Need a/c help with my 1966 427
 

My a/c cools at 56 vent degrees, last year it cooled at 43 degrees. If I turn the stv valve adjusting spring clockwise 1/2 turn, the vent temperature goes back down to 43 degrees. The next time I drive the a/c goes back up to 56 degrees. This sequence of temperature change happens over and over. I need some a/c advice. Thanks, Dutch

Dutch I am a novice at air conditioning, from what you describe you will most likely need to put a set of A/C gauges on to know the pressures so the experts can help you. It would be a help also to know the approximate area you live so consider adding that info to your profile, as elevation and ambient temps might come into play. You stated you turned the STV adjustment so I'm thinking you have a stock A/C system and not one equipped with an STV elimination kit. I'm speculating that the evaporator maybe freezing up. Have you noticed if the A/C starts out cooling OK and then warm up? This will be the extent of my so called help. Hope you have a solution to the problem soon.

Reach out to Larry aka Powershift. He is the go to guy on AC systems. Jerry

how do I locate aka powershift, I need some good a/c advice

Correct, without looking at pressures, you are shooting in the wind...get a set of gauges and see what the pressures are, adjusting the STV is fruitless without knowing how the ac pressures are running.

Dutch - go to the search function in the top right and search for threads started by Powershift then open a thread and click on his name and send him a PM


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Phil

DUTCH:

No need to go thru a lot of trouble. I'm right here.

I need for you to tell me where you live and a bit about your car AC. The where you live part is so I can get a feel for temperature and humidity.

I have helped you over the last few years on the NCRS site with various AC questions, etc. If I recall, your car has a STV bypass kit with a variable rheostat on the firewall for temperature control. Is this correct?? If not, what do you have if different from factory stock?? Do you also have AC gage set to read pressures of the compressor suction and discharge??

Let's start with the basics.

Larry

Sorry I just noticed your message. I live near Harrisburg,Pa. My car is a 1966 427 with original factory a/c. Years ago, a local garage changed the original setup to a stv bypass with a thermostat. I changed the system back to original 2 years ago. DOM rebuilt the compressor, and the partsguy rebuilt the stv valve a few years ago. Last year the vent temperature was 40 vent degrees. This year it’s cooling 50 vent degrees. I am having a time trying to get 33 psig on the low gauge by turning the spring adjustment at the bottom of the stv valve. The high side gauge reads 170 degrees. I have a heater shut off valve on the heater hose and I don’t have the vacuum compensator attached to the stv valve, I use a rubber clamp to block the hole where the vacuum compensator fits. We will start with this information. Thanks, Dutch

Dutch:

Sounds like you got your system changed back to factory style and used good talent to do the work. I am unfortunately not that familiar with all the details of the STV valve, like I am for the later POA valve. Your 40 F vent temp(when you get it) is good. The suction pressure of 33 psig and discharge pressure of 170 (or a bit above) are also good, and tell me you have it charged properly. But do you see any bubbling in the receiver/drier sight glass during AC operation?? You could have a slight refrigerant leak causing some cooling problems.

My gut feel is the STV valve is sticking in a partially closed position, causing the suction pressure to go up to around 40-50 psig during operation. That will cause your cabin vent temp to go up to 50-55 F. Is this what you,are seeing?? Make sure you take all your pressure reading at 1500 -2000 RPM and not at idle, since idle speed will automatically increase suction pressure since the AC compressor is running a lot slower.

Is it possible the clamp you have on the STV valve for the vacuum compensator is leaking and causing erratic signal to the STV valve??

I will send Mike Geary a PM asking him to help me on your problem. He doesn't post here as often as before, but he knows the earlier cars with the STV valve very well.

FOR NOW: I would hookup gages, run AC at 1500-1600 RPM, check drier sight glass, and get a set of pressure readings, including noting what outside air temp is. Put a fan in from of the grill to aid in air movement across the condenser. Inside blower speed on HI. Then make a few adjustments on the STV like you have been doing, and see how that affects the suction and discharge pressures. Report back.

The STV has always been problematic (reason for later POA) and if it is leaking or sticking I cannot give you a magic formula to get it working right......except another rebuild. But let's see what your answers are to my questions and if we can get Mike G involved.

Larry

Larry, thanks for helping my situation, I also feel something is going on with the stv valve. I do have a tight clamp at the bottom of the stv valve, and I keep checking it and the system with my leak detector. I used a fan, but I need to turn the stv’s spring adjustment clockwise a couple turns and try to achieve 33 degrees on the low side before I rebuild the valve. Even though I have my 609 certification, I have a older retired heating and cooling technician to help me charge and evacuate my system when need be. He is not an expert on my 66 a/c system, but knows the basics. I have on bubbles. Dutch

Dutch:

Does your AC cooling start out at around 40 F and then slowly drift up to the 50-55 F duct temperatures before you shutoff the engine or AC?? If so, STV may be sticking or not be working, or it is adjusted for too low an evaporator core pressure (temperature) and the evaporator is freezing up with time. In your old system, you had a provision for this on the firewall which you could adjust. Now you have to do it all with the STV.

However, if the duct temps start and stay at the 50-55F range at engine startup and don't change with time, then STV is sticking or is adjusted incorrectly to produce too high an evaporator core pressure (and associated temperature).

This can be seen with AC manifold gages when first starting engine and turning on AC. Watch the suction pressures: do they start out around 50 psig, come down to 30 psig, and then slowly drift back up to 50 psig over time................ or do they just go to 50 psig and stay there??

Let me know.

Larry

Dutch:

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.

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I don't know what else could be the cause, if pressures check out.

Mike

Dutch

Mike:

Dutch says he is missing something he calls the vacuum compensator. I don't have a 66 Service Manual to check out what this does or does not do. I know the 66 STV has a vacuum hose going to it, in addition to one for the hot water valve. What does not having this item do for his system operation??

Does it fine tune the STV set point due to altitude changes??

Larry

Is he missing the absolute vacuum regulator?

Ed
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No idea what that is, but perhaps it is the vacuum head that controls the STV setting as actuated by a vacuum valve on Corvette and full size cars in turn actuated via the temp selector. Pontiac did it with a Bowden cable direct to the STV, no vacuum head. I have two STV systems that I later gutted out of necessity because they invariably would bind and stick. They suck, GM knew it. It lasted only several years. Even Rolls Royce, who used the GM system OEM gutted the STV as a service update.

Dan

Ed:

That's what I need..........the service manual that describes the STV and its components. I can probably find something online if I look, but am hoping Mike G can help Dutch and I out.

The POA replaced the STV in 1967, and it was supposed to be a single component that compensated for altitude changes. That's why I brought it up. It may not be required for normal operation, but then again.................

Stay tuned. :D

Larry

Absolute Vacuum Regulator
 

Yes Dutch indicated that his STV does NOT have the AVR. He has the vacuum port plugged where the AVR would normally connect.

So what the AVR is supposed to do is maintain a constant pressure in the sealed adjustment spring chamber, regardless of altitude.

This part is truly "unobtainium". But unless you change elevation significantly, it's not a factor as far as I know.

Here's a wild-*ss thought about a possible effect of no AVR -- as the car is driven and underhood temperature rises (big block), the spring adjustment chamber (sealed) would become pressurized, thus changing the set point for STV opening. Might explain why cooling falls off on the road.

Mike

Why is the AVR not used on STV's installed on other GM cars?

Dan

Mike:

An STV is simply a back pressure regulator or back pressure controller similar to the ones we use in the chemical plants. I believe all these regulatore/controllers have an atmospheric vent hole for the diaphragm. You may be on to something.........your guesses are typically accurate.

From what I have read, the AVR is in no way interconnected to the refrigerant cycle, so an opening should not release refrigerant. Pics I see of other type GM STV have what looks like an open cage design for this part. Maybe what Dutch needs to do is to open up this opening. Perhaps to answer Dan's question, the engine compartment where STV is located is is more affected by temperature changes on the Corvette than other GM models that use a different design. So opening the port may still have negative consequences for Dutch..........but what else to try??

Dutch says the setting and pressure change with heat and cool-down.............so that may be the answer.

Dutch can also ask this question on the auto ac forum at autoacforum.com. They have some really sharp auto AC moderators and forum regulars that might be able to provide additional input.

But to test this theory, Dutch should try and set STV at 30-33 psig when AC system is cool at startup. Use 1500 RPM and blower on HI as a standard. Check vent temps. Run car and note any changes in vent temp. Then hookup gages and at 1500 RPM and HI blower see what changed at engine compartment heated up. But make no changes to STV.

I guess the hot water valve could also be in play here as well, so Dutch needs to disconnect the vacuum source to the valve and plug the vacuum hose for now. And make certain (verify) the valve remains closed during all our testing/trials.

Larry