Ac question

mistyjamesprewett · June 30, 2014

Ok ive got a strange one that nobody seems to be able to figure out. I have a 2002 Sierra 1500 ext cab 4x4 automatic transmission. Here is my problem at idle up to 45 mph my ac works great but anything above 45 mph say 70 mph it works just not good you can feel ot blowing ice cold air then warm air then ice cold seems to be on about a 3 second cycle that it goes back and forth any help would be greatly appreciated thank you on advance for any help

8.1HD/ALLISON · June 30, 2014

Weird. That should not matter. Has the system been evacuated and recharged to be sure it is charges to proper freon level? That would be my first check. The maybe test drive with guages attatched to watch pressures. Usually cycling that fast indicates low freon level.

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mistyjamesprewett · July 1, 2014

I had it done middle of last summer didnt do it then but at idle last summer it was just jot airtill yoi started driving but other than that it worked fine I used a Walmart recharge kit to check levels this year and said it was still gine but I know thats not exactly accurate

8.1HD/ALLISON · July 1, 2014

I would evacuate system, put under vacuum for 30 mins and recharge to be sure. Then see how it acts.

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mistyjamesprewett · July 1, 2014

Ill give it a shot I just didnt want to spend the money yo get it done then have to replace parts I just assumed that since it only blew hot last summer after service while idling that something needed replaced as I used to have a 99 silverado before this truck and it blew rather cold at idle and ICE cold any other time

Duelin · July 1, 2014

Do you have a good A/C shop in your area? Remember I said good. If so let them diagnose it. That way you know what you are chasing.

daxr · July 2, 2014

I'm a bit rusty on AC problems, as I've worked in a shop with fairly automatic equipment for awhile. The first thing we do on an intermittent or blows-warm complaint is hook up the machine, do a full evacuate and leak test, then recharge to spec. That takes care of 90% of problems, and you don't really have to diagnose much.

But if I had to think about it I'd say maybe a simple overcharge could cause you're problem. At low rpms the pressures are ok, but at higher rpms you may wind up with higher pressures, enough to engage the high pressure cut out switch. Hooking it up to gauges would give a better idea on that...but then if you brought it into our shop we'd just evac and charge, probably fix it, and not really look at why. If an evac and recharge didn't fix it then there's looking at pressure switches, restrictions in the system and so forth, but that doesn't happen too much.