2 - Fuel Pressure Sensor

Test Step 3. Check for Active Diagnostic Codes
A. Connect Caterpillar Electronic Technician (ET) to the Service Tool connector. Read More: How to Connecting Caterpillar ET to the ECM.
B. Check for active diagnostic codes. Record any active diagnostic codes. Note: Wait at least 15 seconds in order for the Diagnostic Codes to become active.
Figure 1.3 - Fuel Delivery Pressure Sensor (Voltage Below Normal)
C. Determine if the problem is related to an open circuit (diagnostic code -03) or a short circuit (Diagnostic Code -04). In this case, we found Diagnostic Code -04 (94-4).

Test Step 4. Check the Pull-up Voltage at the Sensor Connector
A. Turn the keyswitch to the ON position.
B. Measure the voltage between terminal C (signal) and terminal B (sensor return) at the sensor connector.
C. Turn the keyswitch to the OFF position.
Expected Result: The voltage is 11 ± 2 VDC (The voltage is correct, the signal wire to the ECM from the sensor connector is OK).

Test Step 5. Disconnect the Fuel Pressure Sensor in Order to Create an Open Circuit
A. Disconnect the Fuel Pressure Sensor connector.
B. Turn the keyswitch to the ON position. Wait at least 15 seconds for activation of the diagnostic codes. 
C. Use CAT ET to check for an active open circuit Diagnostic Code (-03).
Figure 1.4 - Fuel Delivery Pressure Sensor (Voltage Above Normal)
→Error Code 94-3 shown on CAT Messenger display: 
Figure 1.5 - Fuel Press Signal Shrt High/ 94-3 (CAT Messenger)
D. Turn the keyswitch to the OFF position.
Expected Result: An open circuit Diagnostic Code -03 is now active for the disconnected sensor.
OK→A short circuit Diagnostic Code (-04) was active before disconnecting the sensor. An open circuit Diagnostic Code (-03) became active after disconnecting the sensor.
Repair→Temporarily connect a new sensor to the harness, but do not install the new sensor in the engine. Verify that there are no Active Diagnostic Codes for the sensor. If there are no Active Diagnostic Codes for the sensor, permanently install the new sensor (CAT P/N: 325-2727). Clear any Logged Diagnostic Codes.
Figure 1.5 - Logged Diagnostic Codes (94-4)
→After replace the new Fuel Pressure Sensor.
Figure 1.6 - Fuel Pressure (Reading Value: 35 Psi)


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