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So yesterday my brother overfilled my AC by at least half a can after borrowing it. AC was working fine before this but “wasn’t cold enough” for his liking I guess. So he drove it last night and said on his way home the AC started blowing outside temp air. He checked it and discovered it was overfilled so he decided to let pressure out via the high side port (no idea why he didn’t vent it from lowside and I also realize this is illegal as shit) by sticking a screwdriver in. This morning when I go to see what’s upc the compressor will now not engage at all. The clutch does not spin. The AC bottle and gauge I have read at like 10-15 psi when hooked up to low-side, so I figured maybe he let out too much refrigerant and go to add more myself. Immediately upon squeezing the trigger of the AC bottle the pressure now jumps to red-line indicating mechanical malfunction.. When I let go of the trigger the pressure now reads at 35. Evacuating the high pressure side with a screwdriver now makes an air escaping sound but little to no green refrigerant or white smoke comes shooting out like it did last night he says.
What could possibly have caused this malfunction? If it was overfilled wouldn’t letting it back out fix the issue or was draining from high side instead of back out the low what caused an imbalance ? Was the compressor fried from overfilling and driving it a couple hours? Clog in system now? Why does high side AC now seem like it’s completely empty?
Sorry for wall of text. I’m pissed my AC was working fine before and my brother just did a number on this one. Trying to retrace his steps to find out what went wrong.
 

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If you have a twin gauge set, What are the pressures on both sides? This will start to answer your problem. Without them it’s only a guess.
The AC system has both a low pressure cut off switch and a high pressure cut off switch. The pressure must be between these 2 values or the compressor clutch will not get energized.
Crude pressure numbers would be
Low side >35- 40psi
Hi-side < 250-300psi

Find or borrow an AC service manual. You can find good manuals for free on line.
 

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The high side port on GM cars are notoriously bad about going bad right after being messed with. For instance, I had checked my ac with proper gauges a few months back. And as soon as I took the high size valve off the port, it wouldn’t seal. The next day I replaced both the high side port and low size Schrader valve.

But let me tell you that if you have released all of the refrigerant, you will want to have the system vacuumed out. You may be able to recharge it, and it may work initially. But it’ll likely caused major issues. The little bit of moisture that will enter the system will cause acid to build up and rot the system from the inside. It may also damaged the seals and evaporator under the dashboard. It can cause damage to the compressor as well. So please have a shop vacuum the system. I usually change $40 to vacuum and recharge the ac system, that a half hour labor we charge.

Not only that to think about, but hopefully he hasn’t used an R134a with leak sealer. That can cause some major problems, like clogging.

I like to recharge the system to the exact amount of refrigerant, if I can. I can’t always. But when I can’t I like to charge the system when it’s fairly hot outside so I don’t overcharge it. Sometimes you may overcharge the system when temps are cooler. Then when it gets hot, the system will start to shut itself off due to the pressure rising with the higher temp.

Venting refrigerant is technically illegal, yes. Believe me that a lot of ac techs do it anyways, whether it’s automotive, residential hvac, or other type of refrigeration. A lot of older guys, who are stuck in their old school ways, just do this as practice. I’m sure there’s worse things to allow to escape into the atmosphere though.

Also, Dorman makes a high side valve to replace the original style. It has a replaceable traditional Schrader valve in it instead of the rubber type seal. Wish I had bought it instead but didn’t want to drive around to find one at the time. Really an extra 20 minutes and I’d have had it. But the part # is 800-955. It was like $9 when I had bought it for a customer vehicle.
 
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