Re: basic a/c question

The AC is controlled by three things---the AC switch in the passenger compartment, a low suction pressure switch and a high discharge pressure switch.
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 gasket.
I hope this helps...

COULD
evap
evaporator
over
of
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 fan speed).
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 miles).
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!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Great Advice and right on the money.
Thomas Moats wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.