Relays and Sensors Description

Relays and Sensors Description

Blower Motor Relays

The blower motor relays direct system voltage either through the blower motor resistor or directly to the blower motor to control speed settings.

A/C Compressor Relay

The A/C compressor relay provides system voltage to the compressor clutch. The relay is controlled by the HVAC Controller, the A/C Compressor Refrigerant Pressure Switch and the A/C Compressor Cycling Switch.

A/C Compressor Refrigerant Pressure Switch

The system is equipped with a high-side pressure cutoff switch.

This switch is normally closed. The switch opens between 2940 kPa (420 psi) and 3150 kPa (450 psi).

This action will cause the following effects:

* The compressor will stop cycling.
* The pressure relief valve will be prevented from discharging refrigerant and oil.

The switch will again close after the pressure drops between 1050 kPa (150 psi) and 1750 kPa (250 psi).

A/C Compressor Cycling Switch

The refrigeration cycle is the on and off operation of the compressor.

A switch controls the refrigeration cycle. The switch senses the low-side pressure, which is an indicator of the evaporator temperature.

This switch is normally closed. The switch opens as pressure increases to 315 kPa (45 psi) and closes as pressure decreases to 182 kPa (26 psi).

The pressure cycling switch is the freeze protection device in the system. The switch senses the refrigerant pressure on the suction side of the system. The switch is mounted on a standard service low-side fitting. This switch also provides compressor cutoff during cold weather.

The operation of the low-pressure cycling system provides additional protection to the compressor.

Low-side pressures are not sufficient to close the contacts of the pressure switch if the following conditions exist:

* A massive discharge occurs in the low side of the system.
* The orifice tube is plugged.

The following actions will occur if there is a low charge when the outside (ambient) temperatures are high:

* Insufficient cooling
* Rapid compressor clutch cycling

The pressure cycling switch may be replaced without removing the refrigerant charge. A service fitting is located in the pressure switch fitting.

When replacing the pressure switch, install a new O-ring seal and the switch.