Case IH 120 – ATC (Automatic Temperature Control): Blower speed select pot open/shorted to power

This fault code indicates a circuit issue with the blower speed potentiometer (the dial or switch in the cab used by the operator to select the HVAC fan speed). The control module is receiving a voltage signal from the potentiometer that is higher than the expected maximum threshold. This happens when there is a physical break in the circuit (an open circuit) or if the signal wire has chafed and is touching a live voltage source (shorted to power).

• STX Steiger
• Magnum MX Series (MX180 to MX305)
• Magnum TG Series
• Magnum Series 4 (235-340)
• Steiger STX Series (STX325 to STX500)
• Axial-Flow AFX8010
• Axial-Flow 7120
• Axial-Flow 8120
• Axial-Flow 9120
• Axial-Flow 20-series

1. 1

   Visually inspect the wiring harness at the rear of the ATC control panel for pinched wires, chafing, or exposed copper that could be shorting to a 12V power source.

2. 2

   Disconnect the plug at the blower speed potentiometer and check for pushed-out pins, loose terminals, or corrosion (green crust).

3. 3

   Perform a potentiometer sweep test using a digital multimeter set to Ohms (Ω) across the switch terminals while slowly turning the dial. Look for 'OL' (Open Loop) or dead spots indicating a broken internal resistor.

4. 4

   Test harness continuity on the signal wire from the switch connector back to the ATC module connector to ensure there is no break in the wire.

1. 1

   Disconnect Power

   Turn off the machine and disconnect the negative battery cable to prevent electrical spikes while working on the CAN-bus/ATC system.

2. 2

   Access the Control Panel

   Remove the cab trim panels/bezels surrounding the climate control unit to access the mounting screws. Pull the ATC panel forward.

3. 3

   Isolate the Fault

   Based on your diagnostic checks, determine if the fault is in the wiring or the switch itself.

4. 4

   Replace the Switch (If faulty)

   Unplug the electrical connector from the back of the blower speed potentiometer. Unclip or unbolt the switch from the control panel housing. Install the new potentiometer and reconnect the harness.

5. 5

   Repair Wiring (If faulty)

   If the harness is damaged, cut out the shorted/open section of the wire, solder in a replacement section of the same gauge, and seal it with heat-shrink tubing.

6. 6

   Reassembly

   Push the ATC control panel back into the console and secure it with the mounting hardware. Reinstall all cab trim pieces.

7. 7

   Clear Codes and Test

   Reconnect the battery. Turn the ignition key to the ON position. Clear the stored fault codes via the instrument cluster or diagnostic tool. Test the blower motor in all manual speed settings and the 'Auto' setting to ensure the code does not return.