Case IH 3518 (DTC 2AC4) - ECU – Engine Control Unit (ECU): Ambient temperature sensor voltage is lower than expected

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

• MAXXUM T4E
• Case IH 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Case IH STX Steiger Tractors (350, 400, 450, 500, 550, 600 HP models)
• Case IH Farmall Tractors (40, 45, 50 with Cab and CVT Transmission)
• Case IH Puma Tractors

1. 1

   Visually inspect the ambient temperature/humidity sensor wiring harness for chafing, melting, or pinch points that could cause a short to ground.

2. 2

   Unplug the sensor and inspect the connector pins for corrosion, bent pins, or moisture ingress, which can drag the signal voltage down.

3. 3

   Perform a voltage test with the key ON (engine off) using a multimeter to ensure the sensor connector is receiving the proper 5V reference voltage from the ECU and has a solid ground.

4. 4

   Check the continuity of the signal wire from the sensor harness connector back to the ECU pinout.

5. 5

   If the wiring, 5V reference, and ground are all intact, the sensor has failed internally and must be replaced.

1. 1

   Power down machine

   Turn off the machine and engage the battery master disconnect switch to prevent electrical spikes.

2. 2

   Locate sensor

   Find the ambient temperature/humidity sensor on the intake piping or HVAC intake housing.

3. 3

   Disconnect sensor

   Press the release tab on the electrical pigtail and gently pull the connector free from the sensor.

4. 4

   Remove sensor

   Depending on the exact mounting style, either unthread the sensor from the intake housing or remove the retaining screws holding it in place.

5. 5

   Install new sensor

   Insert the replacement sensor. If it is threaded, ensure the O-ring is seated properly and do not overtighten to avoid cracking the plastic housing.

6. 6

   Reconnect harness

   Plug the electrical harness back in until it clicks securely.

7. 7

   Clear codes

   Restore battery power. Turn the key on and use the Case IH Electronic Service Tool (EST) to clear the inactive 3518 / 2AC4 code. Alternatively, run the engine; if the ECU detects the proper voltage range, the code may move from Active to Inactive on the cab display.