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HVAC Service Documents

Recommended Charging Procedure
Charge Determination
Basic Troubleshooting
R134a and PAG Oil Handling
Compressor Oil Handling
Compressor Troubleshooting
Blower Motor
Glossary of Common HVAC components

Gauge Diagnostics

Using Gauges for Troubleshooting
Air in cooling system (insufficient suction)
Defective expansion valve
Excessive refrigerant
Insufficient compressor operation
Insufficient refrigerant
Moisture Intrusion
Refrigerant does not circulate

Summary Table of Gauge Diagnostics

Glossary of Common HVAC Components

Cold Control (Thermostat)
Evaporator Core
Fan Switch
Heater Core
Pressure Switch
Receiver Dryer
Thermal Expansion Valve (TXV)

Cold Control (Thermostat)

The cold control or thermostat measures the temperature of the core and engages/disengages the compressor circuit. The cold control has a sensing tube that is placed through the evaporator core. THIS TUBE IS VERY DELICATE.


The A/C pump for the system. The compressor converts the coolant from a low pressure / low temperature gas to a high pressure high temperature gas by adding work to the A/C system cycle via a pulley drive.


The A/C condenser coil converts the coolant from a high pressure, high temperature gas to a high pressure lower temperature liquid. By passing air through the coil and utilizing the latent heat of vaporization or dew point of the gas, a liquid stream can be formed. The core temperature is higher in temperature than the ambient air passing throught it. The condenser is typically near the radiator drawing engine air, or in a remote housing driven by a condenser fan. THE CONDENSER MUST ALWAYS DRAW FIRST AND COOLEST AIR. IF THE CONDENSER IF MOUNTED IN SUCH A WAY THAT THE RADIATOR AIR PASSES THROUGH THE COIL THE SYSTEM WILL NOT WORK.

Evaporator Core

The system evaporator core is the point where the cab’s air interacts with the system. The coil draws heat from the cab. The evaporator core is typically mounted inside the operator cab with air ducted around the cab.

Fan Switch

fan switch
This is the switch which controls the A/C fan. (Typically high med low and off). This is typically mounted into console control panel.

Heater Core

heater core
The heater is the point in a cab where engine heated coolant is exchanged with cab air warming the air in the cab. The heater is typically mounted inside the operator cab with air ducted around the cab.


Air conditioning hoses connect all components in the A/C cycle. Smaller #6 hoses carry liquid from the condenser to the receiver dryer, to the evaporator. #8 A/C hoses carry high pressure gas from the compressor to the condenser. #10 A/C hoses carry low pressure/low temperature gas from the evaporator to the compressor. A/C lines also come installed with steel fittings crimped on them, and often times service ports already installed for A/C charging. Heater hoses carry engine coolant from the engine to the heater core.

Pressure Switch

pressure switch
High, Low, and binary switches are used in the system as a protection device. The pressure switches keep the system from creating too much pressure, or in the case of rapid evacuation, they keep the system from ruining the compressor and ingesting debris. Typically the switches are mounted on the receiver dryer and evaporator core. In some cases a binary switch is used which is mounted on the receiver dryer.

Receiver Dryer

receiver dryer
The receiver dryer in the system is the liquid tank for the system. The dryer has desiccants, and filters inside it to remove any moisture and dirt from the system. MOISTURE IS VERY BAD FOR AN A/C SYSTEM. In most cases where an A/C system has been left open to the elements, the dryer will need to be replaced. Receiver dryer mounting is important. PMI dryers must be vertically mounted. This is a liquid tank with a take up tube running through the center trapping any gasses at the top of the dryer. The dryer is typically where a binary switch will be located.

Thermal Expansion Valve (TXV)

thermal expansion valve
The thermal expansion valve is the point in the A/C system where the high pressure high temperature liquid is flash expanded to a low pressure low temperature gas. This rapid change from liquid to a gas creates an energy sink (cooling effect) removing heat from the air driving the expansion. A typical expansion valve has a pressure tube and thermal measuring tube. (Unless it is a block expansion valve in which case it is internally controlled) The interaction between the tubes and the expansion valves internal superheat spring determines the valve’s superheat. The Thermal expansion valve is mounted on the Evaporator core.

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Perrin Manufacturing is committed to providing you with superior products and services; we renew this pledge through continual improvement of our products and motivation of our staff.

We faithfully comply with all of the requirements of our ISO 9001:2008 Quality Management System, the most current edition of the standard. Beyond that, our highly trained quality staff persistently search for new ways to improve the effectiveness of our system.
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