My wifes a/c is blows cold while driving down the highway, then all of a sudden the a/c indicator light starts flashing and the a/c doesn't blow cold any more. At highway engine speed the low side pressure is about 35-30 and high side is about 250 - 300. I evacuated and recharged the system to find 32 oz of freon evacuated where the factory only says to use 28 oz. I pulled a 30 minute vacuum on the system to ensure there was no air. I then injected an equal amount of PAG oil in that had come out during the vacuum. I recharged the system back to 28 oz as per factory spec. The pressures didn't really change. I don't think the a/c is freezing up, but it's acting like maybe a pressure switch is kicking the compressor off, but this car doesn't have a pressure switch. The only thing I can think of at the moment that could be wrong with it is maybe the expansion valve. I haven't checked to see if the condensor fan is working, would that cause this issue?

 

What kind of car?

You mention that it works while on the highway, when does it stop working? At road speed or at idle/low speed?
That 300 psi on the high side seems too high when going down the road, what is the ambient temperature?

 

Ambient temperature hasn't really seemed to make a difference in this case, its been doing it ever since we bought it, its getting hot now, so the wife is making me look at it. It's a 1994 dodge stealth. I put a thermometer in the vents to see if it had a cut off temp and it maintains 38-40 degrees at highway speed or a idle. when it shuts off, its still at 40. After cut off it maintains 70 degrees. My friend said that light is flashing a code and the owners manual said if it does this then it should be taken to a service center immediately. I've done about all I know to do with a/c systems, so I may end up taking it somewhere.

 

That's all good info, but you didn't answer the questions.
-What was the ambient temp when you got the 250-300 psi reading on the high side?
-Does the system stop just at random or only at idle/low speed?

It almost sounds like the system is shutting down due to high head pressure.
If it does this going down the highway (eliminating the cooling fan) you may have an airflow issue with the condenser. Possibly debris between the condenser and radiator.
If it only does it at idle/low speed, there may be a problem with the cooling fan.
The ambient temp is important. 300psi at ~100 degrees ambient may be normal. 300psi at 70-80 degrees is a problem.

There is a pressure transducer in the system that provides feedback to the PCM. If it is out of range, it can cause the PCM to think there is a problem and shut down the system. This is also a common failure.

 

Sorry, I did mean to put the ambient temp. When I had the machine hooked up, ambient temp was 65 degrees. It's pretty random, it can go a whole day without messing up, or it could go a whole day making us sweat it out. The condensor fan works properly and I blew out the condensor and radiator with very little dirt or debris coming out. I did remove a tumbleweed that was wedged between the grill and condensor, but it didn't make any difference.

 

You may have an issue with the expansion valve starting to not work properly. You also could have an issue where the evap core is starting to freeze up on you. Have you got it to quit working and then shot a temp gun on the evap core? I know that is easier said than done on most any car, but usually you can get some kind of access to shoot it. Also like previously stated, the pressure transducer may be going out. Any of those would cause the pressure spike and shutdown to prevent damage and venting to the atmosphere.

 

250-300 on the high side at 65 degrees is too high. I would expect to see less than 200 at that temperature, actually about 150 if you took the reading at idle. (The general rule of thumb is 2.5 times ambient temp on the high side).

The randomness of the shutdown is puzzling, though.
"... its been doing it ever since we bought it, its getting hot now, so the wife..." Did you maybe purchase the car over the winter and the problem is happening more now that the temperature is starting to go up?

It's starting to sound like something internal. There is a test that you can do to help determine that.
While watching the high side gauge, mist the condenser with water from your garden hose:
-If the pressure drops like a rock, there is probably an airflow issue over the condenser.
-If the pressure drops only slightly or very slowly, the problem is likely internal.

If the pressure drop indicates an air flow issue, flush from BETWEEN the radiator and condenser with Simple Green and a firm spray from your garden hose. You may have to pull the radiator back and/or remove the fan to do this. You would be amazed at the crud that can build up between those 2 parts. Blowing it out from the front usually won't help, that's how the stuff got in there to begin with..
Also make sure the airflow from the fan(s) is going in the proper direction, or that the fan blade(s) are turning in the right direction. I've seen replacement fan motors wired backward causing them to spin the wrong way. I've also seen this happen on a new car twice over the years. It's not common, but if it bites you once you learn to look for it.

If the pressure indicates an internal issue, carefully feel the lines and all of the parts on the high side of the system while it's running (discharge line, condenser, receiver/dryer, liquid line) all the way to the metering block. It should be hot or very warm the whole distance. If you find a cool, cold or frosted over spot, that indicates a restriction. I'm assuming that this a typical Chrysler AC system with a rectangular aluminum metering block at the firewall. (Both lines pass through it and it has what looks like a mini flying saucer attached to it.)

Hope this helps.