95 Tahoe AC Overcharge?

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Caveman
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95 Tahoe AC Overcharge?

Post by Caveman »

Hello everyone. I am new the forum and need guidance on a problem with a 1995 Tahoe 5.7 front AC (only). Had problems for several years with very slow freon leaks, so I recently replaced the Compressor, Orifice Tube, both high and low hoses, the Dryer, and Evaporator. I flushed and pressure tested the Condenser as it was only a few thousand miles old (previous repair). The new Santech compressor came with 150ml of PAG 46 oil, but the instructions called for 8 ounces of PAG 150 oil, so I drained it and added 8 ounces of PAG 150 oil to the system (included in Autozone P/N P96-24000 kit). Evacuated and charged with 32 ounces of R134a. System pressures did not seem high enough (20/140) and the low pressure switch kept shutting the compressor off. I assumed I had mis-read my scale and added freon to bring the pressures up the design pressures. After adding 8 more ounces of freon, the pressures came up to 25/150, but the switch kept cutting out. Problem ended up being the switch cable. Replaced that and the system now stays running, but pressures are still low. I tried adding freon to get to 45/250 which is the spec I have. Adding 8 more ounces brought the high pressure to 240 but the low pressure stays at 25 psi. Outside air is 85F in the shade and the AC air temp is 45F. I really think I am missing something, but don't think I was that far off on my freon charge. If I was originally correct, the system is now 16 ounces overcharged, the high pressure is in spec and the low pressure is 25 psi low.
I sure could use some guidance as to what I have done wrong to result in this situation.
Thanks in advance for your help.
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Tim
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Re: 95 Tahoe AC Overcharge?

Post by Tim »

Start with the OE Spec on refrigerant, 36 ounces. Even being off a little can create odd issues.
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Caveman
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Re: 95 Tahoe AC Overcharge?

Post by Caveman »

Thanks for the reply Tim. Depending on where you look, the spec is either 32 ounces or 36 ounces. Either way, when I started with 32 ounces, I was way low (20/140). I added slowly to bring the pressure up but the low side never went above 25 psi. I agree that things go weird with too much freon, but at some point I had 36 ounces in the system and it still was too low on high and low sides. Conventional wisdom says that this is a low freon situation. I am looking for a root cause for the low side to never move above 25 with all a new compressor and orifice tube. Especially with significant overcharge. Seems like my high pressure should be off the charts with this charge level. If, for some reason, the orifice tube supplied with the kit was the wrong one, could this cause this situation? I would hate to evacuate and recharge the system without having some theory as to what was causing this problem.
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Tim
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Re: 95 Tahoe AC Overcharge?

Post by Tim »

Should be a Condenser mounted black/white orifice tube.
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JohnHere
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Re: 95 Tahoe AC Overcharge?

Post by JohnHere »

I, too, am leaning toward the OT...either defective or the wrong one...not allowing the compressor to build sufficient pressure. It might also be a damaged or missing o-ring, allowing refrigerant to bypass the OT.
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Re: 95 Tahoe AC Overcharge?

Post by Caveman »

Does anybody know what the correct orifice tube part number and specs are? The one I used came as part of a kit from Autozone. It came with an orifice tube and an expansion valve, so I am assuming this is a somewhat generic kit. Also, does the PAG oil go back into solution or stay airborne in the freon? I will be using a reclaimer and will not run the auto ac compressor to evacuate it, and I would like to wait to remove the freon if it will minimize the amount of PAG oil removed. Also, I assume I should replace the Dryer again since I will have to open the system. How much oil should be added for the Dryer replacement? Thanks again for your help on this.
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Tim
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Re: 95 Tahoe AC Overcharge?

Post by Tim »

Black/White cond mounted OT is correct.
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JohnHere
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Re: 95 Tahoe AC Overcharge?

Post by JohnHere »

Caveman wrote: Sun May 23, 2021 9:27 am Depending on where you look, the spec is either 32 ounces or 36 ounces.
For the 1994-95 model years, my references call for the following: All engines without rear A/C - 36.0 ounces net weight of R-134a and 8.0 fluid ounces of PAG-150.
Caveman wrote: Sat May 22, 2021 3:31 pm Evacuated and charged with 32 ounces of R134a. System pressures did not seem high enough (20/140) and the low pressure switch kept shutting the compressor off. I assumed I had mis-read my scale and added freon to bring the pressures up the design pressures. After adding 8 more ounces of freon, the pressures came up to 25/150, but the switch kept cutting out. Problem ended up being the switch cable. Replaced that and the system now stays running, but pressures are still low. I tried adding freon to get to 45/250 which is the spec I have. Adding 8 more ounces brought the high pressure to 240 but the low pressure stays at 25 psi. Outside air is 85F in the shade and the AC air temp is 45F. I really think I am missing something, but don't think I was that far off on my freon charge. If I was originally correct, the system is now 16 ounces overcharged, the high pressure is in spec and the low pressure is 25 psi low.
Those pressure charts can be misleading, and you can't charge by pressure anyway, only by weight. With an ambient temperature of 85 degrees just in front of the grille, the high side should be around 190-210 PSI with a properly functioning condenser and fan setup, while the low side should be around 28-30 PSI. With pressures in that range, you should be able to attain a center vent temp of around 40 degrees or even a bit lower.
Timpost_id=21226 wrote:Sat May 22, 2021 6:48 pm Start with the OE Spec on refrigerant, 36 ounces. Even being off a little can create odd issues.
Tim is correct. If the proper amount of refrigerant is indeed 36 ounces (which I have no reason to doubt), your system is grossly overcharged, as you suggested. I would recover what's in there now, change out the OT, evacuate for at least two hours, and recharge to spec.
Caveman wrote: Sun May 23, 2021 2:13 pm Does anybody know what the correct orifice tube part number and specs are? The one I used came as part of a kit from Autozone. It came with an orifice tube and an expansion valve, so I am assuming this is a somewhat generic kit. Also, does the PAG oil go back into solution or stay airborne in the freon? I will be using a reclaimer and will not run the auto ac compressor to evacuate it, and I would like to wait to remove the freon if it will minimize the amount of PAG oil removed. Also, I assume I should replace the Dryer again since I will have to open the system. How much oil should be added for the Dryer replacement?
As Tim mentioned, if you get the black-and-white OT, the orifice will be the correct size. No part number needed. If you use an RRR machine, whatever oil that might come out on recovery will accumulate in the graduated bottle attached to the machine. All you have to do is put the same amount of oil back in. Replacing the dryer (accumulator) is always a good idea after opening the system. Drill a hole in the bottom of the unit you take off, measure the oil that comes out, if any, and put the same amount back into the new accumulator before you button it up.
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Tim
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Re: 95 Tahoe AC Overcharge?

Post by Tim »

It was claimed the parts are from Autozone. Doing a quick vehicle look-up, the show the Condenser mounted black/white. What they got I would not know. Best to ask the Autozone guy or match it up on the web.

Also, not trying to be vague on a number. OT's are for the most part sized by color.
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Al9
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Re: 95 Tahoe AC Overcharge?

Post by Al9 »

Possible overcharge condition and lower than expected pressures tells me the orifice tube may be smaller than normal or there's another restriction somewhere.

TXVs get their MOP setting according to engine displacement and whether the compressor displacement is computer controlled or not. Above their maximum evaporator pressure they stop opening further and might even close, and this unloads the compressor. Not enough refrigerant coming back to the compressor means not so much refrigerant can get pumped. Hence the "unloading" effect, deliberate in that case.
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