Mechanics

A/C PROBLEMS

2007 Dodge Nitro • 75,000 miles

Hello my AC is not working thinking it might have been something I did I added some freon last week to my a/c I overcharged a little bit about 10psi I let the freon out through the Lo port with pliers I know illegal right anyways it worked fine after that then when went up to the mountains and as we were climbing the A/C went from cold to hot quickly.I checked the lo port when we got back and know its reading relly high and if I let some pressure out it quickly builds up to around the 80-90 psi range any help or thoughts on this would be appreciated thanks, Eric
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Hambone887
July 6, 2012.



Before you do anything else, you need to understand how refrigerant works. You have some other problem causing the system to turn off as a safeguard. When that happens, the high and low side pressures will equalize so the low side is going to read too high, compared to when the system is running. If you bleed some off thinking you're going to lower the pressure, it's not going to happen. The lower pressure will just allow more of the liquid in the system to vaporize and expand, and that will make the pressure go right back up. The pressure in the system will be proportional to the outside air temperature. As you bleed more off, more liquid will vaporize and expand and the pressure will go back up. It will keep on doing that as long as there is still some liquid left to vaporize. Once that point is reached, it will be like a car tire. The more you bleed, the lower the pressure will go. Air in a tire is already fully vaporized so there's nothing in there to expand when you bleed some off.

Also, there is absolutely no way to know how fully charged the system is except by recovering the full amount, pumping the system into a vacuum, then injecting a measured amount. Because of the way refrigerant acts under pressure, reading the high and low side pressures with the system not running is meaningless. Experienced AC mechanics can tell from the pressures how the system is performing when it's running, but even they can't tell if it's slightly over or undercharged.

Caradiodoc
Jul 6, 2012.
That actually makes alot of sense now I took it to a meinike I saw a sign for free AC check and the guy there looked it over pumped some froen in there and said my limit switch I guess didn't turn the compressor off and burned it up prolly when I was climbing the mountain any thoughts on that

Tiny
Hambone887
Jul 6, 2012.
I suspect he's guessing. I don't know which system your vehicle uses but regardless, there's a pressure relief valve in the receiver-drier that would have blown off if too much pressure built up.

Starting somewhere in the mid '90s on some models, Chrysler started using compressors with a variable "wobble plate". It self adjusts according to the low side pressure. When there's more heat load, the pressure goes up due to a rise in the refrigerant's temperature as it leaves the evaporator in the dash. That higher pressure adjusts the wobble plate to make the pistons take longer strokes and pump more refrigerant. That way the compressor doesn't have to regulate by turning on and off. That can be felt and can be annoying.

The other system uses an expansion valve, often called the "H valve". It is a controlled leak that is adjusted by a gas in a sensing bulb on the evaporator that expands when it warms up, and that expanded gas pushes on the expansion valve to open up and let more refrigerant in. Those H valves caused a lot of trouble but they usually got handled when the vehicle was still in warranty. Depending on how they stuck, the valve would not let enough refrigerant through to cool the air or it would let too much through. Then the evaporator would drop below 32 degrees and the condensed humidity would freeze into a block of ice that blocked air flow.

I don't like to argue with a mechanic over something I can't see or diagnose myself, but I'm not sure I follow his reasoning. I find it hard to believe something else in the system is going to damage the compressor. If it is just the clutch that has failed, that is not uncommon. They seemed to have a lot of clutch coil trouble starting around the late '90s to 2000. It's a Japanese part so you'd expect to find better quality, but failure is common.

Caradiodoc
Jul 6, 2012.