Case IH 3518 – Engine ECU (Engine Control Module): Ambient Air Temperature Sensor failure (of Humidity Sensor if installed) - Signal too low

The Engine Control Module (ECM) has detected that the voltage signal returning from the ambient air temperature/humidity sensor has dropped below the minimum acceptable threshold. This typically points to a sensor failure, a short circuit to ground in the wiring harness, or an internal ECU fault.

• Steiger T4B
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Steiger Series (350, 400, 450, 500, 550, 600)
• Magnum Series (235-340 Series 4)
• Puma Series (Puma 240)
• Farmall Tractors (40-45-50)
• WD1505 Windrowers

1. 1

   Visual Inspection: Locate the ambient air temperature/humidity sensor (typically located near the engine air intake or air filter housing). Inspect the sensor housing and the electrical connector for physical damage, moisture intrusion, or corroded pins.

2. 2

   Wiring Harness Check: Since the code specifies "Signal too low" (voltage lower than expected), trace the signal wire from the sensor back to the Engine ECU. Check for chafing, pinched wires, or exposed copper that could be causing a short circuit to ground.

3. 3

   Voltage/Resistance Test: Disconnect the sensor and use a multimeter to verify that the ECU is sending the correct reference voltage (usually 5V) to the connector. If the reference voltage is present and the wiring is intact, the sensor itself has likely failed internally.

1. 1

   Power Down

   Turn off the machine, remove the key, and disconnect the negative battery cable to prevent electrical shorts or accidental ECU damage.

2. 2

   Locate the Sensor

   Find the ambient air temperature/humidity sensor on the engine air intake piping or near the HVAC intake, depending on the specific machine model.

3. 3

   Disconnect

   Depress the locking tab on the electrical pigtail and carefully unplug the connector from the sensor.

4. 4

   Remove

   Unthread or unclip the faulty sensor from its mounting location.

5. 5

   Install

   Insert the new OEM replacement sensor into the housing. If it is a threaded sensor, ensure it is tightened to the manufacturer's specification (do not overtighten, as these are often plastic).

6. 6

   Reconnect

   Plug the electrical connector back into the new sensor until the locking tab clicks into place.

7. 7

   Clear Codes

   Reconnect the battery. Turn the ignition on and use a diagnostic scan tool to clear the inactive 3518 / 2AC4 fault code from the Engine ECU.