CM850 Electronic Control System – 019-261   Air Conditioning Pressure Switch

The air conditioning pressure switch circuit signals the system that the air conditioner head pressure is high and the engine fan
must be engaged. The air conditioning pressure circuit consists of the air conditioning pressure switch signal pin and switch return pin. This circuit is considered “fail safe”, meaning when the circuit is open, the engine fan will be engaged by the electronic control module (ECM).

Locate the air conditioning pressure switch. Remove the electrical connection from the switch. Adjust the multimeter to measure resistance. Touch one multimeter probe to one of the terminals on the switch. Touch the other multimeter probe to the other terminal of the switch.

When the system head pressure is high, the multimeter
must show an open circuit (100k ohms or more). If the circuit is
not open, the switch has failed. Refer to the original equipment manufacturer (OEM) troubleshooting and repair manual for replacement procedures.

When the system head pressure is low, the multimeter
must show a closed circuit (10 ohms or less). If the circuit is
not closed, the switch has failed. Refer to the OEM troubleshooting and repair manual for replacement procedures. If the resistance value is correct, the switch
must still be checked for a short circuit to ground.

When the system head pressure is low, touch one of the multimeter probes to one of the switch terminals. Touch the other probe to chassis ground. The multimeter
must show an open circuit (100k ohms or more). If the circuit is
not open, the switch has failed. Refer to the OEM troubleshooting and repair manual for replacement procedures. If the switch passes all of the previous checks, the circuit
must be checked for an open circuit, a short circuit to ground, a short circuit from pin to pin, and a short circuit to an external voltage source.