Case IH 3987 – Engine Control Unit (ECU): Turbine downstream pressure sensor voltage is higher than expected

The ECU is receiving a voltage signal from the turbine downstream pressure sensor that exceeds the maximum physical limit (typically above 4.5V to 4.8V). This indicates an electrical circuit issue such as an open circuit on the sensor's ground wire, a short circuit between the signal wire and a voltage source, or an internal short failure within the sensor itself.

• tractors
• combines
• N-series loaders

1. 1

   Locate the turbine downstream pressure sensor on the exhaust piping just after the turbocharger and visually inspect the wiring harness for melted insulation, chafing, or tight bends.

2. 2

   Disconnect the sensor and inspect the connector pins for corrosion, moisture, or pushed-out terminals.

3. 3

   With the key ON and engine OFF, verify the 5V reference wire is supplying approximately 5.0 volts.

4. 4

   Check the ground wire for continuity to the ECU ground or chassis ground (an open ground is the most common cause of an SRC High code).

5. 5

   Check the signal wire for continuity to the 5V reference wire or battery voltage to rule out a short circuit.

6. 6

   If the wiring tests perfectly, perform a sensor bypass test as the sensor has likely failed internally.

1. 1

   Power Down

   Turn off the machine, remove the key, and disconnect the negative battery terminal to prevent electrical spikes.

2. 2

   Access the Sensor

   Locate the turbine downstream pressure sensor on the exhaust system. Ensure the exhaust system is completely cool before proceeding.

3. 3

   Harness Repair

   If the diagnostic checks revealed a broken ground or shorted wire, splice and repair the wiring harness using heat-shrink butt connectors. Route the harness away from extreme exhaust heat.

4. 4

   Sensor Replacement

   Disconnect the electrical connector. Use a wrench to carefully unthread the sensor from the exhaust bung. Clean the threads in the exhaust pipe bung. Apply a small amount of high-temperature anti-seize to the threads of the new sensor (if it does not come pre-applied). Do not get anti-seize on the sensor tip. Thread the new sensor in by hand to prevent cross-threading, then tighten to OEM torque specifications.

5. 5

   Reassembly

   Reconnect the electrical harness to the sensor, ensuring the locking tab clicks into place.

6. 6

   Clear Codes

   Reconnect the battery. Connect a diagnostic tool (such as CNH EASy or Jaltest) to the machine's diagnostic port and clear the inactive 3987 fault code.

7. 7

   Validation

   Start the engine and monitor the turbine downstream pressure live data to ensure it reads within normal parameters and the fault code does not return.