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So as I understand the facts, your system worked properly last year and there have not been any parts changes since then.
If your suction/discharge pressures are good, seems like it must be within the STV. That points to the piston/diaphragm assembly, as Larry indicated.
This service part is still out there and the STV overhaul procedure is well described in the service manual supplement for '65. No special tools needed.
I don't know what else could be the cause, if pressures check out.
Mike
Mike, what is the tube in the last picture? Just learning.
Are there any solutions to correcting a hanging or sticking STV? I ask because I don't grasp how changing the diaphragm-piston combo would correct this problem. Was the POA design any better? The POA like the STV only lasted a few years.
To the OP, the late 66 STV did not use a APR. The late 66 STV was fitted with a vacuum actuator pod like the rest of the GM lineup that used the STV. You could locate a STV from 63-66 that has this vacuum pod, remove all the parts from the donor STV body and swap them onto your Corvette STV body. If you do this don't mix the springs, they look alike but according to the parts book are different in some way. I'm guessing maybe spring pressure rating. The 67 P&A 30 parts book I have shows that a 64 Chevrolet STV (5910498) will directly interchange so if you obtain one of those be sure to check the mount bracket hole location and size of the large tube openings. Below photos are from ebay.
POA lasted about 6 years with GM (67-72). Replaced by VIR. STV was very late 63 thru 66 (about 3 years).
POA either worked or it did not. If it stuck or was sticking you could a few tricks to make it work for awhile. POA has adjustment feature for R12 and R134a to optimize each refrigerant. It was not rebuildable by the typical mechanic.
I am not a fan of the STV...........and one that is missing parts makes the task even more challenging to get it to work. But we can try.
Here is a link for auto ac forum. OP may want to spend some time here as well. Good guys, very knowledgeable. But the real old folks that used to help us are gone forever.
POA lasted about 6 years with GM (67-72). Replaced by VIR. STV was very late 63 thru 66 (about 3 years).
POA either worked or it did not. If it stuck or was sticking you could a few tricks to make it work for awhile. POA has adjustment feature for R12 and R134a to optimize each refrigerant. It was not rebuildable by the typical mechanic.
I am not a fan of the STV...........and one that is missing parts makes the task even more challenging to get it to work. But we can try.
Larry
Larry, The STV was introduced with the A6 compressor for 1962. Previously the hot gas bypass valve was used with the A5, was never used with the A6. The POA was phased in to replace the STV which they knew was prone to sticking. I've had my STV's rebuilt and rebuilt again only to gut them and cycle the compressor myself from inside the car, not such a hassle for the amount of use they get. The auto AC website you mention is quite good, but their is no one left that knows about 60's auto A/C controls. I have stock systems in a 63 Impala, 62 Bonneville, 60 Buick, 63 Ford & 68 Chrysler. I am a well educated student of 60's auto air conditioning, but I never knew the STV used in a C2 had a compensator on it! I always say this forum has the smartest car guys in one place anywhere.
Sorry to hear about your A/C issues. If you were down here in FL I would send you to a gentleman named Jon who worked for Frigidaire in the '60's. I met with him yesterday to look over my '66 327/300 A/C system, and in our conversation, he told me that the real Corvette A/C issues were with the big blocks (sorry!).
Anyway, mine is fine, so I was glad to hear that. He had a lot of stories about various Corvettes he worked A/C on at the time. One was the actual Mako Shark; another was a '61 they put a system in when they were developing them back then for the '63.
His two main points to me: The system has many bends and direction changes; this is not good for A/C systems. Corvettes are very small and they had a lot of issues with the design. Next, he brought out a POA valve like they used on later A-Body cars and said that the systems in our cars would be much better had they been designed for that type situation, but it was too late for our cars. He did mention that they begged them NOT to destroy the tooling for the bellows in the POA valve, but they did and that the ones made today were not as reliable as the ones made from the original tooling.
Anyway, he had 100 stores about those days, Corvettes, their A/C system engineers, etc.
Please get a professional to work with you. Based on Jon's points, I don't think it will ever be as good as today's systems, but if it is set up properly like mine evidently is, it will be, as Jon said, "As good as it possibly can be."
Larry, The STV was introduced with the A6 compressor for 1962. Previously the hot gas bypass valve was used with the A5, was never used with the A6. The POA was phased in to replace the STV which they knew was prone to sticking. I've had my STV's rebuilt and rebuilt again only to gut them and cycle the compressor myself from inside the car, not such a hassle for the amount of use they get. The auto AC website you mention is quite good, but their is no one left that knows about 60's auto A/C controls. I have stock systems in a 63 Impala, 62 Bonneville, 60 Buick, 63 Ford & 68 Chrysler. I am a well educated student of 60's auto air conditioning, but I never knew the STV used in a C2 had a compensator on it! I always say this forum has the smartest car guys in one place anywhere.
Dan
Dan:
You always have good information.
But my reference to the STV time period was based on the C2 Corvette only. That is why I said "late 63" when the first AC systems were installed in the 63 model Corvette. I knew STV was used prior for other Makes and Models in the GM line.
But please continue to reply and help me stay "straight" with my information.
The AVR is in the pic to the far right. My STV is broken down as I decide whether to rebuild it or modify it.
The AVR is that the same as the Absolute Vacuum Compensator?
Some years ago all topics about the factory Air Condition on the 1963-67 Corvette was all gathered in 1 topic. Probably a "sticky" Any here other than me that miss the AC sticky?
Any chance to have the factory Air Condition/C60 as a sticky again?
I hope it is okay i post one question in this thread. The 1967 Coorvette with factory A/C do not use a STV valve but a POA valve.
Any know if the bracket for the POA valve on the 67 Corvette sit located different compared with the STV valve bracket the 1963-66?
Thinking of the 2 bolts that fasten the STV - POA valve are located on different place on the outer AC evaporator box/lid. The POA valve bracket will not fit the 2 bolts that sit on the 1963-66 outer AC evaporator box/lid?
I hope it is okay i post one question in this thread. The 1967 Coorvette with factory A/C do not use a STV valve but a POA valve.
Any know if the bracket for the POA valve on the 67 Corvette sit located different compared with the STV valve bracket the 1963-66?
Thinking of the 2 bolts that fasten the STV - POA valve are located on different place on the outer AC evaporator box/lid. The POA valve bracket will not fit the 2 bolts that sit on the 1963-66 outer AC evaporator box/lid?
I have a couple of photos which might help answer your question. The 67 box has an additional bracket welded to it in addition to the weld studs being in a different location.
Thank you for posting this. After reading it, I have a much better understanding of both the STV and the AVR. STV is standard stuff and I know and have worked with these back pressure control valves many (many) times in years gone by. The AVR is a bit unique, but now I understand where GM was going with it. It was a fine tuning addition to the main STV adjustment due to elevation differences. Way more complex than probably needed, since other makes and models from GM did not include it. Probably a poor initial attempt by GM to have their flagship car (Corvette) have all the latest "bells and whistles". Obviously the POA was STV + AVR all in one package.........which was better........but still not perfect.
Hopefully we can get Dutch straightened out, but I agree with Mike G that plugging the AVR attachment hole is not a thing we should do. It simply sets the STV up for problems. And if a refrigerant leak occurs, then the STV diaphragm is ruptured and needs replacement.
Larry
EDIT: Richopp: You need to get your AC friend to get on our forum and provide his knowledge and experience. I would love to talk with him. I am certain others would as well. So, sign him up.
Last edited by Powershift; 07-20-2019 at 01:15 PM.
The AVR is that the same as the Absolute Vacuum Compensator?
Some years ago all topics about the factory Air Condition on the 1963-67 Corvette was all gathered in 1 topic. Probably a "sticky" Any here other than me that miss the AC sticky?
Any chance to have the factory Air Condition/C60 as a sticky again?
AVC /AVR. Compensator. Regulator. I think it all refers to the valve at the bottom of the STV. Might be wrong.
I am in no way an A/C expert. Hell I am not even an A/C novice.All I know is i am having all my A/C stuff rebuilt and gonna try to get it going down the road.
Probably should wait until I have my motor built and in the car. One thing at a time.
Vitaminmopar. Thanks for postin the pictures of the difference og the 1963-66 Corvette and the 1967 Corvette outer avaporator/blower motor boxes
I have a spare outer AC assembly and that is for sure a 1967 outer AC evaporator housing
Powershift thanks for letting me know about the AC FAQ topic Strange i have not seen it when searching for Air Condition threads Just searched the threads an the Air Conditioning 101 thread is not there if that is the thread you meant?
I have been working with Dutch by email and phone this past week to try and get things resolved. Back in Post #18 Mike Geary stated that we might need to "uncover" the STV to AVR port to keep from building pressure on the internal diaphragm as the engine heats up. I reviewed the cutaway drawings and illustrations from the Chevrolet Service Manual that "VitaminMopar" posted up, and agreed with Mike G. I also got Dutch to revise his STV to accomplish this.
Here is his email to me from last night (Sunday July 28):
Larry, you (and Mike G.) were right, by removing the plug at the AVR port. The vent temperature went down to 40 degrees, the low side gauge was reading 20 psi, and the high side was 150 degrees or 225 psi. the RPMs were 1500. Then I turned the adjustment spring many turns, I do not think it will turn any more. The RPMs were 1500, and the low side gauge was 25 psi. I could get 33 psi if I lowered the RPM s. Now I’m getting vent temp in the low 30s without freeze up, it’s a traveling ice box.
There is a very slight freon detection at the STV-AVR connection hole. That is the latest on my STV problem. Dutch
I am posting this to update everyone on where things currently stand. I hope this trend continues for Dutch and his AC works better. Appears that STV diaphragm might also have a slight leak or tear, which will eventually deplete the R12 refrigerant in the system.......and will need repair. Also I do not like the many turns that he has to make to adjust the suction pressure. There may be other issues as well with his STV.
Dutch told me he has two AVR valves as well, but that they will need checking/rebuilding if he uses them. He was told years ago that AVR was not necessary for him, so he removed it. But one way or the other, that internal STV diaphragm cavity does need to be at atmospheric pressure (or an altitude compensated atmospheric pressure) to work correctly.
A big thanks to Mike G. for pointing this out early in the game.
I will continue to update progress if Dutch does not come back and do it himself. I think this is a good learning thread.
07-19-2019, 09:20 AM
And if a refrigerant leak occurs, then the STV diaphragm is ruptured and needs replacement.
