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Q: A/C issues

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My 2000 Chevy Tahoe has no cold A/C. So I went to the store today and bought an A/C recharge kit to recharge it. It directed me to disconnect the hose and check my level before connecting it to the A/C coolant bottle. So I did this and I immediately realized that it was in the middle of the red zone, obviously meaning it was severely overcharged. I have never recharged this system before, so there should be no reason it is overcharged. But then I realized that when my A/C compressor kicks on, the pressure drops down to the blue, meaning that it is undercharged. It makes no sense that it goes from being overcharged to undercharged. Does anybody know the solution to this or why this would be happening? Thank you for your help in advance.

My car has 135000 miles.
My car has an automatic transmission.

When you do not have the A/C on, you will have pressures on the low and high side be at about 90 to 105 on both sides. When you turn on the A/C with engine idling, the low pressure will run around 25 to 35 and the high side will be 175 to 210. This is normal. If you are using a store bought can, it should be used as a guess for correct pressures. Add a little bit of refrigerant at a time until you feel the A/C getting cold, and the pressure is around 25 to 35 with the A/C blower on the low blower setting. Do not overcharge it. Stop when it is getting cold. If you had to add more than a few ounces of refrigerant, then you have a leak somewhere and should get it checked by a mechanic.

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