New Holland 121 – HVAC / BCM: Short Circuit on Power Supply or Open Circuit of the Temperature Selector Potentiometer

The fault indicates a short circuit on the power supply or an open circuit in the temperature selector potentiometer. This variable resistor controls the cab's HVAC system. A short circuit suggests the voltage signal is higher than expected (potentially due to a wire shorting to a power source), while an open circuit means there is a break in the electrical path. In either case, the control module cannot properly read the user's desired temperature setting, leading to improper operation of the climate control system.

• T4 Series
• T5 Series
• T6 Series
• T7 Series
• TM Series

1. 1

   Visually inspect the wiring and connectors associated with the temperature control knob for any visible signs of damage, corrosion, or loose connections.

2. 2

   Test the potentiometer by disconnecting its electrical connector and measuring resistance (Ohms) between the center wire and one of the outside wires. The resistance should change smoothly as the knob is turned. If it drops to zero or shows infinite resistance (open circuit), the potentiometer is likely faulty.

3. 3

   Check for a short to power: With the potentiometer disconnected and the key on, measure DC volts between the signal wire pin on the harness connector and a good chassis ground. It should not show system voltage.

4. 4

   Check for an open circuit: With the potentiometer disconnected, test for continuity between the signal pin at the potentiometer connector and the corresponding pin at the HVAC control module connector.

1. 1

   Access the Control Panel

   Remove the A/C or HVAC control panel in the cab to gain access to the potentiometer.

2. 2

   Disconnect and Remove

   Carefully disconnect the electrical connector from the back of the potentiometer and unfasten it from the control panel.

3. 3

   Install the New Part

   Install the new potentiometer, ensuring it is securely fastened.

4. 4

   Reconnect and Test

   Reconnect the electrical connector and test the operation of the HVAC system to ensure the temperature can be adjusted correctly. Note that some systems may require a recalibration procedure after component replacement.

5. 5

   Reassemble

   Once functionality is confirmed, reassemble the control panel and any other trim pieces that were removed.