Automotive Air Conditioning System Theory........... Procedures...........Diagnosis.......... // -->

"Servicing The World With Automotive Air Conditioning Theory and Diagnosis"


Low Side
Cycling Switch Operating Parameters:

-Automotive Air Condition Is Our Only Business-

Type of System:
Thermostatic Cycling
15 - 30 psi
Cycling Pressure GM
25 - 44 psi
Cycling Pressure GM
21 - 44 psi
Cycling Pressure Ford
25 - 46 psi
Cycling Pressure Ford
23 - 46 psi
Cycling Pressure Chrysler
21 - 44 psi

These are pressures at which the compressor clutch is set to cycle on and off. Actual normal low Side Pressures will Depend on the Ambient Temperature, Humidity and sun Load and the Type of Vehicle.

GM Variable Displacement System

Low Side Operating Parameters: 25 to 35 psi

Ideal High Side Pressure

At 70
140 - 210 psi
At 80
160 - 235 psi
At 90
210 - 290 psi
At 100
255 - 365 psi
At 110
280 - 385 psi

These are what would be considered normal high side pressure when connected to a service port located between the compressor and the condenser.

Remember; There is no "single" normal low or high side pressure. Normal low and high side pressure changes with a change in temperature, humidity, and sun load.

Remember; If the service port is located between the condenser and the orifice tube the pressure will be about 20 psi to 30 psi lower than if it is located between the compressor and condenser

Switch used to control evaporator core temperature either by system pressure or an electrical sensing bulb. Clutch cycling switch controls electrical current to the compressor clutch preventing the evaporator core from freezing during operation

The compressor will cycle for the following reasons:
Ambient temperature low, refrigerant pressure low as a result of low ambient temp.
Ambient temp high, cabin temp low, evaporator exchanges little cooling because cabin is already cool.
Ambient temp warm to high, fan set on low, same reason as #2.
Ambient temperature high, cabin temperature high, system charge low.

 The system is cycled to maintain the most useful pressure for commonly encountered temperatures. With a FULL charge, the system will cycle very frequently in cooler temperatures, and sometimes will not have the pressure to EVER come on. That is how the CCOT systems work, and why they cycle the clutch a lot. The thermal expansion valve systems cycle the clutch less. If the charge is proper, in high temperatures, you might never see a time when the compressor clutch cycles off, until the cabin is cool, or the temperature drops outside. If the charge is too low, it'll cycle frequently to keep ice away.

If the charge is too high, you will blow the system up. That's why you need to know how much refrigerant is in the system. In very high ambient temperatures, it's occasionally advantageous to have a slightly low charge, only because the pressures of the system are maintained at a more optimum level despite the huge outside temperature differential. But that same system will be largely ineffective if the temperature is lower.

The whole picture is, you should make sure you have the proper charge as often as possible, especially if the system has a leak. That way, you'll be assured proper cooling, good compressor life, system integrity, and for you, sanity. Fortunately, A/C work is NOT very hard if you take the time to read about it a little bit and learn about what causes it to function as it does.