Case IH 3518 (F-171-04) – Engine Control Unit (ECU): Ambient Air Temperature Sensor failure (of Humidity Sensor) - Signal too low

The control module is receiving a voltage signal from the ambient air temperature/humidity sensor that falls below the minimum acceptable threshold. This condition typically points to a short to ground in the sensor circuit, an open circuit, or an internal failure of the sensor itself.

• VESTRUM CVX
• Magnum Tractors (235 - 340 Series 4)
• Steiger Tractors (350 to 600 horsepower)
• Puma Tractors
• Axial-Flow Combines (240 Series, 7230, 8230, 9230)
• Wheel Loaders (821F, 921F)

1. 1

   Locate the combined Humidity and Ambient Air Temperature sensor mounted at the engine air intake / air filter housing

2. 2

   Inspect the sensor's wiring harness and connector for visible damage, chafing, or corrosion

3. 3

   Check for a short to ground or an open circuit in the wiring between the sensor connector and the Engine ECU

4. 4

   Verify if other codes are present (e.g., 3519 for CAN signal failure, 3517 for signal too high) to determine if the sensor has failed internally

1. 1

   Turn off the machine and disconnect the battery

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

2. 2

   Locate the sensor

   Locate the ambient air temperature/humidity sensor on the engine air intake piping or air filter housing.

3. 3

   Disconnect the electrical wiring harness

   Carefully depress the locking tab and disconnect the electrical wiring harness from the sensor.

4. 4

   Remove the sensor

   Remove the mounting hardware or unthread the sensor from the intake housing. Cover the turbocharger/intake opening immediately to prevent debris from entering the engine.

5. 5

   Install the new sensor

   Install the new ambient air temperature/humidity sensor into the intake housing.

6. 6

   Reconnect the electrical harness

   Reconnect the electrical harness, ensuring the connector clicks securely into place.

7. 7

   Reconnect the battery

   Reconnect the battery.

8. 8

   Clear the fault code

   Turn the key to the ON position and use the Electronic Service Tool (EST) or onboard diagnostic display to clear the 3518 fault code.

9. 9

   Verify repair

   Start the engine and verify that the code does not return and that the engine operates normally.