The AC is controlled by three things---the AC switch in the passenger
compartment, a low suction pressure switch and a high discharge pressure
When the fan is set on LOW, there is less air moving across the evaporator
so less heat is absorbed. The condensor keeps getting colder and colder and
the suction pressure gets lower and lower until eventually it trips the low
pressure switch and disengages the AC clutch. This prevents the evaporator
from icing up.
When the fan is set on HIGH, there is more air moving across the evaporator
so more heat is absorbed. The refrigerant is hotter, causing the discharge
pressure to rise. Eventually, the high discharge pressure switch trips and
the AC clutch disengages. This prevents the compressor from blowing a head
I hope this helps...
I picked up one of those low-side pressure gauges (along with a can of
R134a to recharge with) at Advance Auto last year. I'm just a little
concerned about how to read it and I wanted to be absolutely sure
before adding any more charge to the system.
I really thought this would be a common experience, but have not found
the answer after a fair amount of hunting around (the folks at Advance
Auto didn't seem familiar with what I described to them). This thread
gave me the most & closest info, so I'm confident you folks will know
the answer! Here goes...
I connect the gauge and let's say at that moment the compressor is
running. I get a reading of between around 25-35 pounds. Wherever it
reads, it drops down eventually to something below 25 and sits there
for "awhile" (maybe 15-30 seconds, maybe longer, perhaps depends on
Then the clutch kicks out (I see it stop spinning so I believe the
compressor is off). The pressure then within a few seconds climbs to
45 lbs, at which point the clutch kicks in again. The pressure drops
quickly back to around 35 lbs, and then within a short time (a think a
bit longer than the climb up) drops down to a bit below 25 lbs. Then
cycle then repeats (it sits there for "awhile"...).
From what I've read I suspect I'm just low on charge, and after a
period of time at low pressure the system is turning off the
compressor for fear of freezing up. Then the fact that it is hot and
needs to run, eventually runs the pressure back up, at which point a
high pressure cutoff causes the compressor to run again. Am I close?
More information -- The A/C hasn't worked well for awhile. I think 3
years ago I brought it to a guy who does radiators and A/C locally who
has a good reputation. He checked the system (he didn't find any
problem), and put a dye in the system to help detect leaks. A year
later it was performing really badly again, I brought it in and he
told me he did not see any evidence of the dye leaking and suspected
pinhole leaks (maybe in the evaporator, I'm not positive where he
said?), and that it would be around $1000 to repair -- and probably
cheaper to just periodically recharge. He recharged it and felt it
was working well. It certainly was better but probably not great
(though I don't think it ever was -- BTW it's a '95 Windstar with 175K
The following year (last year) is when I decided that if it was just
going to require an annual recharge I would just do it myself. I put
R134a in last year that claimed to have some stop-leak in it. I think
it's held up better going into this year, but it definitely is not
doing a great job.
So I've put some stuff in that is just stop-leak, and I should have
recharged it at the same time... but I've been reluctant because of
the readings I've gotten. I've read that overfilling can kill the
compressor, cause it to seize up -- I've always worried too that
overfilling might cause a leak (is it enough to cause it to
"explode"?) -- so I wonder why these systems wouldn't have some sort
of overfill pressure relief valve?
So mostly I just want some confirmation that the high reading when the
compressor stops is a normal phenomenon (in fact if I shut the A/C off
I get that reading as well), and it's the sustained reading when it is
running that I care about. Of course any other words of wisdom are
greatly appreciated too!
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