How do I test my car AC compressor?

A/C compressor Test. Compression ratio test.
The A/C compressor is considered the heart of the refrigeration system. The term that best describes a compressor is vapor pump, because it actually increases suction pressure level to the discharge pressure level.
Compression ratio is the technical expression for pressure difference; it is the high-side absolute pressure divided by the low-side absolute pressure. Absolute pressures rather than gauge pressures are used when figuring compression ratio in order to keep the calculated compression ratio from becoming a negative number. Absolute pressures keep compression ratios positive and meaningful.
Compression ratios are used to compare pumping conditions for a compressor. When compression ratios become too high, the refrigerant gas temperature leaving the compressor rises to the point that oil for lubrication may become overheated. Overheated oil may turn to carbon and create acid in the system.
To check a/c compressor compression ratio, note on paper the low and high side gauge readings and add atmospheric pressure to both readings. Divide the high side reading (with the atmospheric pressure added) by the low side reading (with the atmospheric pressure added).The result should be between 6.5:1 and 7.5:1. A compression ratio of 8:1 or higher may cause compressor failure.
If the A/C system operating pressures of your car gives you out of range values Click here.

What could cause high A/C compressor compression ratio?

A high compression ratio can be caused by:

1. Overcharged A/C system.
2. Inefficient condenser.
3. A/C condenser insufficient air flow.
4. Dirty evaporator core.

In the past, not much attention was paid to dirty evaporators. Technicians are now beginning to realize that low evaporator pressures cause high compression ratios.
A dirty evaporator will cause the suction pressure to drop. When the suction pressure goes below normal, the vapor that the compressor is pumping becomes les dense and gets thin (sometimes called rarified vapor).The compressor performance declines. Low suction pressures also cause the high-pressure gases caught in the clearance volume of the compressor to expand more during the piston’s downstroke. This gives the compressor a lower volumetric efficiency. These gases have to expand to a pressure just below the suction pressure before the suction valve will open. Because more of the downward stroke is used for re-expansion, less of the stroke can be used for suction. Suction ends when the piston reaches bottom dead center.

R-134a

Ambient temperature	High Side Pressure	Low Side Pressure
90 F/32 C	250-270 psi /1724-1862 kPa	45-55 psi / 310-379 kPa

Calculation:
Calculation example:
Compression ratio = Absolute Discharge
Absolute Suction

CR= 250 psig + 14.7 atmosphere
45 psig + 14.7 atmospheres

CR = 264.7
59.7

CR = 4.43:1

Understanding the A/C compressor compression ratio results.

A compression ratio of 4.43:1 would indicate to a service technician that the absolute or true discharge pressure is 4.43 times as great as the absolute suction pressure. Either an increase in head pressure or a decrease in suction pressure will cause higher compression ratios.
Any time a system has high compression ratios the compressor’s discharge temperature will also be elevated. This is because of the higher heat of compression during the compression stroke associated with higher compression ratios. When compression ratios are high, more energy is needed to raise the pressure of the suction gases to the discharge pressure. This means more heat is generated as heat of compression during the compression stroke.

How do I know if my compressor is bad?

Easy A/C compressor diagnosis by sound.
Growling noise with compressor disengaged: A worn compressor pulley bearing or clutch bearing will cause a growling noise.
Growling noise with the compressor engaged: A/C compressor internal bearings damage.
Easy A/C compressor diagnosis by temperature.

1. Identify the A/C compressor under the hood of your car.
2. Feel the suction line temperature (The thicker line coming out from the A/C compressor).After 5 minutes of ac operation it should be cold.
3. Carefully feel the discharge line temperature (The thinner line coming out from the AC compressor. ***The discharge line works at a really high temperature. ***.After 5 minutes of ac operation it should be hot.
4. After 5 minutes of A/C compressor operation feel the ac compressor it should be warm but not hot or extremely hot.