Relays and Switches

Blower Controls
Battery voltage is supplied at all times to the No. 1 blower relay through the HTR A/C fuse and the RED wire. When the control assembly is "OFF," all blower relays are de-energized. In this state there is no voltage path through the relay contacts to the blower motion The blower motor does not run.

With low blower speed selected, current flows through the No. 1 relay, to the top of the resistor board (A) and both resistors.

With medium #1 blower speed selected, current flows through the No. 1 relay, to the No. 2 relay, to the center of the resistor board (A) and one resistor to the motor.

With medium #2 blower speed selected, current flow is the same as in medium #1 in addition, a parallel current path goes through resistor (B) and No. 3 blower relay to the blower motor. This provides a parallel path with a value less than one alone, but more voltage than for medium #1 blower speed.

With high blower speed selected, No. 1 and No. 2 blower relays are off. Relay No. 3 is on, and provides a parallel battery feed circuit to the blower motor.

Compressor Pressure Switch
This system is equipped with a compressor pressure switch that is normally open. It activates at 2069-2482 kPa (300-360 psi) and releases at 1724 kPa (250 psi).

Its function is to drive the recirculation door to the closed position to reduce the system load and decrease system high-side pressure.

High Pressure Cut-off Switch
The system is also equipped with a high pressure cut-off switch. This switch is normally closed, and opens at 2827-3103 kPa (410-450 psi) to interrupt the voltage to the compressor clutch coil. This will stop the compressor from cycling and prevent 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 standard service 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 fining. During replacement of the pressure switch, a new O-ring seal must be installed and the switch assembled to 4.5 N.m (40 lb in.).