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.

And yes.......it certainly appears you do. > > > > > Well SORTA! > > > Except it's a pressure switch in the car's accumulator... but you COULD > > make > > > a case that refrig gas pressure reflects the temp of the air over the evap > > > coil... so, in a way, you are correct. > > > > > > But not quite. > > > > Would a better case be the actual temp of the refrigerant in the evaporator > > reflecting the pressureof the refrigerant, instead of the airflowing over > > the evaporator? When you are talking about the actual refrigerant, > > temperature and pressure are proportional to each other. When the temp of > > the refrigerant is at a measured degree, you can figure out the pressure > > with fair certainty. And the other way around. > > > > If I understand it correctly, that is. > > > > >
Reply to
Bales_Family
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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!

Reply to
BarryLL

High and low side pressures very widely for a number of reasons. The temperature, amount of moisture in the air, sun load, air flow over the coils. The compressor is turned on and off by a pressure switch on the low side of the system. There are charts available that show relative high and low side pressures for out side air temperatures. With the engine rpm at about 1500 and the A/C set to max you should see about 35 to 40 degrees coming out the registers. Again there are charts that show the range of temps coming out the registers for a given out side air temperature.

There is a leak. Look at the fittings, any sight of dirt build up at any fitting is where the system has been leaking and dirt is now sticking where the system oil has come out.

That's because the system needs to be opened up, sealed and evacuated. There is a good possibility of air in the system.

Over filling can cause liquid to get to the compressor, that will cause the compressor to hydraulic and break. Not good. The system will not allow a overpressure situation. The compressor has a high pressure relief valve. Electrically there is a high pressure shut off switch. The processor uses this to first turn the high speed cooling fan on, if pressures do not fall, it then cycles off the compressor.

Reply to
Thomas Moats

Great Advice and right on the money.

Thomas Moats wrote:

Reply to
Mike Graves

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