Automotive Air Conditioning Information Forum (Archives)

Viceroy on Tue September 30, 2014 11:36 PM

Year: 1992
Make: Chevrolet
Model: Caprice
Engine Size: 5.7
Refrigerant Type: R12
Ambient Temp: 85
Pressure Low: 38
Country of Origin: United States

First, let me start off by saying that I used a considerable amount of information from this forum in solving my problem. Been a lurker, but finally posting. I have an R12/R22 gauge set, but for whatever reason, the high pressure hose does not fit my R12 AC port, so no high side readings, unfortunately. I can only rely on the high side temps I took, and attempt to extrapolate pressure from that.

A little background first. I live in South Florida, just north of Fort Lauderdale. We have some of the worst conditions for the performance of AC, as we not only get blistering temps, but that is constantly backed up by humidity that is generally a minimum of 80%, and routinely hits 95%+.

The problem I have been having is that when temps are at their hottest, I get solid 42-45 degree vent temps, and then as it gets cooler outside, or rains, my vent temps climb to 60 degrees. Not that I'm complaining that when it's 99 outside, with humidity over 90%, I am sitting in my wagon feeling like I need to grab a wool coat and mittens. It just rankles me that if I get on the highway, if the temps are cool, or if it rains, the AC gets warmer. It's still cold enough that I want to point the vent away from my knuckles, but still.

So, today, with temps hovering between 85-88 degrees, with humidity of 85%, I decided to test my system in a few ways. Here's the details:

Ambient: 87 (average)
Temp Entering Condenser: 89
Temp Behind Condenser: 124
Cold Line Temp: 44
High Line Temp: 160
Vent Temp: 49-50
Low Side PSI (@ Idle): 38 psi

Obviously, the high temp was quite ridiculous, looking at the R12 chart. So I decided to experiment. First thing I did was idle the engine up to 1500 rpm. Low Side pressure dropped to 22 psi. OK, so maybe bad compressor? Maybe. But I have been deeply suspicious of my OEM fan for some time. Besides the fact that the fan appears to be a pure centrifuge, sending exactly no air in backwards flow, but flinging it outward instead (which is nice, since a good percentage of that flinging air remains in the fan shroud), I also discovered under smoke test that air actually flows forward to the engine fan. So, I decided to do the mist test to see what would happen. I barely cracked open the spigot, and let a fine mist travel toward the condenser. Results? Low side dropped to 21 psi, hovering just above cutoff. Next, I idled up the engine to 1500 rpm. Never made it. AT 1100 rpm, low side pressure sank to 18 psi and the compressor cut off.

No big shock here, because early this morning, when the air was kind of cool, I heard the compressor cycling.

So, next step was the paper test. I took a small piece of paper and set it close to the condenser to see how easily it would take. It didn't take the paper real fast, and didn't stick real hard, either. My Ram and Charger would take that paper right out of my hand. Clutch fan roars like a lion when the heat hits it, so it's not that. This steamboat paddle wheel of a fan just sucks, or doesn't suck, rather.

So, the long story made short here, is that any time the condenser is actually getting decent airflow, the true state of being undercharged is revealed, and the compressor just cycles it's butt off, causing temps to rise. Rain, cool ambient, highway, whatever. Warm, warm, warm.

Well, fortunately, I already have a set of truly high flow electric fans ready to go in, and then I can get this system charged up the right way.

Thanks, I couldn't have gotten this far without you guys.

Cussboy on Tue September 30, 2014 11:58 PM

You need a "GM type" high pressure side flare adapter for your 1992. These came into being after some unknowledgeable folks couldn't tell low side from high side when both were 1/4 inch flare fittings.

Of course Ford/Mazda used a different type of high side fitting then, needed a different type of adapter. Toyota used same as GM. Luckily, I have both.

Contact AMA at 602-233-0090

Edited: Wed October 01, 2014 at 12:03 AM by Cussboy

Viceroy on Wed October 01, 2014 12:27 AM

Awesome. Thanks.

mk378 on Wed October 01, 2014 12:32 AM

Loss of cooling in cool, humid weather could be evaporator freeze-up. Next time it happens, pop the hood and look for ice on the lines. This is caused by the cycling switch not opening in time.

Also the switches can fail where the pressure goes way up before the compressor starts again. This will cause insufficient cooling once it starts to cycle. The compressor should stop around 25 psi and restart before 50. You may be able to test this even in hot weather by setting the interior fan on low and revving the engine.

If the shrader valve pin in the high side is flush with the top of the port (like the low side is) you need a standard 3/16 high side adapter. If it is well recessed (about 3/8 inch), you need the "GM deep throat" adapter. That type of port (intended to be especially "idiot proof" I guess) was used by GM late in the R-12 era. Before about 1987 the ports were both 1/4 inch, and like Cussboy said, that caused problems with people connecting their cheesy can and hose rig to the wrong port and blowing up the can.

Edited: Wed October 01, 2014 at 12:44 AM by mk378

Viceroy on Wed October 01, 2014 1:30 AM

No ice on the lines. Doesn't seem to be freeze over, because the drain keeps pouring like a faucet.

The NAPA near here is owned by a refrigeration guy, so I'm going to see if they have what I need.

Viceroy on Wed October 01, 2014 5:47 PM

OK, today, I took the electric fan set that I am about to install in my car, and set it loosely in front of the condenser to see what would happen. Low and behold, low side pressure immediately dropped 10 psi. With them set up in their most efficient state, behind the radiator, they should be kicking butt and put this problem to bed for good.

Solution confirmed. Poor airflow through condenser due to inferior engine fan.

GM Tech on Wed October 01, 2014 11:04 PM

If your low side hit 10 psi- then there is your problem-- the cycling switch (evap freeze avoidance switch) is stuck closed. The compressor should cycle off at 22 psi or so to avoid evap freeze-up. If very humid, your evap will freeze over- and it is a law of physics that you can't blow air through ice. Cycling switch is the one on the accumulator

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Viceroy on Thu October 02, 2014 3:07 AM

No, I mean the low side pressure was reduced by 10psi. It went from 40 to 30, just like that. Can't wait to get these e-fans in. Should make life alot more liveable.