shows how the rotary switch (system on-off) is connected to the thermostat and blower motor assembly. The condenser motor is wired to the thermostat and Trinary™ switches through a relay. The resistor con- trols voltage to the blower motor assembly.
Chapter 5 – System Controls
3. Combination and Supplemental Controls
Combination and supplemental controls may protect the major compo- nents (primarily the compressor) from damage, control engine tempera- ture and insure condenser efficiency.
When used in system design, combination and supplemental controls help maintain the pressure and temperature balance inside air condi- tioner and heater systems. They can make safe system operation possible in severe or varied climate conditions, and maximize heat energy move- ment under less than ideal conditions.
Two or three function pressure switch assemblies (Red Dot Binary™ or Trinary™ switches) may be installed on the liquid side of the AC system between the condenser and expansion valve. Both switches are illustrated in Figure 5-12.
RED DOT BINARY™ SWITCH
Low pressure protection Between 3-15 PSIG (falling pressure) 40 PSIG maximum (rising pressure) High pressure protection Between 270-330 PSIG (rising pressure) 80-120 PSIG below open pressure RED DOT TRINARY™ SWITCH
Low pressure protection 22.5 ± 7.5 PSIG (falling pressure)
40 PSIG maximum (rising pressure) High pressure protection Between 270-330 PSIG
(rising pressure)
80-120 PSIG below open pressure Shutter fan/override Opens 35-60 PSIG
below closing pressure
200-230
PSIG (rising pressure)
OPENS CLOSES
OPENS CLOSES
The two function Binary™ switch prevents the compressor from oper- ating if there is no refrigerant in the system. It also stops the compressor if head pressures increase to unsafe levels. The switch resets when pressure drops to normal.
Red Dot’s Trinary™ pressure switch performs three distinct func- tions to monitor and control pressure inside the AC system. The switch is installed between the condenser and expansion valve, usually on the receiver-drier. All three functions automatically reset when the proper pressure is achieved.
• The low-range pressure function prevents compressor opera- tion if the refrigerant charge has been lost or ambient tem- perature is too cold.
• A mid-range pressure function activates the engine fan clutch or radiator shutter assembly. As system pressure reaches mid range, the switch engages the fan clutch (fan motor on roof top condenser units) and/or opens the radiator shutter. This in- creases air flow to the condenser and stabilizes or lowers system operating pressures. The switch cycles on and off to maintain operating pressures.
• A high-range pressure function turns off the compressor if pressure is to high.
Figure 5-12
Red Dot Binary™ and Tri- nary™ switches with switch operating pressures given.
• Multi function switches other than Red Dot’s are being used in some applications. The concept is the same as described above.
4. Fan Clutches, Radiator Shutters & Override Controls
An engine fan clutch and radiator shutter assembly may be used to control and maintain engine (coolant) temperature. Engine fan and radia- tor shutter operation are closely related to radiator mounted AC con- denser efficiency (heat energy movement). This is the main reason for fan clutch and radiator shutter override controls.
There are three types of fan clutches:
• Viscous Drive • Air Actuated • Electric
All but the viscous type may have a fan override control as part of the AC system. The viscous drive fan clutch has a high viscosity silicone fluid that moves inside the clutch to increase or decrease fan speed. The fluid is controlled by a temperature sensor, valve and centrifugal force.
Air actuated fan clutches are either on or off. Control is by a
thermostatic valve which measures engine coolant temperature. When coolant is hot, the valve opens and air pressure enters the fan body causing the clutch to engage. Some air actuated clutches use air pressure for the off condition.
The electric fan clutch has a preset engine coolant temperature
sensor. When the set temperature is reached, the clutch is engaged for increased air movement through the condenser and radiator. When cool- ant temperature drops the clutch disengages the fan.
Radiator shutters control air (outside air) flow through the radiator.
These shutters are opened and closed by thermal or air activated spring loaded valves. Both types of valves sense and maintain engine coolant within a narrow temperature range (4 to 6 degrees). The narrow 4 to 6 degree Fahrenheit temperature range selected may fall anywhere be- tween 160 to 220 degrees Fahrenheit. Thus, override controls are re- quired to bypass radiator shutter controls for efficient or safe condenser function and AC system operation.
Chapter Review
• Basic system controls begin at a control panel inside the cab or sleeper compartment. We can decide on heating or cooling and where and how much air flow we want. We can select a temperature range for the inside of the cab or sleeper. • When a heater, air conditioner or HVAC system is turned on,
the automatic control devices take over system control. They are designed to keep the system in balance and maintain the operator selected temperature range inside the vehicle.
Chapter 5 – System Controls
• A balanced system is more efficient in moving heat energy into or out of the air in the cab than an unbalanced one. Balance means maintaining pressures and temperatures in- side the system so that heat energy moves easily and effi- ciently from one area to another.
• In most systems one or more heat or pressure sensitive switches may be installed. These usually protect the com- pressor if something goes wrong inside the system or the refrigerant leaks out.
• The more advanced system designs use air pressure or vacuum for system and component control. Electronic con- trols are also used. The advantages of advanced controls are more even cab temperature and faster control response. • The engine fan clutch and radiator shutter are controlled by
engine coolant temperature. In an AC system with an underhood condenser, override controls are used to govern fan clutch and radiator shutter operation.