Ok, I'll give the symptoms and what I have done and hopefully you give me an answer before I have to take it somewhere and get totally ripped off!

First, the truck is an 02 crew cab.
Now I noticed a/c took a long time to cool off. Seemed to blow warm and then gradually cool off. I chalked it up to 95 degree 95% humidity. But it is cooler now and it still does the same thing.

So I put guages on it. Low side reads 25psi, High side reads 150. So I add a 11oz. of 134. Well the guage reads the same, so I add another can and the guages reads the same. Now I am wondering what is going on. So I get a sniffer and the only thing I can find is a small leak at the drier/low side valve. I tighten it and no leak. Now I add a third can.... I am up to approx 30 oz. and the low side pressure is just starting to climb. At cut out is around 45 and at cut in it drops and holds 30 for a short while and then starts to drift back to 25 psi. The high side is still at 150-160. So I check the guages with the engine off. They read 85psi both high and low, so I figure I have overfilled a slight bit and bleed a couple of ounces off to equalize at 75 psi.
Now the od part. While I am running at highway speeds the temp at the vent is 45-47 and when I am in residential it drops to 42-43 degrees and when parked I have seen 36-40 degrees.

So now I am totally confused because the principles of a/c are not adding up here. I am by no means an expert but I want to learn because the last time I went in for a/c repair it cost me over $1000 and I later found it was just a leaking hose.

 

First off you don't want to over fill it. Usually the way to check for freon amount is you fill it till the big line to the compressor gets cold. Aids in cooling the compressor. So did you have a leak? or was it at a connection you made? If you had a leak then the system was definetly low. Pressures are related to outside temps and difficult to gauge. I would think it would be colder going down the road. Unless you were somehow sucking fresh air into the system adding to the cold air by having the blend door or other door messed up. Just guessing.


Dick

 

In cold weather I can believe that it would be colder sitting still because the compressor will cycle frequently going down the road. If it where me I would have the system reclaimed, evac, and charged to the correct charge.

 

The high side pressure should be 2.2 to 2.5 times the outside temp with fan on hi and engine at 1500 rpm. Those temps you got are good by the way.

 

Sounds like the system is still low. I add freon untill I feel suction line coming out of the acumulator getting cold. Thats with the compressor running and the fan on high. If you have the proper charge in the system the compressor should'nt cycle on the low pressure switch,(if it does you have another problem) it should only cycle on the temp control. That is as long as your not trying to charge the system while the outdoor temp is cold and there is no load on the system. I think your normal operating pressures should be around 35 to 45 psi and more on the low side depending on the heat load in the cab. The low side cut out should be around 25 psi and the cut in around 55 to 60 psi. The high side pressures can have a pretty good spread (165 to 250 psi) and more depending on the ODT. All this came to me at a Holiday Inn last night so you may wish to verifiy this info.

 

The best way to determine proper charge (not the easiet ) is to detrmine if you have the proper amount of superheat temperatue coming out of the evaporator coil; ~6-10 degress F. Not enough and the discharge air temp raises and the compressor short cycles, to much and you can have liquid carryover to the compressor, not good.

Superheat temperature is the sensable heat gain above the saturation (Boiling Point)temperature for a given liquid. If the saturation temperature of 134a is 20 then reading 30 on the line exiting the evap coil would indicate 10 deg F superheat, Oh if it only were that easy. Because its impratical to direct read the outlet of the evap coil you are left with indirect measuring which; includes cheking system pressure Hi & Lo side, cab supply temps, If the unit is providing cool air under set conditions, IE CHARGING the system with Hi/LOgauges installed and monitoring cab air temp while idleing but the temps very under other operating conditioins it may point to problems outside of the AC system, like the heat control damper, some are Vacuum controlled and have caused me problems in the past, to troubleshoot it I blocked the heater supply line by using a hose clamp and 1/2 pipe cap clamped to it. Cab temp dropped 10 degrees and I knew which direction to go. Sorry for the long post...

 

The best why to charge an auto air unit would be to reclaim the freon, pull a vaccuum on the system and then weigh the proper amount of charge in. Make sure you have found and repaired the leak frist.
Like smecomark1 said charging by the superheat method can be difficult. Because your compressor changes RPMs with your engine, you may have 10* @ 1000 rpm and 0* superheat @ 2000 rpm. When you have 0 or minus degrees of superheat you will most likely be getting liquid freon back to your compressor. Very bad on the compressor parts. Try to find an honest repair shop, there should be one some where near you. I know that it can be tough to find someone. Get recomendations from friends or co-workers. Try a school or jr college shop class near you. some them have AC/R classes that probably have students more up to date than your local mechanic and they work for free, you buy the parts and material. At they used to do it around here. Good luck..

 

Hey there 79 CJ, I agree you do need to take a class on automotive a/c and heating. It's not that hard. Biggest problem you have now is just how much freon you have in your system right now. Your outside temp will reflect your pressure readings. If the outside temp is "example" 45 degrees, pressure readings will be low. If your outside temp is 70 degrees or higher, pressures will be higher. Try to get with a friend in your home town that works on a/c because you do need to start from the beginning, reclaiming your freon and putting the right amount of freon in your system. from there he can walk you thru.......because it might be as simple as your blender door being partly open allowing heat to come in from the heater core, which by the way you can still have a normal pressure reading on your gauges.