Refrigerant Charging Procedure in Automotive A/C systems
We 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. Altough 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.
Before charging you need to get small length of electrical wire and jumper the AC
cycling switch (low pressure switch) which is located on top of the AC receiver drier
on the passenger side firewall. Just unplug from sensor and connect the 2 prongs
together with the jumper wire. This will make the AC clutch kick on. You cannot
charge the system if the AC clutch is not on. I can’t seem to find my receipt for
the vacuum pump but do a “Google” for Robinaire and you will get many hits –
again mine was $58 shipped. You need 34 oz of refrigerant (2.2lbs). Get 3 12oz
cans and you’ll be squared.
*MORE REFRIGERANT DOES NOT MEAN COLDER TEMPERATURES*
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 por 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. *Do not use the sightglass (if so equipped) in R134a-based systems as if were R-12 based.*
Make sure what is the required amount of gas. From factory, all systems have a decal under the hood that give the data. 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. 
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- “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.
- 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.
- 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.
- 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.