Case IH 3938 – Engine Control Unit (ECU): Air Temp in AFS: Signal range high on the period of PWM signal

The Engine Control Unit (ECU) is receiving a Pulse Width Modulated (PWM) signal from the Air Flow Sensor (AFS) or Air Filter Temperature Sensor that exceeds the maximum expected period or range. This is typically caused by an electrical problem in the wiring (such as an open circuit or a short to power) or a drifted/defective temperature sensor.

• Magnum
• Steiger
• Puma
• Maxxum
• Optum
• Farmall
• Axial-Flow series

1. 1

   Visually inspect the Air Flow Sensor (AFS) / Air Intake Temperature Sensor, electrical connector, and wiring harness for visible damage, corrosion, or loose pins.

2. 2

   Using a multimeter, check the wiring harness between the AFS and the ECU for continuity, open circuits, or a short to voltage on the signal wire.

3. 3

   Verify that the sensor is receiving the correct reference voltage (typically 5V) and has a solid ground connection.

4. 4

   If wiring is intact, monitor the PWM signal coming from the sensor with the key ON using an oscilloscope or advanced multimeter to see if the signal period remains out of range or flatlined high, indicating internal sensor failure.

1. 1

   Power Down

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

2. 2

   Locate the Sensor

   Find the Air Flow Sensor / Air Temperature Sensor located on the intake manifold, air filter housing, or intake piping.

3. 3

   Disconnect

   Carefully unplug the weather-pack electrical connector from the sensor.

4. 4

   Harness Repair

   If your diagnostics revealed a broken wire or corroded connector, splice in a new connector pigtail or repair the harness using heat-shrink butt connectors.

5. 5

   Sensor Replacement

   If the sensor is defective, remove the mounting hardware (usually Torx/Allen screws or a threaded hex body) and pull the old sensor out of the intake tract.

6. 6

   Installation

   Insert the new OEM sensor. Ensure any O-rings or seals are properly seated to prevent unmetered air leaks. Tighten to the manufacturer's torque specification.

7. 7

   Reconnect

   Plug the electrical connector back in until the locking tab clicks.

8. 8

   Clear Codes

   Reconnect the battery, turn the key to the ON position, and use the CNH Electronic Service Tool (EST) or an equivalent diagnostic scanner to clear the inactive 3938 fault code. Start the engine and verify the code does not return.