08/21/2016
Recharging AC Systems the Proper Way!
How AC works
Before you can start recharging car air conditioning, it’s important to understand how it works. Your car AC is a closed system, meaning it never needs “topping off” or “charging up” if it’s working properly. It does not use up refrigerant. If you find it is low, you have a leak that you should eventually fix.
The sealed system has a few major components: the air compressor, condenser, receiver/drier, expansion valve, and evaporator core. The entire system has refrigerant in it under high pressure, about 40psi or so at rest. There are two “sides” to the AC system, the high and low. The air compressor will take refrigerant from the low-pressure side and compress it, pushing it into the high-pressure side, which also makes it heat up.
The warmer, higher-pressure refrigerant then flows through the condenser, which is basically a radiator. The condenser will cool the warmer refrigerant as much as possible before it flows into the drier. The drier filters moisture and humidity out of the line.
Then, the refrigerant goes into the expansion valve. The expansion valve is a one-way valve that allows the high-pressure refrigerant to expand and become lower-pressure. This shift in volume, if you recall from physics, will result in the refrigerant becoming very cold. The cold, low-pressure refrigerant passes through the evaporator core, which is also basically a radiator. A fan blows over the evaporator core and hopefully cools down your car. It then continues back to the air compressor, where it is compressed again.
As you can see, there is no need to ever refill the refrigerant in a properly sealed system. If you have a leak, you can find it by adding UV dye and use a blacklight to find the source.
Equipment required
Short List:
AC Manifold Gauges
Vacuum Pump
Cans of Plain Refrigerant
Can Tap
New Receiver/Drier for your car (Always replace if the system was opened)
PAG Oil (Only required if replacing system components)
Oz Scale (Optional)
Bag of AC O-Rings (Needed if the seals in your car are old)
Recharging AC Systems
Recharging car air conditioning requires a few specialized tools. The most important thing is a true set of manifold gauges, pictured above. I have a set of gauges that I got for about $120 and they work great. The gauges allow you to see what the pressure is on the high and low sides of the system, as well as safely add coolant. The parts store will sell cans of refrigerant that have a cold-side gauge on them, but that really doesn’t give proper insight into whether the system is really working.I would add that 90% of the AC compressor I have had to replace were due to someone recharging without using the proper set of manifold gauges and overcharging their system!!
You also need a vacuum pump. I bought a standalone unit that I’ve had a lot of success with, and cost about a hundred bucks. They also sell cheaper units that work using an air compressor, so that could be an option if you already have a big compressor in your shop. The vacuum pump is critical if you’re filling a completely evacuated system. It allows you to totally vacuum out any air in your lines. The vacuum action also boils out any moisture in the lines. If this step is not performed, the AC won’t work properly and the lines can eventually be ruined from the inside-out from moisture.
You also, of course, need refrigerant and a can tap. The refrigerant required depends on the car – factory AC prior to 1994 used Freon. If your car is this old and hasn’t been converted, it must be filled by a licensed professional – nobody else is allowed to have Freon. That said, you can convert Freon systems to the new refrigerant that is more environmentally friendly, r134a. This requires replacing several key components in the system and is outside the scope of this article.
When you buy r134a, make sure you buy a can that doesn’t have any stop leak! Stop leak is great for gunking up your expensive AC parts and ruining your repair. Just say no. The above pack of cans was from Walmart, but all the parts stores carry plain unadulterated r134a. Also, if you are putting in a new compressor or replacing other components, you may need more PAG oil. AC requires a small amount of oil in the system. You can google how much oil to add for your car depending on which parts you are replacing. If you are doing a simple refill, no additional oil is required.
The can tap allows you to connect the can to your manifold gauges.
This is optional, but really helpful. If you are filling an empty system, your car will have a specific amount of refrigerant to put in, in ounces. The Miata I’m working on takes 21. I found that a simple kitchen scale allows me to zero out the scale with the can on it prior to filling, and then ensure I’m putting in the right amount. You can also fill to a given pressure, depending on ambient temperature, but putting in the right amount of refrigerant is the right way to do it.
Lastly, you may want a bag of o-rings for your car. Every connection in the AC system has an o-ring on it. If you know yours are probably all original, getting a mixed bag of replacements is a good idea. You may fix some small leaks you don’t realize you have, and it helps keep you from repeating this procedure.
Milo's Mobile Auto Repair is offering a special on AC recharge repairs for the month of August $80 (most cars) for evacuate and recharge including r134-a refrigerant and 10% off on any repairs!
Call today to make appointment and take advantage of this special offer!
858-692-4437
