Air Conditioning

CIRCUIT OPERATION

Compressor Controls

With the A/C-Heater Select Control in any mode except VENT or HEAT, current flows from the HTR-A/C fuse through the Evaporator Pressure Control Switch, the High Pressure Cutout Switch, and the Coolant Temp. Cutout Switch to the A/C Compressor Clutch and the A/C Compressor operates. At the same time a signal is sent to the Electronic Control Module (ECM). This A/C Request signal increases engine idle speed.

If the A/C system pressure drops below 25 psi, the Evap Pressure Control Switch opens stopping current flow to the A/C Compressor. When the A/C system pressure rises to 45 psi, the switch closes and current again flows to the A/C Compressor.

The RPO L19 engine is equipped with two additional compressor cut-out switches. If the engine temperature gets too high (usually during idle after high loading conditions), these switches shutoff the A/C compressor, reducing the load on the engine and preventing engine overheating.

One switch, a Coolant Temp Cutout Switch, is located in the thermostat housing. When the coolant in the top tank reaches 124°C (255°F), the switch shuts off the compressor. When the coolant temperature drops below 116°C (240°F), the switch allows the compressor to reengage.

The second switch the High Pressure Cutout Switch is located in the back of the A/C compressor. This switch senses compressor outlet pressure. When the pressure exceeds 2,827-3,103 kPa (410-450 psi), the compressor is shut off until the pressure drops below 1,448-1,724 kPa (210-250 psi).

The A/C Compressor Clutch Diode is connected across the terminals of the A/C Compressor Clutch. Whenever the clutch is de-energized, the magnetic field around it collapses, generating an induced voltage in the clutch coil. The Diode provides a path for the current resulting from the induced voltage so that other circuit components are not damaged.

Blower Motor Control

In all blower speeds except HI, power to the Blower Motor is supplied by the HTR-A/C fuse. In HI blower speed, the blower motor is supplied by Fuse Link C.

The Blower Motor operates whenever the A/C-Heater Select Control is in MAX A/C, NORM A/C, BI-LEVEL, BLEND or DEFROST. Power through the A/C-Heater Select Control is available to the Blower resistors through the 51 (YEL) wire. The current then flows through all three resistors and the normally closed contacts of the High Blower Relay to the Blower Motor and the Blower Motor runs at very low speed. With the A/C-Heater Select Control in VENT or HEAT no power is available to the Blower Motor.

With the Blower Switch in LO, current flows from the HTR-A/C fuse, through the Blower Switch to the 63 (TAN) wire at the Blower Resistors. Current then flows through two of the resistors and the normally closed contacts of the High Blower Relay to the Blower Motor. The Blower Motor then runs at Low speed.

With the Blower Switch in MED, the blower operates the same as in Low Speed, except that the current is directed to the Blower Resistors through the 72 (LT BLU) wire and passes through only one resistor and the Blower Motor runs at slightly faster speed.

With the Blower Switch in HI, current bypasses the Blower Resistors and flows to the High Blower Relay. This current energizes the relay and closes the normally open contacts of the relay. This allows current to flow from Fuse Link B through the relay to the Blower Motor and the Blower Motor runs at High Speed.