Case IH 3985 – Engine ECU (ISM-IDI Engine ECU): Exhaust gas pressure sensor voltage is higher than expected

The exhaust gas pressure sensor is a differential sensor that measures the pressure difference between the exhaust gas at the intake and the outtake of the Diesel Particulate Filter (DPF). A "voltage higher than expected" condition means the Engine Control Unit is receiving a signal voltage from the sensor that exceeds its normal operating parameters (the sensor's normal voltage limitation is between 600 mV and 4400 mV). This typically indicates a short to a high-voltage source in the wiring harness, a failed internal sensor circuit, or extreme exhaust backpressure maxing out the sensor's reading.

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1. 1

   Check for associated symptoms such as engine power loss (derate), unnecessary DPF regenerations, or dashboard warning lights for the particulate filter/pre-heating.

2. 2

   Inspect the 3-pin electrical connector at the sensor. Look for chafed wires, melted insulation, or corrosion.

3. 3

   Disconnect the sensor and turn the key to the ON position. Check the signal wire on the harness side for a short to power (e.g., shorted to a 12V source or the 5V reference wire).

4. 4

   Inspect the two pressure tubes running from the exhaust pipe to the sensor. Ensure they are not blocked by soot, kinked, or melted, as severe blockages can cause erratic pressure readings.

1. 1

   Turn off the machine, remove the key, and disconnect the battery to prevent electrical shorts.

2. 2

   Locate the exhaust gas pressure sensor, which is typically mounted near the DPF assembly with two hoses routing to the exhaust stream.

3. 3

   Disconnect the 3-pin electrical connector from the sensor.

4. 4

   Carefully loosen and remove the two pressure hoses/tubes connected to the sensor.

   Note their orientation (high-pressure intake side vs. low-pressure outtake side) so they are not swapped during reassembly.

5. 5

   Unbolt the sensor from its mounting bracket and remove it.

6. 6

   Install the new exhaust gas pressure sensor and secure it with the mounting hardware.

7. 7

   Reconnect the DPF pressure tubes, ensuring they are tight and free of leaks.

8. 8

   Reconnect the 3-pin electrical connector.

9. 9

   Reconnect the battery.

10. 10

    Connect a diagnostic scan tool (such as the Case IH EST) to clear the fault codes.

11. 11

    Start the engine and monitor the sensor voltage to ensure it sits within the normal 600-4400 mV range.

    A manual/stationary DPF regeneration may be required to verify the system is functioning correctly.