STICK AR 5088449 - Caterpillar

Adjustable Thermostat
A Heating Ventilation Air Conditioning (HVAC) system that is controlled by an adjustable thermostat contains two temperature control knobs.The first temperature control knob is for the air conditioning system and the control knob is connected to the adjustable thermostat.A thermostat is a switch that opens and closes. The thermostat switch opens and the thermostat switch closes because of the changes that can occur in the temperature of the sensing element. A sensing element is also known as a capillary tube. Sensing temperatures for the element less than the switch point on the thermostat will open the thermostat switch. Sensing temperatures for the element more than the switch point for the thermostat will close the thermostat switch.When the first temperature control knob is in the OFF position, the electric circuit that connects the adjustable thermostat and the magnetic clutch is OPEN. In the OFF position, the compressor does not operate. The temperature of the air and the position of the temperature control knob determines if the thermostat switch is CLOSED. When the thermostat switch is CLOSED, the current moves through the switch and the coil assembly in the magnetic clutch, and this causes the compressor to operate. The temperature of the air through the evaporator coil is monitored by the capillary tube that is placed between the fins of the evaporator coil.The second temperature control knob is for heating. This knob is connected to the heater control valve, which controls the flow of coolant through the heater coil. The temperature of the cab is maintained by the position of the air conditioning and heating temperature control knobs.Nonadjustable Thermostat (Freeze Control System)
Illustration 1 g01510274An HVAC system using a nonadjustable thermostat contains one temperature control knob. The temperature control knob is connected to the heater control valve, which controls the flow of coolant through the heater coil. The temperature of the cab is maintained by the position of the temperature control knob on the heater.The temperature of the air that crosses the evaporator coil is kept at the coldest possible temperature allowed by the nonadjustable thermostat. The thermostat switch opens at a temperature of approximately −1.1 °C (30 °F) in order to prevent the condensation from freezing to the evaporator coil. In addition, the thermostat switch closes at approximately 2.2 °C (36 °F). Condensation that is frozen will restrict the air flow through the evaporator coils. When the switch opens, the electric circuit that connects the thermostat switch to the magnetic clutch is OPEN and the compressor does not operate. When the switch is CLOSED, the current moves through the switch and through the coil assembly in the magnetic clutch. This causes the compressor to operate. In order to operate the system in the correct manner, the heater core supply must not be closed. Also, in order to operate the system in the correct manner, the return coolant line valve must not be closed.ATC System Operation
Illustration 2 g03090876
Typical HVAC Components
The Automatic Temperature Control (ATC) system includes all of the standard HVAC