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Cool Runnings: Recharging the A/C

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Air-conditioning (A/C) systems used to be much more forgiving for the DIYer, and professional technicians too. Compared to today’s systems, the different refrigerant and operating characteristics made them easier to charge. The refrigerant amount didn’t have to be as precise, and they used to have a sight glass through which you could see the refrigerant flow. Eliminate the bubbles and you were in good shape.

Is it possible for a DIYer to have success charging their own A/C system? The answer is yes – but with caution. They don’t have to be an A/C master, and as long as they keep a few details in mind, they’ll be blowing cold air in no time. We’ll get into the caution part down the page.

The Basics

Modern A/C systems are very dependable. It’s when they require service that many problems occur. The efficient operation and longevity of today’s R134a and R1234yf systems hinges on the accuracy of the refrigerant and oil charge. This is the most important point to keep in mind.

A topic that can easily be a freight train out of control, why a system is being recharged is a factor in the charging itself. Is the system just a little low or completely empty, and why? Was a component replaced due to a collision or a leak? Was the compressor replaced and was the system properly flushed?

The bottom line is these are factors that must be known to determine the proper amount of oil, if any, that should be added. The amount of oil to be added depends on the component that was replaced, and there’s no set standard. It varies for every component in every car. The manufacturer service information, as well as information included with replacement compressors, must be referenced to get it right.

The Refrigerant

R134a and R1234yf are similar in operation, but R1234yf is much friendlier for the environment, which is why it soon will be the only refrigerant available in new automobiles. R134a eventually will be phased out, but for now it will still be around for a long time to service the vehicles that originally came with it. Even though there are two different types, there’s no decision to be made: Use what the vehicle came with. It’s illegal to use R134a in anything that didn’t originally come with it, and the service fittings are different between the two to eliminate the possibility of cross-contamination.


Since it’s illegal to dispel any refrigerant into the atmosphere, if refrigerant needs to be recovered for a repair, a DIYer will have to take the vehicle to a shop to do it. That’s assuming they don’t have recovery equipment at home next to the lawnmower, and that’s one assumption I’m willing to make. If the system is completely empty, it will need to be evacuated prior to charging. There’s no way around this and it requires a vacuum pump. As a general rule of thumb, a 45-minute evacuation period is adequate.

In addition to the refrigerant, a DIYer will need to purchase the proper valve and hose, based on their refrigerant, to connect to the A/C system. Cans of refrigerant come with or without them, since they can be transferred between cans. With the proper amount of oil added and the vehicle running with the A/C system on, the refrigerant can be added to the system. It takes time and patience, but the system eventually will draw all refrigerant in.

The tricky part is the amount. It must be precise for proper performance. Again, unless they have charging equipment in the corner, they’ll have to estimate. Professionally, this isn’t acceptable, but it can be done with a little patience.

The easiest way to do this is to buy the cans that get you as close as possible to the proper amount. If you put a complete can of refrigerant in but still need 4 more ounces and you have an 8-ounce can, you just have to guess based on weight.

Many of the charging valves/hoses come with a small gauge. Since you charge from the low side (fittings are also different between low and high side), the gauges will have a “green” area that represents the range of correct low-side pressure for a properly charged system. This is an added tool to indicate proper system operation, and it can be used in conjunction with the amount, but don’t go too deep down this hole. It’s another freight train.

Ambient temperature has an effect on A/C outlet temperature, as well as how easy it is to charge the system. This can throw you off. The best thing to do is charge it at the hottest point of the day, put a thermometer in the dash outlet and monitor the temperature.

If it’s cold on the thermometer, it feels cold coming out of the dash and you’re close to having the right amount of refrigerant, it’s a good time to stop.

Proceed with Caution

When adding oil to an A/C system, it’s imperative to use the correct oil. All oils are not compatible, and they can wreak havoc on an A/C system. Even more important, many electric and hybrid electric vehicles use electric compressors that require a non-conductive oil. To avoid cross-contamination, a DIYer never should use the same can of refrigerant on two different vehicles

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