How to Repair Automotive Air-Conditioning & Heating Systems. Jerry Clemons
suction line and then compresses it to raise the pressure and temperature. The refrigerant exits the compressor as a high-pressure gas.
2. The refrigerant is routed to the condenser through the dis charge line. The refrigerant enters the condenser as a heated high-pressure gas. Airflow through the condenser fins causes the refrigerant to cool down, which causes the gas to condense into a liquid. The refrigerant exits the condenser as a hot, high-pressure liquid. The temperature of the liquid is typically 20 to 50 degrees cooler at the exit than it was on entry of the condenser. The temperatures at the condenser inlet will vary greatly because of the wide variance of pressures that can exist at the condenser. On very hot days, the pressures can be as high as 300 psi, which would create inlet temperatures in the 275 to 300°F range. On a 70°F day, the pressure would be approximately 150 to 175 psi, which would create inlet temperatures in the 150 to 175°F range.
It is helpful to understand the flow of refrigerant as it passes through each section of the AC system. The low side of the AC system runs from the exit of the orifice tube up to the entry point of the compressor. The high side of the AC system runs from the exit point of the compressor to the entry point of the orifice tube.
3. The refrigerant is routed to the orifice tube through the liquid line. The orifice tube acts as a restriction device since the passage pipe is much smaller than the liquid line. High-pressure liquid enters the orifice tube and low-pressure atomized liquid exits the orifice tube and then is routed into the evaporator core. Atomized liquid will have small droplets of bubbles due to the pressure drop as it exits the orifice tube.
4. The refrigerant enters the evaporator core as a cool low-pressure atomized liquid. Duct box air is routed through the fins of the evaporator core, which causes the liquid refrigerant to begin to vaporize into a gas. This process of latent heat of evaporation absorbs the heat from the duct box air, which causes the air to be cool and dry as it exits the evaporator fins. The refrigerant exits the evaporator core as a low-pressure gas and is then routed to the accumulator dryer.
5. The refrigerant enters the accumulator dryer as a cold low-pressure gas. The accumulator dryer stores and dries the refrigerant and prevents any liquid refrigerant from being routed to the compressor. The exit point is near the top of the accumulator to prevent any liquid from being sent to the compressor through the suction line.
On late-model vehicles, the mechanical valve that adjusted the output of the compressor has been changed to a solenoid, which works much better. The engine computer monitors the conditions of the pressure and the temperature and adjusts the command to the solenoid, which adjusts the output of the compressor. Both of these variable-style compressors did not need to be cycled on and off to control the temperature in the evaporator core.
| Normal Operating Characteristics |
| The key characteristics to look for under the hood while an orifice tube system is running are: • The suction line will be cold and likely have water droplets on the line. • The accumulator dryer will be cold to the touch and likely have water droplets on the device. • The discharge line, condenser, and liquid line will be hot and should be checked with a laser thermometer/pyrometer. • There will be water draining from the duct box drain tube after the system has run for several minutes. |
The compressor is driven by the engine drive belt and is energized when power is sent to a stationary coil that creates a magnetic field to pull the front clutch plate into the drive pulley. This action causes the internal parts of the compressor to begin compressing the refrigerant.
Variable-displacement compressors mechanically adjust their output by monitoring the temperature of the suction gasses that are drawn in from the suction line. When low temperatures are sensed by the internal valve, the angle of the swash plate is changed, which reduces the stroke of the pistons inside the compressor. Many manufacturers used this type of compressor as a method to keep the evaporator temperature from getting too low and causing ice to form on the surface of the evaporator core.
It is common to hear a clicking sound when the compressor cycles on and off during the normal operation of a cycling AC system. The clutch coil creates a magnetic field that pulls the front plate into the rotating pulley. This causes the internal compressor components to begin operating.
The suction line will be cold and have water droplets on the surface when the AC system is operating as designed. This line contains cold refrigerant gas that is being routed into the compressor. The cold surface of this line collects moisture from the surrounding air.
The accumulator dryer will be cold and wet on an AC system that is operating in an efficient manner. One point to keep in mind is that there should not be any ice or frost on any of the lines or components during operation. Visible ice or frost is a sign that the deicing systems are not working correctly.
It is common for the suction line to be cool or cold when the AC system is operating as designed. It is a good practice to check the temperature of the suction line to see if it is cold as a diagnostic step.
The accumulator dryer will be cold and wet when the AC system is working well. The accumulator is located between the exit of the evaporator core and the entry point of the AC compressor in orifice tube systems. If the accumulator is not cold while the system is operating, then the system is not functioning properly.
The high side components of the AC system will be warm or hot when the system is operating correctly. The compressor raises the pressure and temperature of the refrigerant gas, which is then routed to the discharge line that connects to the condenser. The liquid line connects the exit of the condenser to the inlet of the evaporator core. All of the components in the high side of the system will be at a high temperature while the system is operating.
The evaporator drain tube will have water droplets dripping when the AC is used on hot days. The water originates from the hot, humid air that is pushed through the evaporator core. As the heat is absorbed into the boiling refrigerant, the water that was in the hot air sticks to the surface of the evaporator and then runs down the bottom of the duct box where it drains out of the drain tube. Refrigerant boils at very low pressures/temperatures. The refrigerant is turning from a liquid to a gas in the evaporator while taking on the latent heat of evaporation, which is why the duct air turns cool and dry after it passes through the evaporator core.
Operation of Each Major AC Component
The following list explains what each component of the AC system is and what it does.
Compressor: The AC compressor serves as the pumping device that causes the refrigerant to move through the system when the AC is commanded to run. The different compressor types include the piston, scroll, and vane pump styles. The compressor is needed to pull in cold, low-pressure gaseous refrigerant into hot, high-pressure gaseous refrigerant. The temperature of the gaseous refrigerant needs to be increased in order to use the ambient