A/c intermittantly blows warm

[csledge]

Hey all,
new to the forums, hopefully you guys can shed some light on this so I don't have to get raped at the mechanic. I have a '99 Blazer LS, 2dr 4wd.. I'm having problems with the A/C. It will work great for 20-30 minutes or so, but then suddenly it will start blowing warm. If I cut it off for a while it'll work fine again and the cycle repeats. I pulled over and looked under the hood once when it did this, to see that the compressor wasn't cycling at all, which I assume would be the problem. But the compressor itself must be fine or it wouldn't work at all, right? I'm guessing it's some sort of switch or something, but I dunno.. what do you think?

[87wildside]

Welcome to the Planet!

[Hammer Head]

Welcome to the Planet. When's the last time you had the system charged & tested? It sounds like you may need some freon.

[04silveradomykk]

I'm with HHead, have your mechanic put the guages on it to read the PSI of the charge in your A/C system...

[Mikz86TA]

Welcome to the PLANET......I agree with the A/C needing charged up...you might have a leak.

[csledge]

it's just recently (within the past 2 weeks) been charged and looked at. It wasn't working at all and I got a guy to look at it. Said he had to replace some relay, and went ahead and charged it while he was at it. I hooked up a gauge and it reads fine.. compressor works, evaporator cold, everything's gravy until it's been running for a good 20 min or so, then it all goes south.. still blows, but warm air.

[Hammer Head]

Maybe the clutch on the compressor is toast. Seeing as it only works for a limited time then.

[T Man]

seeing as how it works for a while, I would think that maybe there is an obstruction in the orafice tube that is allowing it to cycle the refrigerant for a while then blocking it off. I will do some checking with our lead tech today to see if he backs up my theory

[csledge]

anything I can do to help figure it out? I'm trying to get it to do it again, so I can have the gauge hooked up to it when it does it, to see if/how the pressure changes, but it's not doing it.. I'm thinking it only does it on pretty hot days, when it did it last it was 95+, right now it's in the 70's and it doesn't seem to want to do it

[04silveradomykk]

On that extremely warm day the PSI would be abnormally higher than usual... it's possible the PSI is too high and the Cut-off switch is just keeping the compressor off until the pressure drops.... that doesn't explain the warm air though.

[csledge]

I've been driving around all day today.. low to mid 80's no problems at all. intermittent problems are a PITA

[csledge]

any ideas?

[Mikz86TA]

Like MYKK said, it could be there is a little bit higher than needed amount of freon in the sys abd the hotter days are causing a peak and the switch is shutting it off. Also again it could be a clogged oriface screen.

[csledge]

I will check on that.. It's about 73 degrees outside, and low side pressure is hovering around 35, regardless of the setting I put the AC on. with the AC OFF, low side pressure is 85.. Not to push the subject, but I noticed something, peeking around under the hood. Anytime the compressor cycles off, the clutch will still move a little for a few seconds, but in the opposite direction. And when the truck is off, I can spin the clutch (not the whole pulley) in that same direction by hand. Maybe I'm just a car-hypocondriac, but is that normal? lol

[Sparky2263]

Anytime the compressor cycles off, the clutch will still move a little for a few seconds, but in the opposite direction. And when the truck is off, I can spin the clutch (not the whole pulley) in that same direction by hand. Maybe I'm just a car-hypocondriac, but is that normal? lol

Yup, totally normal.

Need to find out if the compressor is engaged when it starts blowing warm. i.e. hook up a test light of some sort to the compressor feed wire.

[Sparky2263]

"I do know that the clutch is not spinning when it starts blowing warm, is that what you're getting at? I've watched it do it, everything will work great, then the clutch stops spinning, as if it's just cycling off, but then never cuts back on"

One of 2 things. Either the relay or associated wiring is the problem or the air gap on the clutch is too great. When it gets hot and cycles off, the magnetism created by the clutch coil is too weak to pull the clutch back in.

Still need to bust out with the test light and wire it into the clutch feed. Need to take a voltmeter and measure both ground and feed sides when duplicating the problem.

No need for a fancy, golly-gee-whiz test lamp. Just your basic, good quality test lamp, voltmeter and a jumper lead will work fine.

Basic electricity, nothing to it.

[csledge]

So how do I check if it's the orifice tube w/o having to just replace it? I can recharge it myself if need be. I will do those other tests first though, if I can ever get it to do it again. It seems to only do it on really hot days when it's been running for a while, and I haven't run into either situation the past few days.

[JONGO8]

I have this same thing. When it is hot out side and I'm drivimg for awile and stop to go in a store and come back out to start the truck the ac won't come on.

I have to open the hood and find a stick and push the clutch in and it will work untill I stop.

[mack]

I've had the same problem for 2 years now. Mine will turn back on when I rev the motor. I guess that beats having to get out of the truck :)

[Sparky2263]

I have this same thing. When it is hot out side and I'm drivimg for awile and stop to go in a store and come back out to start the truck the ac won't come on.

I have to open the hood and find a stick and push the clutch in and it will work untill I stop.

That's the air gap. Adjusts with different thickness shims under the clutch.

[csledge]

looks like mykk got it on mine - I read low side pressure on a pretty hot day, and after running a little while, the PSI was getting a little high. I got a bit of the freon evacuated and haven't had the problem since. I had just taken it to an AC guy a week or so before because the AC wasn't working at all. He had replaced a relay and added a bit more freon, I suppose he incidently overcharged the system just a tad. Note to AC newbies: more freon doesn't = better performance! In my case, actually worse! :)