Charging your Air Conditioning System

Just in time for the heat of summer, this writeup will cover charging your Jeep's A/C system. A little history, after a recent wheeling trip I turned on the A/C as I was getting on the highway. I heard a grinding sound for a few seconds and there was nothing but hot air blowing from the vents. My wife (following me in her Jeep) said she saw a puff of smoke. After some troubleshooting, I found this:

Turns out one of the ties that held the line in place broke. When I turned on the A/C and the system pressurized, it kicked the line right into the fan. One new refrigerant line later (dealer only part, so I kissed $150 goodbye) I had the system ready for a charge.

You'll need 3 things to do this properly:

1. A vacuum pump
2. A thermometer (an instant meat thermometer works very well)
3. A manifold gauge set

Now, if you did not have to open the system and are only adding refrigerant you can skip the vacuum pump. However, if the system has been opened you need to evacuate it (or create a vacuum) BEFORE you add refrigerant.

Be careful when getting the gauges. First off, there are two different types of refrigerant used in vehicles, R12 and R134a. Make sure to get the gauge set compatible with the refrigerant used. Also, there are a lot of "recharge kits" out there that will not allow you to accurately judge the performance of the system. You need a set that reads both the high side pressure (leaving the compressor) and low side pressure (coming to the compressor). It should also have either manual or automatic backflow valves to prevent refrigerant from being released into the atmosphere.

The manifold set should look something like this:

I got that one at AutoZone for $50. Follow the directions that come with the specific gauge set you have to connect and operate it.

In the case of a R134a system (I think it's the same for R12, but not sure):

1. The blue hose connects to the low pressure (suction) port, which is located between the accumulator outlet and the compressor inlet
2. The red hose connects to the high pressure (discharge) port located between the compressor outlet and condenser inlet.
3. The yellow (in some cases white) hose is used to recover, evacuate and charge the system.

After hooking up all the hoses, the first thing I had to do was evacuate the system...basically, create a vacuum. With the engine off, I did this by connecting the vacuum pump to the yellow hose and pumping the air out until my vacuum gauge read 26 in. Hg. I was then ready to charge. I stuck the thermometer in the center vents, started the engine and with the windows down turned the A/C to max. I connected a can of R134a to the yellow line, opened the low side valve slightly to start dispensing it into the system and paid close attention to the gauges. As low side pressure would climb the compressor would click on. High side pressure would then go up bringing low side pressure down, so the compressor would shut off and the whole thing would start over. This cycling continued for a few minutes until there was enough refrigerant to maintain adequate pressure on both sides to keep the compressor running.

Now comes the fun part...making sure that the A/C performs to specification. There are 4 factors/items that need to be checked:

      1) Ambient air temperature (the outside temp)
      2) Center vent output temperature (what the A/C is putting out)
      3) Low side pressure
      4) High side pressure

Basically, at a given air temperature, the vents should push air at a certain temperature while the high and low side pressure should be in certain ranges....therefore I just shot refrigerant in until it read within spec.

So in my case it was 80°F out while I was doing this (okay...81...close enough for gubment work). So, according to my Factory Service Manual, the A/C should blow air between 33-44°F, with a low side pressure 32-41psi and a hi side pressure 200-260psi. I got it to blow air at 39°, with a low side pressure of 38psi and a high side pressure of 250psi.

I'd like to point out the the chart included with the gauge set specified readings that are different than my FSM. They stated the high side should have a lower psi and that the low side should have a bit more pressure. They also made no mention of temperature coming from the vents...very key if you are trying to troubleshoot a problem. These types of charts are very generic...you are always better off finding out what the specific temperatures/pressures should be for your vehicle.

Having said that, if for some reason you cannot get something within the vehicles specs...it most likely means you've got a problem with one of the components in the system.

As for myself, call me frosty...'cause mine is blowing ice cubes.

Rock On!
Steve