Year: 1994
Make: Buick
Model: Roadmaster
Engine Size: 5.7L
Refrigerant Type: R134a
Ambient Temp: 74 F
Pressure Low: 40?20?
Pressure High: 130
Country of Origin: United States
I am a newbie to this forum, and seeking help since the AC in my beloved Buick is getting feeble. I know some about these systems, but evidently not quite enough. I have the GM shop manual for the car, but it is pretty sparse when it comes to actual troubleshooting. (If I wanted to tear down and overhaul my compressor, tho, it has all the info!)
I have a gauge set, which is where the above readings came from. Now, I have to confess one really naive point: Is the low side reading the number when the compressor cuts in (20 PSI) or is it the number just as the compressor cuts out (40 PSI) ? The compressor on-time is about 6 - 7 sec, during which the low side is pulled from 40 down to 20 PSI. The off-time is about the same before it cuts in again. The receiver gets cool on the outside but never really cold. The measured air temp at the center dash vent, AC on max, is 57 degrees. Relative humidity during these measurements (thx to local weather station) was 45%.
This is an orifice type system. I rebuilt it myself about 4 or 5 years ago, with new orifice, compressor, and receiver. It worked the way I expected until this Spring.
Anyway, am I just low on R134a? I hate to just throw refrigerant in blindly. Or could the compressor be conking out already? It doesn't seem to make any odd noises.
If the situation is not obvious, are there other checks or measurements to make?
Thx.
Oops! Wait a minute! The compressor cuts on at 40 PSI and off at 20 PSI. Sorry about that!
Sounds like you have a typical low-charge situation. In addition to the pressures, feel the inlet and outlet of the evaporator (I know this is a pain with the evap inlet down by the frame rail). The outlet should be as cold or colder than the inlet. If it's warmer, which I suspect you will find, then you are low on R134a. There's a long thread just a few posts above this one (Superheat!) which explains a lot of the theory, if you want some reading. I think your best bet is to recover the charge, evacuate, and recharge the system by weight.
I agree, probably just leaked out. Best way is to recover and weigh what came out to confirm. You could also just add some refrigerant and see if performance improves. Some effort should be taken to look for leaks-- install UV dye if it doesn't have it already. You can buy a can of R-134a with UV dye included. This is the only additive can you should ever buy, and just use one dye can per car.
Edited: Tue August 23, 2011 at 8:52 AM by mk378
Typical low charge scenario-- number one leak location on your R-4 compressor is shaft seal, number two is body o-rings-number three is the suction hose from accumulator rubs on upper A-arm and rubs a hole in rubber hose through the foil insulation..
Fix your leaks and evac and recharge and be cool.
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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......
I have one of the fancy electronic leak detectors, but getting the probe down and around the compressor - especially at the rear - is an extreme PITA. What were those GM engineers thinking? Can one of you folks enlighten me as to which is the correct low-side pressure number? Just as the compressor kicks in (40 PSI) or just when it cuts out (20 PSI) ?
Low pressure should be around 25-35 (constant running) under moderate heat load.. 75-85 degf Heavy heat load- near 40-45
low heat load, low blower, compressor will cycle to avoid evaporator freeze-up.
rapid cycling as yours is, is an indicator of a massive loss of refrigerant, (more than half of factory spec is gone) due to a leak....
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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......
I will check for leaks. I rebuilt the engine-bay part of the AC four or five years ago, including that hose. So hose is relatively recent.
Yeah, you are right- '93 was the last year for R-4 on a B-body-- '94 was an HD-6-- still shaft seal and belly o-rings are highly suspect on those..
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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......
Well, I have fixed the problem and have learned some things. One thing I learned was that the compressor should be running constantly at engine idle and AC on max, and that compressor cycling under those conditions is clear evidence of low charge.
I evacuated the system and refilled. When I had one can (12 oz of the required 28) in, AC operation was about what it was when I began this thread. After a few more ounces from the next can, the compressor began to run constantly at engine idle. A bit more (maybe 18 oz or so in the system) the receiver became very cold to the touch, as it should. With two full cans (24 oz of the nominal 28) the system seems to be operating well. I just need to top off with a partial can. I just realized that I can set the can on a scale as I fill the system the rest of the way (D'oh!) to get the right amount in.
Edited: Thu August 25, 2011 at 3:45 PM by Rustbucket Ranger
You haven't fixed it----
If your tire was flat- do you consider it fixed if you put air in it?
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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......
Well, to use the tire metaphor, if a tire lost 5 lbs a month, I would probably live with it for quite awhile. Actually, I did have a tire like that on a different car earlier this year, and that's what I did.
As for the AC, I could not find any leaks with my electronic leak detector, at least in the engine bay. So I don't know what I would fix. Given what a PITA it is to work on this car, I certainly don't want to start dramatic reconstruction without some evidence of a serious as opposed to a slightly annoying leak. If it takes another three or four years to leak down this far again, I can live with it. If it gets to the point where it leaks down in one season, then might reconsider.
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