A/C Blowing Hot - Compressor Turned On When Freon Pressure Too High

[campbmic]

2003 GMC Yukon that was blowing cold, then after sitting for 6 months began blowing intermittently hot and finally only hot. Took it to a shop and;

1. Truck had no freon
2. Compressor did not engage (but I could turn the compressor by hand)

I got the truck home turned on a/c and put in about 6oz of freon to 40 psi. I jumped the compressor directly from the battery and it engaged! Freon pressure would rise to about 70 psi and the compressor would kick on for 2 seconds. That would cause the psi to drop down to 40. It would gradually rise (for 4 sec back up to 70psi and cycle over and over.

So I let out some of the freon and now its sitting at a constant 35psi and the compressor is not turning on at all.

 

[sumo]

So after a recharge the compressor wound not engage? There are a list of things needed to be seen by the ecu in order to command the compressor on. Things like eng temp, ac pressure sensor. Jumping it verifies you have a working compressor clutch and cool. Could be a bad pressure sensor.

 

[campbmic]

The AC had ZERO freon in it. I put in about 2 oz and got the psi to around 35 and no compressor.

Then I jumped the compressor but it would only stay on when it was hooked up.

So I added more freon (about 6oz total) and now my gauge reads 70psi right at the edge of the warning zone. The compressor kicks on by itself for 2 sec and lowers the pressure to 35psi, then the compressor shuts off for 4 secs and the psi rises back up to 70.

This cycles over and over, from what I have read, some people say thats a symptom of not having enough pressure. So maybe my cheapo gauge is funky or the system needs higher pressure than I thought?

Can anyone who has done this chime in?

 

[sumo]

Gauges that come with refill cans are garbage. Static pressures lo side should be around 70-80 and hi side around 100. Your best bet would be to take it to have it evac and recharged using a professional ac machine

 

[01ssreda4]

Not to be rude but you dont sound like you have the slightest clue what you're doing. An average automobile freon fill is like 20+ ozs. I dont know if you just arent aware of this or dont know how to read a can or what. But 2-4 ozs of freon aint gonna get the job done. If it shows 35-70psi the compressor should be running, period. If it isnt, youve got a faulty high or low pressure switch or a blockage somewhere.

 

[campbmic]

Not to be rude but you dont sound like you have the slightest clue what you're doing. An average automobile freon fill is like 20+ ozs. I dont know if you just arent aware of this or dont know how to read a can or what. But 2-4 ozs of freon aint gonna get the job done. If it shows 35-70psi the compressor should be running, period. If it isnt, youve got a faulty high or low pressure switch or a blockage somewhere.

Thanks. I do actually know how to read a can of freon believe or not.

Does your can of freon have writing on it that tells you the freon capacity for your vehicle? Mine doesn't.

Before I started I looked up the freon capacity here: http://www.techchoiceparts.com/refrigerant-and-oil-capacities/gmc-light-truck
That chart says its 48oz.

Like I said, right now the system has about 6oz of freon in it. The pressure starts at 70psi and the compressor kicks on and drops the pressure to 35. When the pressure hits 35 the compressor turns off and the pressure rises back up to 70 causing the cycle to start over again.

Im trying to figure out if I should keep filling it or what, I'm guessing the gauge isn't super accurate.

 

[01ssreda4]

Well, if it was empty, enough that the compressor isnt coming on, its very very low. Now, knowing that, and add some common sense, adding 6 ozs into a system thats very low but requires 40+ ozs, well it seems self explanatory to me. Compressors cycle, depending on pressure, if its high it will force cycle off the high side switch to keep the system from exploding. If its low it could cycle off the low side switch. If you arent looking at high side pressure (sounds like you are just using a refill gauge) then you are only seeing half the picture. The minimum you can do here, is try to get the correct amount of freon in the system. Once you get close to doing that, a little more diagnosis can be done. Until then we are pissing in the wind.

 

[sumo]

Since you don't know how much you have and no real way of telling, you should have it professionally evac and recharged to spec. If you overcharge you can damage your compressor, to little no ac or not cold enough.

 

[Bowzer]

If it was flat and you recharged it without pulling a vacuum then then you have non condensibles which raise head pressure. It may be cycling off on the high pressure switch.

My 01 with rear coil takes 48oz

 

[mizzouguy]

Since you don't know how much you have and no real way of telling, you should have it professionally evac and recharged to spec. If you overcharge you can damage your compressor, to little no ac or not cold enough.

THIS RIGHT HERE ^^^ especially if it was empty...it now has air because there is a leak somewhere. You have to have it vacuumed down, and then you can tell if there is a leak if the vacuum bleeds off. Fix the leak, vacuum it down again, and refill by weight. If you are using stuff you can buy at an auto parts store, at least get a manifold gauge set and a cheap vacuum pump. Harbor freight has then, or autozone rents them.