Relays and Switches

Blower Motor Relay

The blower motor relay is used to provide battery voltage to the blower motor assembly only during high blower motor assembly speed. The blower motor resistor is used to provide power to the blower motor assembly in all other speed settings.

High Pressure Cutoff Switch

The system is equipped with a high pressure cutoff switch. This switch normally closes at 1035-1379 kPa (150-200psi), and opens at 2827-3103 kPa (410-450 psi) to interrupt the voltage to the compressor clutch coil. This stops the compressor from cycling and prevents the pressure relief valve from discharging refrigerant and oil.

Pressure Cycling Switch

The refrigeration cycle (on and off operation of the compressor) is controlled by a switch that senses the low-side pressure as an indicator of evaporator temperature. The pressure cycling switch is the freeze protection device in the system and senses refrigerant pressure on the suction side of the system. This switch is mounted on a low-side fitting. This switch also provides compressor cutoff during cold weather.

Additional compressor protection results from the operating characteristics of the low-side pressure cycling system. If a massive discharge occurs in the low side of the system, or the orifice tube becomes plugged, low-side pressures could be insufficient to close the contacts of the pressure switch. In the event of a low charge, insufficient cooling accompanied by rapid compressor clutch cycling will be noticed at high outside (ambient) air temperatures.

If replacement of the pressure cycling switch is necessary, it is important to note that this may be done without removing the refrigerant charge. A service fitting is located in the pressure switch fitting. During replacement of the pressure switch, a new O-ring seal must be installed and the switch assembled to the specified torque of 4.5 N.m (40 lb in).