CLAAS 772Ah – ECM / ECU / A15 module: Engine Coolant Temperature Voltage High

The voltage signal received by the engine control module (A15) from the engine coolant temperature sensor (B45) is above the expected maximum threshold. This typically indicates an open circuit in the sensor's wiring or a faulty sensor, causing the system to read an illogically low temperature. To protect the engine, the system defaults to a preset coolant temperature of 90°C and may enter a degraded mode with reduced engine power.

• AXION 960
• Axion series
• Lexion combines
• Jaguar forage harvesters

1. 1

   Check the engine coolant level to ensure it is filled properly, as low coolant can cause incorrect readings

2. 2

   Visually inspect the engine coolant temperature sensor, connector, and wiring harness for damage, corrosion, loose connections, or coolant leaks

3. 3

   Test the sensor circuit voltage by disconnecting the harness, turning the ignition on (engine off), and measuring for a 5-volt reference signal from the ECM

4. 4

   Check the signal wire in the harness for a short circuit to the chassis ground

5. 5

   Test the sensor resistance with a multimeter when cold and hot; resistance should decrease as temperature increases (NTC sensor). Infinite resistance indicates an open circuit

1. 1

   Confirm the Diagnosis

   Complete the diagnostic checks above to confirm that the sensor itself is faulty and not the wiring or connector.

2. 2

   Prepare for Replacement

   Allow the engine to cool down completely to avoid burns and minimize coolant loss. It is advisable to relieve any pressure in the cooling system by slowly opening the radiator or expansion tank cap.

3. 3

   Locate and Remove the Sensor

   The coolant temperature sensor is typically threaded into the engine block or thermostat housing. Place a drain pan underneath to catch any spilled coolant.

4. 4

   Disconnect and Unscrew

   Unplug the electrical connector from the old sensor. Using a deep socket wrench, turn the sensor counter-clockwise to unscrew and remove it.

5. 5

   Install the New Sensor

   Ensure the threads and sealing surface are clean. Screw the new sensor in by hand to avoid cross-threading, then tighten it to the manufacturer's specified torque. Do not overtighten.

6. 6

   Reconnect and Refill

   Securely plug the electrical connector onto the new sensor. Top off the cooling system with the appropriate type of coolant to replace any that was lost.

7. 7

   Clear Fault Codes

   Use a diagnostic tool to clear the 772Ah fault code from the ECM's memory.

8. 8

   Verify the Repair

   Start the engine and monitor the coolant temperature on the dashboard or with a diagnostic tool to ensure it is reading correctly and rising as the engine warms up. Check for any coolant leaks around the new sensor.