I bought a gauge set a while back, and here's the way I do it.
Vacuum applied via the center (yellow) hose with both high and low
(red and blue) valves open. After an hour or so, close both valves
and see what the low side pressure reading does. Must be at least
27" Hg, and stay there for at least half an hour with no needle
movement. If you want to wait, it should hold with minimal change
overnight, although you may get some slight pressure from water
diffusing out of the dryer material. If it won't hold for 30 min
with no change, it's not gonna hold refrigerant, either. May still
leak under pressure but not vac, but that's a different issue.
Once you get it to hold a good vacuum, turn off the pump, attache
your freon source (can or tank), pressurize the yellow line, then
unscrew it from the guage set enough to bleed air and freon off under
pressure. 30 sec of audible hiss is enough, you don't want to blow
the can out, just purge the air out of the line. I then invert the
can and open the high pressure valve, allowing liquid freon to blow
into the high side. Tons of room, after all, and you cannot get a
slug of liquid into the compressor that way.
Close the high pressure valve and leave it shut. Open the low
pressure side and start the engine and switch on the AC.
Pressure should rise on the high side and plummet on the low, usually
drops well below 30 psi. Close the lower pressure valve when it gets
low and the can is warm -- if it's freezing cold, you are still
boiling freon.
Close the low pressure valve, remove the can, attach another one,
then puncture the seal and pressurize the line. Bleed off as for the
first one, although if you are fast you won't need to . You can also
crack the low pressure valve and bleed off at the can if you want.
Open the low pressure valve and dip the can of freon in hot water.
This will boil the freon out MUCH faster than letting it get cold!
High pressure side should start around 100 psi or so and climb as the
freon goes in. Once you get to 275 or so the low pressure line
should be cold (not frozen) and cool air should be coming out of the
vents. Won't be cold so long as you have freon in the can, usually
55 psi or so. Once that can is empty, the pressure should be around
250-275 on the high side in the summer and the low side 34-45 psi.
You will get decent cooling, but need to add more freon, usually I do
a "guestimate" partial can. High side should be no more than 350 if
you have air flow, although it can climb above that.
Fans come on around 275 or so, depending on temperature and circulation.
If the fans are not on and the high side is 450, you have a defective
high side switch or fan relay/fan/wiring. I believe the newer ones
are high side safety, too, and will shut the compressor off if the
temp or pressure is too high, check the wiring diagram.
If this is the case, you don't have anything wrong with the AC, you
have a bad fan cut-in switch, bad relay, or broken wire or dead fan.
Peter
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