Thermostatic Switch
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| Illustration 1 | g01077022 |
The thermostatic switch that is in the compressor electrical circuit cycles the compressor. This allows the operator to adjust the amount of coolness that is desired and prevent the evaporator from freezing.
The thermostatic switch consists of a stationary contact and a pivoting frame that is attached to a capillary bellows assembly. The capillary tube is filled with R-134a or a similar refrigerant. The capillary tube is inserted between the evaporator core fins. The refrigerant that is in the capillary tube expands or the capillary tube contracts. This depends on the temperature of the evaporator.
When the refrigerant in the capillary tube expands and contracts, the bellows expand and contract. The expanding and contracting bellows cause the pivoting frame to pivot.
Part of the wire to the compressor clutch coil is connected to the stationary contact, and the other part is connected to the pivoting frame. The contact and pivoting frame must come together in order for the switch to close and operate the compressor clutch.
The operator regulates evaporator cooling by varying the space between the stationary contact and the pivoting frame. Moving the contact and pivoting frame farther apart (decreasing cooling) causes the bellows to expand farther before closing the thermostatic switch. Moving the contact and pivoting frame closer together (increasing cooling) causes the thermostatic switch to close with less bellows movement.
Adjustable thermostats have provisions for regulating the range between the opening and closing of the switch. The adjustment screw is located under a removable cover. If the adjustable screw is not found in this location, the thermostat is nonadjustable.
The nonadjustable thermostat system contains one temperature control knob.This system is called a Freeze Control System. The knob is connected to the heater control valve. The heater control valve controls the flow of coolant through the heater coil. The evaporator air flow temperature is controlled by the nonadjustable thermostat. The cab temperature is maintained by monitoring the air flow across the heater and evaporator coils. When air flow across the heater and evaporator coils reaches 2.2 °C (36 °F), the nonadjustable thermostat turns the compressor ON. When air flow temperature decreases to -1.1 °C (30 °F), the nonadjustable thermostat turns the compressor OFF.
Compressor Clutch
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| Illustration 2 | g01077023 |
The clutch is driven by the engine crankshaft through a belt to the pulley assembly on the magnetic clutch. The pulley assembly turns on the bearing. The pulley assembly is not connected to the shaft. The drive plate is splined through the hub to the shaft. The coil assembly is mounted on the frame of the compressor. The coil assembly does not rotate.
The electrical current from the thermostat creates a magnetic field in the coil assembly. The magnetic field pulls the drive plate against the pulley assembly. The pulley assembly then turns the drive plate, the hub, and the shaft in order to operate the compressor.
Low Pressure Switch
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| Illustration 3 | g01077026 |
Illustration 3 shows the low-pressure sensing switch (arrow) that is threaded into the receiver-dryer. The low-pressure sensing switch is used to protect the system from damage due to the lack of oil. The low-pressure sensing switch is located in the electrical circuit to the magnetic clutch, the switch opens when system pressure decreases below 175 kPa (25 psi) and shuts off the compressor. The low pressure switch may be located on one of the following components:
- The dryer
- The expansion valve
- The liquid line
- The compressor
A similar high pressure switch (not shown) is used on some machines to shut off the system before system pressure reaches the high pressure relief valve setting. The high pressure switch is located in the electrical circuit to the magnetic clutch. High system pressure opens the switch. This shuts off the compressor.
High-Pressure Relief Valve
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| Illustration 4 | g01077027 |
The high-pressure relief valve is located on the compressor and/or the receiver-dryer. The high-pressure relief valve (arrow) allows the refrigerant to be released to the atmosphere if system pressure increases above 3450 kPa (500 psi). On todays systems, the high-pressure relief valve opens a high pressure switch. This prevents refrigerant from being vented into the atmosphere.
Moisture Indicator
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| Illustration 5 | g01077028 |
Illustration 5 shows the moisture indicator. The moisture indicator is located in the line between the receiver-dryer and the expansion valve. The moisture indicator measures the relative moisture in the system. A moisture reference color chart is on the face of the indicator. The color blue represents a dry system. The color pink represents a wet system.
The moisture indicator should be checked at the end of each shift. To check the moisture indicator, look at the indicator ring (2) through the sight glass (1). If the indicator ring is blue in color, the system is dry. If the indicator ring is pink in color, the system has moisture. The moisture must be removed and the receiver-dryer must be changed.
Note: Since 1999, moisture indicators have been removed from most of the systems because of the inaccuracy of the color change and misinterpretation of the color meaning by field personnel.