Front

GENERAL DESCRIPTION
The Cycling Clutch Orifice Tube (CCOT) refrigeration system is designed to cycle a compressor on and off to maintain desired cooling and to prevent evaporator freeze-up. Passenger compartment comfort is maintained by the temperature select bar on the controller.

Control of the refrigeration cycle (on and off operation of the compressor) is done with a switch that senses low side pressure as an indicator of evaporator pressure. During air temperatures of 60-80°F, the equalized pressures within the charged air conditioning system will close contacts of the pressure switch.

When an air conditioning mode is selected, voltage is supplied to the compressor clutch coil. As the compressor reduces the evaporator pressure to approximately 25 psi, the pressure switch will open, de-energizing the compressor clutch. As system equalizes and the pressure reaches approximately 46 psi, the pressure switch contacts close, re-energizing the clutch coil.

When the engine is turned off with the air conditioning system operating, the refrigerant in the system will flow from the high pressure side of the expansion tube (orifice) to the low pressure side until the pressure is equalized. This may be detected as a hissing sound for 30 to 60 seconds and is considered a normal condition.

CONTROL DOOR MOTORS









Three reduction gear motors are used to position the control doors based on input from the control panel. The motors are the permanent magnet type and use a feedback potentiometer to indicate exact door position.

The direction of motor rotation is controlled by a series of four transistors called an "H" gate. When the transistors marked "A" are forward biased, the motor will rotate in the forward direction. When the transistors marked "B" are forward biased, the polarity in the motor windings is reversed and the motor will rotate backward.

The feedback potentiometer is driven by the final drive gear. A five-volt input signal, feedback circuit and ground are used to signal door position.

The feedback circuit is a self-balancing bridge. If the input voltage and the feedback voltage are balanced, there will be no current flowing in the motor control circuit. When input voltage is changed, a voltage imbalance condition will exist in the motor control circuit. This will cause one pair of transistors in the "H" gate to be forward biased, driving the motor forward or backward as needed. As the motor runs, it moves the wiper in the feedback potentiometer until the control circuit voltage returns to a balanced condition. When the voltages balance, current flow in the motor circuit stops.

The three control door motors are wired to the microprocessor with several common circuits. Circuit 361 is the common grounding point for the feedback potentiometers while circuit 363 supplies the five-volt input to each. The feedback circuits are separated and wired individually to the control head.