Air Conditioning Troubleshooting

Important Note:Always wear safety glasses when working with refrigerants. Always use a set of Manifold Gauges when servicing an Automotive Air Conditioning System.

Typical Gauge Set

Discharging or Reclaiming from the System:
Attach your Manifold Gauges to the high and low pressure fittings respectively, attach the fill hose to your reclaiming equipment. Turn on the reclaimer, and slowly open the low and high side valves. The system is empty when the reclaimer does not cycle for 10 minutes. Note: the car should NOT be running during this procedure.After discharging the system, you should measure any oil that was removed from the system and replace with the same amount.

Flushing the system:
Flushing an AC system involves using a flushing agent such as R-11, being blown through the system under pressure. A compressor should NEVER be flushed as it tends to remove the oil. The orifice tube should be replaced whenever any debris is found in the system.
Completely reclaim all the refrigerant from the system. Disconnect the refrigerant lines. Remove the orifice tube. Fill the flushing cylinder with the flushing agent and pressurize the cylinder with approx 100psi of air pressure.Attach the flushing gun to the cylinder and open it to blow the cleaning agent throughout the system. Perform this procedure on all components except the compressor. Flushing should be continued until there is no evidence of refrigerant oil or debris being carried out with the flushing agent. Once the system has been properly flushed, it can be reassembled. Replace the Orifice tube, and replace the accumulator. All O-rings in the system should now be replaced as well.

Evacuating the System:
Evacuation is performed with a Vacuum Pump which literally sucks the air and moisture out of the system. To properly evacuate a system, the pump must be capable of producing 29"hg of vacuum. At 29"hg any moisture in the system begins to vaporize sufficiently to be drawn from the system. The industry norm is to leave the vacuum at29"hg for 30-45 minutes.
Once the connections have been made, turn on the evacuation equipment, open both high and low side service valves on the gauge set.After approx 5 minutes, the gauge readings should be steady at 29&quothg. Continue evacuating for another 45 minutes. After this period, shut off the valves in your gauge set, turn off the vacuum pump. Note:It is now that I recommend checking for leaks. Your gauges should hold this vacuum point for at LEAST 20 minutes. If there is any reduction in vacuum readings there is a good possibility you have a leak.Again, with R-12 as expensive as it is, you can't afford not to check for leaks.

Recharging the system:
Recharging alone is NOT a repair. Any leaks should be located and repaired prior to recharging the system.

It is extremely important to know the correct amount of refrigerant to add to a system. Overcharging as well as undercharging can reduce a systems efficiency. If the system has NO refrigerant left it is easy to determine by looking at a capacity chart. On the other hand if the system isn't empty, adding the proper amount an be tricky. ONe way to remove any doubt is to reclaim the remaining refrigerant and start over from 0. Our cars have a tag on the evaporator case which indicates the amount of refrigerant to completely charge our systems. Just for reference sake the amount is 3.25 lbs.

Charging Methods:
The High Side Liquid Charging Method involves the use of a refrigerant heater to increase pressure enough to overcome the pressure in the AC system. This charge is done with the engine off and the compressor NOT running.
The Low Side Gas Charging Method involves adding refrigerant in vapor form to the low pressure side of the system with the compressor RUNNING.
The Low Side Liquid Method is used only on our type cars with an Orifice Tube type system. And is performed by adding liquid refrigerant to the suction accumulator.

Note: On our cars the Low Side Liquid procedure is specified in the manuals.

You will need either a sufficient amount or 16oz. cans (which you can't buy without MACS or ASE certification) or a Cylinder.Connect your manifold gauges to the high and low side fittings. Connect your charging hose to the Can tap or charging cylinder. (It is a good idea to use a scale when charging from a cylinder.)
Slowly open the valve to the can or cylinder, crack the charging hose at the manifold gauge set to purge any air. Slowly open the low side service valve and allow refrigerant to enter the system until you reach approx 50lbs on the high side and low side, and close the valve. Now, its time to start the car. Turn on the AC, set the temp to cold, and turn the blower on a medium speed.Slowly open the low side valve taking notice of the low and high pressure readings. The Compressor will cycle less and less as the system is closer to a full charge. If at any time the high pressure gauge goes above 250psi or the low pressure gauge goes above 70psi. STOP IMMEDIATELY AND DETERMINE THE CAUSE OF THE READING!! If all is going well and you're approaching a full charge, you should notice gauge readings at approx 35psi and 180psi on the low and high sides respectively. Additionally, the cooling fan should be running. On some newer cars the fan is controlled by a pressure switch, to come on a 225psi and turn off around 175psi. But not on ours. The temperature out of the vents should be in the 35*-40* area on a 80* ambient day.At this point I personally begin to &quottrim"the charge to attain the coldest vent temperature. Using my charging and reclaiming equipment, I'll play with the charge usually no more than a few ounces until I get the coldest reading I can. Now, you're done. With the car still running, disconnect the Low pressure hose. Shut the car off, let the system pressures equalize. Then, using a shop rag or towel wrapped around the fitting, slowly remove the high pressure fitting, being careful not to get freon on your skin. (It will give you frostbite on contact!!) Re-install the fitting caps on the valves, close the hood, and enjoy the cool air! You're done!

Normal Gauge Readings:

Low gauge readings give a good indication of evaporator temperature at the evaporator outlet. Typically, at 25 psi and above, outlet temperature is approx 3 numbers higher in degrees. (ex. 30 psi=32*F)And will rise with ambient temperature. High side readings have no real relation to evaporator temperature, but will rise with ambient temp as well. Be sure when diagnosing pressure problems, the ambient temperatures taken into account. Below is a small table for noting pressure values and the associated possible problems:

Low Pressure Low
High Pressure Low
Low refrigerant Charge
Low side normal
High Side Low
High Side Restriction
Low side normal/high
High Side Normal/High
System Overcharged
Low side high
High Side high
Moisture in the system
Low side Low
High Side Normal
Faulty Cycling Switch
System will not operate Stuck open Cycling Switch
Low side high
High Side high
Inoperative cooling fan, blocked Condenser

I hope this information proves useful to you. If you would like help to a specific problem that is not addressed in any of these pages, please feel free to post in the GNTTYPE mailing list or email me at

By Jim Testa
Certified Master Automobile Technician
MACS Certified in AC Repair~#152865