Coolant Temperature Sensor / Switch HVAC: Description and Operation

A/C Pressure Transducer / Engine Coolant Temperature Sensor

The pressure transducer protects the refrigerant system from extremes of system pressure and in conjunction with the engine coolant temperature sensor, controls the speed of the engine coolant fan. The pressure transducer is fitted in the high pressure/temperature line of the refrigerant circuit on the AC pipe connecting the condenser and thermostatic expansion valve. It is installed in the left rear corner of the engine compartment, and is fitted to the pipe in a threaded coupling.

The transducer is connected through the vehicle harness to the EMS2000 where the signal is processed to calculate the pressure in the high pressure line. Because the compressor is lubricated by oil suspended in the refrigerant, the EMS2000 prevents operation of the compressor unless there is a minimum refrigerant pressure, and thus refrigerant and oil, in the system. When refrigerant pressure increases to a value that indicates additional condensing is required, the EMS2000 requests an increase in cooling fan speed. The signal from the pressure transducer is a varying voltage type. Higher voltages indicate higher pressure, lower voltages lower pressure. Normal operating range is approximately 1.2 - 2.0 volts.
An open in the transducer circuit will cause the EMS to see maximum system pressure and disable the compressor from further operation.