A/C Blows cold then warm

[josterbauer]

I vacuumed the A/C system down and recharged it with 2 lbs of r-134. It will blow really cold and cycle as it should, then the pressure on the low side kicks up high when the compressor kicks off. It will stay there for about 5 minutes and blow warm air. Then the compressor will kick on (low side down to 30, high at 200 - 75 degrees) and cool as it should. Any one have any ideas what would cause this?

[Mopar1973Man]

Compressor shouldn't cycle. It should remain locked up and solid. So the low side pressure is falling below 30 PSI and kicking out right? So that means you need more freon. Table included so you can get enough freon loaded up. I typically try for about 40 PSI on the low side. The only time the compressor should kick out is of the high side gets to hot and the pressure is well over 400 PSI or if the low side gets to cold and the pressure is below 30 PSI.

[josterbauer]

With the compressor in the unlocked position the low pressure side is at 60 psi. When it kicks in it will kick the low pressure side down to around 30 psi or a bit lower. So I should add more then the 2 lbs that I already added to the system? With the a/c on max the compressor should always be engaged and the pressure should be at around 35-40 psi on the low side?

[Mopar1973Man]

You should add enough to keep it from cycling. Even Sanden Compressor specs say it shouldn't cycle more than 3 time a minute other than that clutch damage can occur. Also too low on the low side during compressor operation will cause freeze up problems. A normal house refrigerator will have a low side of 2-5 PSI to get cold enough for the freezer. So over charging causes it to be warmer.Disregard the lines I goofed... But take notice to the temp on the inside ring for R134a you want the needle close to 35-40*F. 30 PSI is 28*F and will cause freezing hence why the compressor shuts down.

[josterbauer]

Sounds good. I will add more r-134 and see if it stays around the pressure that I need it to. I just did not want to add too much to the system as I already added the 32 oz (2 lbs) that is stated on the sticker. Thank you. I will post an update as soon as I add some more.

- - - Updated - - -

I have updated information. I added more r-134 to the system today (75 degrees). Here are some pictures to show how the gauges are reading.

Low and High side at idle -

Low and High side at 1500 RPM -

The question I have is this good enough, or do I still need to add more to the system. The truck was at a stand still for all pictures. No fan blowing at the condenser. I have almost double the amount that the truck states to have for the initial charge.

[josterbauer]

Bumping up the thread. Seeing if anyone has any ideas on the pressures and adding more r-134? I have already added over what the sticker shows on the truck. The pressure is still a bit low, and seeing if I still need to add more as it is getting close to double what the sticker is showing. If there is truly 12 oz worth inside of the cans of r-134.

[Wild and Free]

More important is how is the A/C performance now? Pressures look good, I have been doing A/C work on all types of equipment for over 20 years and A/C charts are nothing more than a very "General" rule of thumb not to be depended on totally, what I find odd is that you have that much Freon in it, the factory amount is about the only "Close" spec that can be depended on, but some systems can be extremely finicky.

[josterbauer]

Testing it with the gauges, the compressor stayed locked up the entire time. I was just a bit worried when the pressure was at around 28 psi when at 1500 RPM. It just seems that it may be getting too close to the low side shut off. I just want to know what the A/C gurus think of running the way it is, or add a bit more r-134 to the system to get it up to the 35-40 psi mark? It was only around 75 degrees out when I hooked up the gauges, so it should go up with the temp.

[Mopar1973Man]

Yeah it about right on. Thing is as it get warmer in weather the pressure will increase. Now what is your vent temps like? You should see roughly about 35-38*F if the compressor stay locked.

[josterbauer]

I will leave everything as is then. The other problem that I noticed today was the high pressure side was reading low. I do not know what would cause that. The low pressure is the same as it was yesterday, but the high side only got up to 75 psi today and never moves from there. I don't know if that had to so with the 60 degree temps. It still blew cold inside the cab. Would that be normal?

[Mopar1973Man]

It's all based on current weather conditions like trying to recharge a A/C system in a the dead of winter is going to have much lower numbers than charging A/C on a 100-110*F summers day. So I would hold for now and see how it performs on a hot day if the cycling problem returns then check your pressures again. Wild & Free is right there is a balance to how much liquid and gas freon you want in the evaporator too much liquid or too much gas and you lose the cooling abilities over a range of weather.

[josterbauer]

I will call this one good. I will check it again once we have warmer temps that we need to use the A/C. Thanks for all the help with this one. It has helped become more aware of how the A/C pressures work and function.