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Definitely get a/c gauges I've got the cheapest set you can find. All your wanting is the pressure gauge that reads r 134 psi. A brand new set on e-bay is 29.99. The low side with compressor clutch running will always be between 21 to 25 psi. That's the compressor job to keep the low side at that psi. The high side is where your able to read what's going on. For example hooking the hvac line up to the high side will show you the psi flux. With the a.c running a/c clutch engaged the high side will climb to 280psi then the a/c fan will kick on to relieve head pressure within 5 minutes of a idling engine at normal operating temp. If your watching the high side while idling and the pressure climbs to 250 and stalls there and dose not hit 280 And the a/c fans never turn on your freon is to low. The a/c system is designed to have it hit 280 and flux to 230ish.
On the other side of the spectrum if you watch your pressure climb to 280 and hear your a/c fan turn on you will see the high side pressure start dropping if the pressure drops to 250 and stalls there and dose not drop any lower you have to much freon in the system. I'm sure everyone knows how to add freon. But to extract some While your watching the high side psi stalled at 250 psi crack open the valve on the hvac manifold and bleed a little freon off the high side to get the pressure to drop to 230. Just a spit of high side freon at a time then close the valve then watch the pressure. The system needs to hit 230 to turn off the a/c fan.
Having to much freon will destroy the compressor. Fun job swapping them, a brand new compressors comes pre filled with ester a/c oil no need to add more. Your old dryer has oil in it from your last compressor and if it's not replaced the added oil from your new compressor will accumulate to way to much a/c oil. Compressor and dryer must be replaced at the same time.
When installing a new compressor the system needs to be vacuumed before adding freon. There's a very good vacuum pump on eBay for 99$ that will vacuum the system down to -18 psi. The vacuum pump hooks up to the hvac manifold so at all time your reading what's going on by looking at the high side gauge. Looking for leaks is at this point, turn on the vacuum pump and drink a few beers. After 15 or 20 min the system will be at -18. Close the vacuum pumps gauge on the manifold, turn off the pump. Your a/c system should read -18 forever. If not you've got a leak and you will hear it hissing.
Checking for leaks on a charged system leave the car off and hook up to the high side. You should read right around 125psi indefinitely. Any gauge will show you if your loosing pressure the psi I'm referring to is the r 134 gauge. The hardest thing buying a hvac manifold is getting the a/c hookups that fit the high side and low side of the car. Most manifolds come with adapters that fit your houses a/c unit.
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