A/C Charging

[John B Kobberstad]

Basic Charging Procedures:
REFRIGERANT CHARGING PROCEDURE IN AUTOMOTIVE A/C SYSTEMS

Recommend reading this procedure completely before charging. You may need information before charging you want to keep handy. Have this procedure with you while charging. Although it can be done by a single person, we recommend having a helper around.

This procedure outlines the steps to perform a complete charge. These steps are not to be used to refill or make partial charges. Toping off requires system parameters monitoring and knowledge, specially in R134a-based system. Excessive gas will harm your system and will keep it from cooling properly. MORE REFRIGERANT DOES NOT MEAN COLDER TEMPERATURES

CHARGING IN GAS OR LIQUID – Please read step 10

Do not use the sightglass (if so equipped) in R134a-based systems as if were R-12 based.

SPECIAL EQUIPMENT REQUIRED

1) Vacuum pump

2) Gauges (R12 or R134a)

3) Service port adapter (as required in most R12-based systems) R134a systems do not require adapters other than the couplers in your gauges in the majority of applications

4) Refrigerant gas (R12 or R134a)

5) Optional: 2-3 ounces of specified oil.

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1. Make sure what is the required amount of gas. From factory, all systems have a decal under the hood that give the data. If the decal is missing or you are not sure, please specify make, model, and the type of refrigerant used and whether your system is a factory or an after market unit here. It is very important to know if your system has oil. Oil starvation is the main reason of compressor failure. Oil can be added to the system in two ways: with oil injectors or through the low side port under vacuum. The procedure to add oil through vacuum is described here. Some refrigerant charge and other useful specifications are provided here.

2. Connect both blue and red gauge hoses to the system’s service ports. The discharge port (red hose) is located somewhere between the compressor and expansion device, either before the condenser or after it. In R134a systems, the port is the thicker of both, while in R12 systems is the thinner one. In some aftermarket systems, the port is located in the back of the compressor. If your ports are located in the compressor, the low side is marked by an “S” and the high side by an “H”. Port caps have an “L” for low or “H” for high.

3. Open both gauge valves. Connect the common hose (yellow one) to the vacuum pump. Make sure both gauge needles are zeroed down. Needle is adjusted by turning a fine screw inside the dial. You must remove the plastic lens to do this.

4. Turn the pump on. You’ll notice that both gauge readings begin to drop. The blue gauge’s needle will even dip into negative values. The desired low side reading should be less than -25, while the high side will remain at 0.

5. After approximately 5 minutes, close both gauge valves. Turn the pump off, and observe the needles. Any movement will indicate vacuum loss. The faster the movement, the greater the loss. If after two minutes you don’t see any needle movement, open both valves, turn the pump on, and continue vacuum for not less than 30 minutes. Close both valves then turn the pump off.

6. Get ready to charge. Have the necessary refrigerant amount handy. Cans make charging easier and more precise unless you have a charging cylinder or a precision scale.

7. Disconnect the yellow hose from the vacuum pump and connect it to the can tap or charging cylinder valve. You can charge in two ways: liquid (can upside down) or gas. Liquid charging is a lot faster but not recommended unless you extreme care or have experience. Traditional (gas) charging is slower but safer.

8. Once you’ve decided, and with the can or container connected, unscrew the top yellow hose connection (at the gauges) allowing refrigerant to escape for a couple of seconds. This will purge the air out of the hoses so you make sure that all you dispense is refrigerant. You can do this venting in liquid form so you can see when a fine, steady stream of refrigerant escapes indicating that all the air is gone from the hose. Don’t breathe refrigerant and don’t charge in a flammable environment.

9. “Flood” the system with liquid refrigerant (can upside down) by opening the red valve (high side) until it won’t take anymore. Close the red valve. Jump the low pressure cycling switch (if so equipped). If you don’t know what this is or where it is, ask for help here.

10. Turn the engine on. Turn the A/C into MAX on its third or higher blower speed. You’ll note that both gauge readings are now positive. The red gauge should read between 100 and 150, while the blue gauge between 5 and 15. If you are charging with cans, don’t forget to purge air out the hose after hooking a new can as described in step 8 above. Now, charge will continue through the low (blue) side (see note below) . If you haven’t jumpered any switch (you don’t have to), you’ll notice the compressor cycling frequently on and off. That is normal. The cycling will disappear as you dispense more refrigerant. The only disadvantage of frequent compressor cycling is that charge will take longer. Needles will move up and down with every cycle. This is normal. Do not jump any switch unless you are absolutely certain!.

WARNING: DANGER

NEVER OPEN THE RED VALVE WHILE CHARGING. The red valve is to be opened only during pre-charge, vacuum, or when a system is evacuated. Its function is keeping an eye on the high side only. It must be closed at all times. If you have any doubts, wait until you receive information. Severe injury or death may occur. Remember: ALWAYS wear safety goggles. NEVER charge your system in a closed environment.
GAS OR LIQUID CHARGING?

There are two ways to charge: gas (can or cylinder up), or liquid (can or cylinder upside down). A compressor is designed to compress gas. Direct liquid charge will harm the compressor if suction pressure is not controlled. Liquid charging is faster, yet is riskier. Liquid charging should be made by professionals or under supervision. Liquid charging can be done if the suction port is away from the compressor (some compressors have the suction valve in its body).
Liquid charging can be done never exceeding a 50 PSI suction pressure. If you can’t control it, charge in gas form. In cooler weather, cans may freeze. You may immerse them in water and shake them while charging.
11. Once about 2/3 of the charge has been dispensed into the system, spray water in the condenser to optimize heat exchange and speed the process. When you get the condenser wet, vent temperature is likely to raise. This is normal. You’ll also note pressure drop in both gauges.
12. Once the specified amount has been dispensed, close the blue valve. Let the system run for a minute. Turn the A/C off and then the engine. Wait another minute and disconnect the couplers from the service ports. Disconnect the low side first. If you jumpered a switch, reconnect it too.
Optimum cooling performance is attained after 10 minutes of operation. Our own acceptance criteria is at least 50°F in the center vent to the driver side at idle after 10 minutes or less. Remember: cooling increases while the vehicle is in motion.
VERY IMPORTANT

Due to the physical properties and chemical composition, R134a and R12 charging amounts are different. Never, if you are retrofitting, charge the same or specified amount of R12 with R134a. If in doubt, please e-mail us here. Do not use this procedure if you are using any other refrigerant. This is just for R12 or R134a. Although procedures and parameters may be similar, we do not use nor recommend alternative refrigerants.
HOW TO DETERMINE OPERATING PRESSURES

Every vehicle has its own operation parameters specification. Depending on whether or not the vehicle has a factory or an after market system, and whether it was retrofitted or is still original, among others. There is no established calculation to determine the exact operating pressures.
The low pressure (blue gauge) should be 35 or less at idle, regardless of the type of gas. Only if you’re running a dual system, the low pressure may be between 45 and 50 at idle.
The high pressure, for starters, is directly related to ambient temperature: the higher the temperature, the higher the higher the pressure.
Bear in mind that dispensing a complete charge without lowering the pressures with water will result in higher readings. This is normal, and that is why you should spray water in the condenser at 2/3rds of the charge or once you have completed it. You’ll note an immediate pressure drop when you spray water in the condenser.
To obtain a ballpark high side value, multiply ambient temperature by 2.2 if you have an R12 system. If you have a factory R134a system, use 2.3. To convert °C to °F, use this formula: °C X 1.8 + 32.
Remember: this is only a ballpark. If in doubt, we have factory charts to help you determine the correct pressures. Please have your vehicle’s make, model, refrigerant type, and year and click here.
Factors like a an obstructed or very dirty radiator and condenser, weak or inoperative fan clutch, weak or inoperative radiator fan(s), either electric or mechanical, will make pressures go up and impair cooling, even in mild days.

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Four Seasons – Tech Tip

Charging with Cans, Guess Again?

Unlike the old days when an R12 system could be ½ pound on either side of full and still
produce cold air, an R134a system is not as forgiving. The acceptable operating window for
R134a cooling is about +/- 2oz of full. This is why technicians should recover and recharge
to O.E. specifications rather than attempt to “top-off” a system that is being serviced for a
low charge.

The charging method you choose will make a difference in the accuracy of the final
charge amount. For greater accuracy a charging station or a 30lb cylinder and an
electronic scale is the preferred method of charging. The charging accuracy is
questionable with the use of 12oz cans.
One problem with can charging is a misunderstanding as to what is meant by 1 pound
of refrigerant. Many technicians today still speak of 1 pound cans, which have not been
produced since the late 1960’s. The industry also saw the use of 14 ounce refrigerant
cans until the mid-1980’s. Today when we speak of 1 pound of refrigerant that amount is
equal to 16 ounces. When we speak of 1 can of refrigerant that is an amount that is equal
12 ounces.

When recharging with a charging station or with a 30lb cylinder and an electronic scale,
you need to add 2oz to the system capacity specification. This extra amount is necessary
to compensate for the amount of refrigerant retained by the equipment or manifold gauge
service hoses, whether you are vapor or liquid charging. So, if the vehicle calls for 28oz
you will need to program in 30oz to ensure a full charge into the vehicle’s A/C system.
NOTE: Some charging equipment requires you to add 2oz to the total charge amount,
while others designs add the amount automatically. You will need to be familiar with
how your equipment works. If in doubt read the equipment documentation or contact the
manufacturer.

If the method of charging R134a is with 12oz cans, then you have to compensate for the
amount of refrigerant to an even greater extent. You will still need to allow 2oz for the
service hoses and since you can never completely empty a can of refrigerant you will
need to allow for an additional 1oz per can. This occurs because the can’s pressure and
the vehicle’s A/C system low side pressure equalizes, this stops the transfer of
refrigerant. So, if the vehicle specifies 30oz, you would require the use of three cans.
This would be 36ozs, less 2ozs for the hoses and 1oz per can times 3. That would put
you within the operating window at 31oz. True, the final charge is 1oz over the required
specification, but it is as close as you can “guess” with the use of small cans.
The drawback to using 12oz cans is when the specification calls for an amount requiring
less than a whole can. Example – a system capacity of 24oz. You may believe 2 cans
would give you a full charge, but actually only places about 20oz into the system. A 3rd
can is then required to finish charging the system but you can only “guess” at the 4oz
needed. You could easily over charge the system. You will also have an undetermined
amount left in the can, leaving you to “guess again” at the charge on the next vehicle
being serviced.

The amount of refrigerant retained in a 12oz can is dependent on the temperature of the
refrigerant in the can. Whereas the amount of refrigerant retained in the service hoses is
dependent on the length of the hoses and temperature of the refrigerant. The above
examples are consistent with the refrigerant at room temperature around 85°F. When the
component temperature is below 70°F, the amount retained could be twice as much.
Heating the refrigerant with warm water (maximum 115°F) will increase the pressure of
the refrigerant in the can and reduce the amount retained in the can to about ½oz.

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[John B Kobberstad]

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