Car Freon Charging for the Novice

The car air-conditioning is getting weaker; ice was seen formed in the evaporating coil. One guessed the Freon was low. Can one recharge the Freon DIY at home? This article will try to answer some of these questions.

Why Freon leaks?
Freon often leaks from piping joints, cracked pressure switch , expansion valve or corroded evaporator. The most common cause of leaks is from corroded evaporator which is often made from thin and flimsy aluminum material for some reasons. Leaks from piping joints are often caused by swelling or damaged O-ring which allows Freon to escape under the hot sun. Sometime, the Freon leakage is so bad that a full charge of Freon would completely disappear over night. This will happen in the case of a corroded evaporator.

What are the Synthoms of Low Freon?
When Freon level drops below 90%, the cooling efficiency would be reduced by about 4%. As the level drop below 80%, ice will start to form often in the evaporator. Further drop in the Freon level until high pressure of the compressor reaches around 20-30 psi, the low pressure switch will start to cut-in and stop the compressor from running to prevent further damage.

What are the Other Causes for Poor Air-conditioning?
Faulty expansion valve and compressor can also cause poor air-conditioning; especially when one has just charged the Freon and there are enough Freon in the system.

How Do I Confirm if Freon is Low without Pressure Gauges?
Open the bonnet; look for the Filter/Dryer. On top of the Filter/Dryer there should a small glass window(sight glass) not larger than a 5 cent coin. Observe the color of the window carefully (one may need a torch light). If the car is short of Freon, the window would be whitish in color even when the engine is speeded up with the air conditioning switched on.

Why Window in Whitish Color?
If the car is short of Freon, the Freon will bubble when it passes through the window. The small tiny bubbles are whitish in color.

What if the Window is clear and the Compressor is not on ?
Quite likely the low pressure switch has activated and prevents the clutch from engaging to turn on the compressor. Test if there is Freon in the system by momentarily pressing the nipple valve provided on the suction (larger) or the discharge (smaller) piping of the compressor with the engine switched off. There should be Freon discharge. If not, Freon could have completely drained. In that case, air moisture could have entered into the system and one should vacuum the system first before charging it with Freon.

I’m Prepare to Charge Freon; What do I need?
One will need the following:-
a) A tank of correct type of Freon, either R12 or R134a depending on the age of the car;
b) A pressure connecting hose;
c) Some amount of correct type of compressor oil for air-conditioning system;
d) A strong torch light

Where can I get the supplies?
From the car air-conditioning spare part shops. Retail Freon comes in 25lb cylindrical tank, the oil in 1 litre bottle and the pressure hoses, in a set of 3. There are also recharge kits available in the Web that provides a short length of pressure hose, Freon gas and also compressor oil.

What type of Freon for My Car?
Pre-1990 models probably come with Freon R12 and the newer ones, Freon R134a. Check out from the Car Agencies what type of Freon is used for your car because Freon R134a cannot be used in an R12 system and vice versa.

Can I tell the difference between Freon R12 and Freon R134a?
Both will be in gaseous state in discharged form. There will be no way to tell by sight. Instrument will detect all types of all refrigerant and blends.

Why I cannot mix the Freons?
Oil used in R12 system is incompatible with the type used in R134a systems. Oil must mix well with Freon for it to be returned to the compressor for lubrication. R12 systems use mineral oil (YN-9) and R134a, synthetic oil (PAG YN-12 or Double End Capped PAG).

How much Freon do I need?
It varies from car to car. A modern car nowadays has roughly about 600 cc to 1200 cc. One could loss more than 50% of the Freon.

How much oil do I need?
It will depend on the amount of leakage. Minute leaks escape from the joints may need less than 5%. Assume 15-20% loss for normal leaks. The complete system has about 300 to 350 cc of oil

Is there an oil level gauge?
System is not designed to have level gauge because it is a pressurized and closed system. The amount of oil required to be charged will much depend on experience and guessing work.

What if I overcharged the System with Oil or Refrigerant?
There is no indication that slight overcharging will harm the system; however, it is expected that one should not have over-charged the system with oil or refrigerant because oil is not compressible and too much refrigerant builds up pressure in the system. Over charging of oil can clog the air-conditioning components and starve the compressor of oil; undercharging of oil can destroy the compressor as well.

Should I fix the Freon leakage first?
One should always fix the Freon leakage first because it will be a waste of time and money if these leakages are not fixed; but, there are times that leakages are minute due to regular use of the car under the hot sun.

Air-conditioning has high pressure; Is it dangerous?
Any pressure vessel is dangerous to handle but air conditioning system has limited volume of Freon. Most cars come with pressure relief valve that will release Freon in the event of excessive high pressure but understand that the industry is doing away with this valve. Wear safety glasses if necessary.

Is Freon Dangerous to handle?
It is not toxic as well as flammable. It is quite safe to handle.

Next Step
Charge the pressure hose or use a proper oil injector with estimated amount of compressor oil. Connect up the tank of Freon as shown attached.

The pressure hose should be connected to the nipple valve of suction piping of the compressor (the larger of the two piping going into the car compartment).

With the engine running in idling speed, turn on the valve on the Freon tank slowly and observe the sight glass on the filter/dryer carefully. If the Freon is not adequate, the sight glass window will be whitish in color. Further addition of Freon should clear the bubbles especially when one revs up the engine a little (see animated picture).


On occasion that the sight glass window is still whitish or the Freon bubbles are still visible after some time of charging, suspect some trouble with the system or the compressor.

Important Note: Do not charge liquid Freon into the compressor as that will harm the compressor. When the Freon tank is turned upside down, most likely Freon will enter the compressor in liquid state rather than vapor state.

Testing and Commissioning
Do a test on the air conditioning system, the air vent should now blow cooler air if the system is fully charged up with Freon.
To ensure proper work is done:
a) Make sure the engine and air-conditioning are running smoothly;
b) Check the suspected area of leaks leak using soap bubble test if necessary.

Other Related Sites
1. Car Aircon Compressor Repairs
2. Car Air-conditioning Part 1
3. Car Air-conditioning Part 2

4. Car Air-conditioning Part 3
5. Car Safety Inspection and Check at Home
6. Replace Engine Oil by Vacuum
7. Wheel Alignment Using Sideslip meter

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