John Deere 000109.03 – ECU: High Coolant Pressure Sensor Voltage

The Engine Control Unit (ECU) has detected a voltage in the coolant pressure sensor's signal circuit that is above the normal operating range (SPN 109, FMI 03). This specific code indicates an electrical fault (Voltage Above Normal or Shorted to High Source). It typically points to an open circuit caused by a faulty sensor, a break in the wiring harness, or a poor connection at the ECU, rather than actual high physical coolant pressure.

• 6 Series
• 7 Series
• 8 Series
• 9 Series
• 6R
• 7R
• 8R
• 9R
• Construction and Forestry equipment
• loaders
• excavators

1. 1

   Carefully inspect the coolant pressure sensor and its wiring harness for signs of physical damage, such as chafed, pinched, or broken wires.

2. 2

   Check the sensor's electrical connector for corrosion, damage, or a loose connection.

3. 3

   Disconnect and reconnect the coolant pressure sensor's electrical connector to ensure it is securely seated.

4. 4

   Trace the wiring from the sensor back to the ECU, looking for any damage to the harness along its entire length.

1. 1

   Verify the fault

   Cycle the machine's power to see if the fault code clears and then reappears. If it reappears, it is an active fault that requires further diagnosis.

2. 2

   Test sensor reference voltage and ground

   Disconnect the electrical connector from the coolant pressure sensor. With the key on and the engine off, use a multimeter to check for a 5-volt reference signal and a good ground connection on the appropriate pins on the harness side of the connector. If both are present, the issue is likely an internal failure within the sensor itself.

3. 3

   Test the signal wire

   If the reference voltage or ground is missing at the connector, there is a break in the wiring between the ECU and the sensor. Using a John Deere technical manual wiring diagram, check the continuity of the signal wire from the sensor connector back to the corresponding pin at the ECU.

4. 4

   Inspect the ECU

   If the sensor and wiring are confirmed to be in good condition, the issue could potentially be with the ECU itself. ECU failure is less common and should be considered the last resort after all other possibilities have been exhausted.