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torch

join:2009-04-30
P1H 3J4
reply to dolphins

Re: [Help] AC Pressures

You cannot fill an AC system by pressure. The static pressure (ie: engine off) will be the same if there is one ounce of liquid as with 20 oz of liquid -- the static pressure is the vapour pressure of the liquid at a given temperature. It varies by temperature, not by quantity. The cooling capacity varies by the weight of liquid in the system and you cannot determine that by pressure.

At 78°F, the vapour pressure of R134a is 83 psi. Your static reading shows you had at least a little liquid in the system.

With the system running, a CCOT type R134a system low pressure will cycle between around 20 psi and 40 psi. As the compressor runs, the low side pressure drops, chilling the liquid refrigerant. When it drops below freezing, the evaporator fins could freeze over, obstructing air flow, so the low pressure switch cuts off the compressor, allowing the temperature to rise back up above freezing.

At 78° it may not cycle at idle -- these things should actually be checked at 1500 to 2000 rpm -- whereas your video seems to show a rapid cycling typical of much lower ambient temperatures, a drastic undercharge or an obstructed orifice tube. The car sure isn't going to cool well when cycling that fast. If the system was as undercharged as that rapid cycling at idle suggests, it wouldn't have to leak very much to quit completely. BTW: Don't run the AC when undercharged like that -- the refrigerant circulates the oil so you are starving your compressor of oil.

Since you can't find a leak, have the system evacuated, vacuumed and recharged by weight (as indicated on the underhood sticker). See if it cycles normally after that. Then get an electronic leak detector and track down any leakage. Since you can't find it visually, there's two likely places. First, put the blower on low and check the vents inside the car. If the evaporator is leaking that's where you'll get a hit. Second, take the fan belt off and put a plastic bag around the compressor overnight. If the detector reacts to the bag contents in the morning, the shaft seal is leaking.