testing your A/C without gauges and know if your low on refrigerant

[Drew]

for this you will need a good thermometer, either one of those digital laser deals, one that uses a thermistor or equivalent.

most vehicles today use R-134a as the refrigerant for the A/C systems. This tech article is based on R-134a.
As with any gas there is a point where it will liquify. This is following Charles' Law which states as the temperature of a gas increases in a sealed container so does the pressure. However every gas liquifies at different temperatures and pressures. We will once again only be referencing R-134a. If you know the characteristics of a gas you can closely estimate the internal pressures inside a sealed system just by knowing the temperature of the sealed system. By knowing these little facts we can now check to see if your system is operating properly.

 All automotive A/C systems are designed to operate the exhaust air from the evaporator at 40F. To achieve this on a R-134a system the low pressure side normally operates between 25 and 40 PSI while the high side is running 200 to 225 PSI. These are hard numbers that we will be trying to achieve. Now you may be asking if I know the pressure, then what temperatures should I be looking for. For this test we only need to worry about one side, the low pressure side. Following the Link http://www.glacierbay.com/ptchart134a.asp it shows us that to achieve internal pressures on the low side between 25 to 40 PSI a R-134a system will have a temperature between 29F and 45F. these charts can be found with a google search. just type in your refrigerant of choice and you can see the P/T chart

to check the temperatures properly make sure the outside ambient temperature is above 50F. Turn the A/C system on and let it run for 10 to 15 minutes to let the system stabilize. If you are using a thermometer with a thermistor, you can tape it to the low pressure side of the system near the dryer to get more accurate temps.  After 10-15 minutes check the temperature of the lines, if the temperature is between 29F and 45F the system is working properly.

If the system is still not blowing cold enough or for low temp situations adding R-134a yourself is possible, but be warned. Dont go to your local auto parts store and pick up the first can you see. A/C system are VERY picky about the amount of oil in the system. My suggestion is to find the bottle with no added oil, just straight R-134a. getting the bottle that states it can make your system what ever percent cooler is just a sales gimmick. If you add too much oil to the system you run the risk of burning up the compressor. this is why if your system is just a little low, all that needs to be added is just the refrigerant and nothing else.

If the temperatures are a bit high check the cabin filter, evaporator or other parts for obstructions, also check the condenser on the front of the vehicle to ensure proper airflow. if obstructions are found and cleared let the system run for another 5 min and recheck the temps. however if temps are WAY out of whack then it is time to visit your local A/C shop as you have a busted hose, a hole in the compressor, the dryer is not operating properly and so on. The tech will be able to figure this out.

Hopefully this will help some of you from spending large sums of money when you dont need to for A/C repairs.  Just remember, if you are adding refrigerant, find the bottle of R-134a that contains NO oil.

On a side note, Due to regulations R-134a is being phased out. Mercedes and BMW are using CO2, Yes Carbon Dioxide (a known greenhouse gas. . .what ever that means). The reason behind moving away from CFC's, HCFC's and HFC's is due to their Ozone depleting properties, however CO2 doesnt deplete the Ozone and it occurs naturally in the air.

BTW, this information I got out of the A/C classes I took while at school here at Ft. Lee. nd yes I am certified to work on vehicle A/C systems. . .what ever that means

[tnram2500]

That's great info Drew, thanks for posting it. Will need to check my ac pretty soon.

[FroneDaddy]

So if I pick up a can of refrigerant to "top off" my system should I use the gauge that comes with the can to add the correct amount or should I watch for the temps to come into range?

[Drew]

So if I pick up a can of refrigerant to "top off" my system should I use the gauge that comes with the can to add the correct amount or should I watch for the temps to come into range?

you can use the gauge. . .the system runs at such a wide band that as long as you are within the temps / pressures your good.

I was just posting this up to show another method of testing your A/C system without hooking anything to the system.

[FroneDaddy]

Another reason I ask is because 1) a lot of those cans don't come with gauges and 2) most of them, once you pierce the can you are flowing refrigerant until either the can is empty or you pull it off. I find those cans very difficult to work with. I also don't think the gauges that come with the can are very accurate because once you connect the hose, if you push down on it a little on it you can get a different reading. I have yet to use one of those cans where it actually made the A/C colder. Maybe it's me though.

[Drew]

yea, most of the gauges just have a green area where the needle goes to to tell you if the system is fully charged. you do have to remember you are working with a 15 psi window, which is quite a bit of wiggle room. however as you were saying. . .they can be pretty crappy.

[BigRedFireTruck]

Cool post I hate doing A/C I got stuck doing it at work now