SW Vette

I'm in the process of sorting out and optimizing the factory A/C system on my '65 L76 coupe. A little background information:

When I purchased the car 2 years ago the seller told me he'd had the A/C system recharged with R12 and, as I recall, he said it was previously filled with R134. This is the part I'm unsure of. Since I've owned it, I've occasionally turned the system on just to keep the seals from drying out, but not more than once every few months. Weather around here isn't hot enough to really gauge how efficiently the system was working, but it felt like the air was blowing cool if not cold, and the rpm dropped by at least 200 when the compressor engaged so I assumed there was a good charge of freon.

Fast forwarding to this past July, we drove a 600 mile round trip from LA to Vegas in 100+ degree temps with A/C engaged full time, cruising at 3000-3500 rpm for hours on end. By the time we got back to LA, the air was blowing lukewarm and there was no drop in rpm with compressor engaged. Using my brilliant powers of deduction, I concluded it was time for a recharge.

I brought the car to a local shop that was recommended as one of the few places still working with R12. Hooked up the charging equipment, brought revs up to 2000, and just as system was almost full and air was beginning to really blow cold..... BOOM! A hose blew off the 90 degree fitting at the expansion valve. Here is where the sad part of the story starts :-(

The mechanic proceeded to remove the fitting from the TXV and reattach the hose. Unfortunately the hose was just a little bit short, so he rerouted it under the heater hose instead of over as it ought to be. Charged the system again, but it just didn't feel that cold to me. I drove home with A/C on but had the feeling something wasn't right. After shutting the car down, I heard a hissing sound from the area around the TXV/STV and said..,......"Awwww sheeeeeite!"

Sure enough, next day there was no rpm drop with A/C on and air blew warm. I looked closely at the work the A/C guy had done and didn't like what I saw. I cleaned up all residual oil around the area, ran the A/C again and rechecked it. More oil showed up around the TXV hose connection. Called the shop and mechanic got very defensive; that was when I decided it was time to add A/C system repair to my resume

So I increased my stated value on the policy, recharged the system with propane, checked for leaks with a blowtorch, and hey, problem solved!

.....just checking to see if anybody's still with me!

I acquired a good quality gauge set and 2-stage vacuum pump, bought the required quantity of R12 in cans, replaced and properly rerouted the hose from drier to TXV, and pulled 29" of vacuum for 45 minutes. Vacuum appeared to hold steady for 15 minutes so I precharged one can with engine off, then started it, and as I charged the system with the remaining 4 cans I noticed both gauges fluctuating from low to high in a cyclical pattern and the sight glass going from visible bubbles to clear. This didn't seem normal to me but I put the full 3.25 pounds of R12 in. Air temp at the center vent got down to about 48 degrees but ambient was only 68 or 70. When all the R12 was in I shut the A/C and then the engine off. Disconnecting the gauges, unfortunately I lost some refrigerant before the fitting was fully off.

​After driving around the neighborhood with A/C on, I realized things weren't right. Again I heard a hissing noise after shut down in the vicinity of the TXV/STV. Listening with a length of hose to my ear I decided it wasn't leaking from the 90 degree fitting, but from somewhere around the absolute vacuum regulator. At this point I discharged the little remaining refrigerant and pulled off the STV, suspecting a leaky diaphragm.

After disassembling the STV on the bench, I found no real evidence of a leak in the diaphragm, but have ordered a rebuild kit anyway. Also I've purchased a rebuilt drier from pacecarJeff (thanks Larry ) and an electronic leak detector (Mactools AC5500) on eBay.

My questions for the experts (finally!) are, should I flush the system and completely drain the compressor of oil, then refill it? I'm not sure if there was ever any R134 in the system but all the o-rings are black and the valves are R12 style Schraders. How do I make sure there's no contaminated oil lurking in the evap, condenser, and TXV? What's the best way to drain and refill the compressor oil?

67vetteal

There is plenty of information available in the Stickies section. Also try checking at, ACFORUM.COM . Not too hard to flush system and install all new "O" rings so you know where you stand. Al W.

SW Vette

Thanks Al, but I'm not sure why you needed to quote my OP in its entirety to tell me that. Prior to posting I read all the relevant A/C topics in the stickies section and didn't find answers to my specific questions.

midyear

STV Valves are known to be leakers, on my driver AC cars (66 and earlier) I usually replace them with the upgrade kit that uses a cycling switch and leak problem solved.

SW Vette

Yes, my research and reading of stickies suggests that the update kit is a viable option. There are those who swear by it, however, there are also those who've tried it and claim the original setup works better. All things considered I'm inclined to keep it original. My feeling is that if there are no contaminants in the system and all mating surfaces are clean and flat, there's no reason the STV will leak, for the next few years at least. I believe careless assembly could be the cause of at least a few STV leaks - the preload on that spring is pretty strong.

My main concerns at this point are thoroughly purging all the old oil from the system and refilling with the proper quantity of R12 mineral oil, and testing for leaks prior to pissing away $150 worth of R12 into the atmosphere

Powershift

At this point it might be best to "go thru" your AC system and get things right. You also said you wanted to learn.

I do not know if you need to flush your system, but perhaps you might want to do this on the condenser first and see how it goes. You can buy flushing agents/cleaners from the auto parts store that are for auto AC systems. Then disconnect the condenser in and out and flush it out following the directions. Then blow it dry with nitrogen or dry shop air.

Disconnect the various hoses, etc and replace the o-rings with the green HNBR type. Use some refrigerant oil on the new o-rings. For R12, this would be R12 type mineral oil. NAPA or others will carry it or can get it. 525 SUS viscosity.

Remove compressor and drain it of all oil. The Chevrolet Service Manual Section 1A has a procedure for this. The A6 compressor also has a drain valve on the lower side of the main compressor case you can open as well. You can check to see if this oil is mineral oil (R!2) or Ester (R12 or R134a) or PAG (R134a only). See if it mixes with the mineral oil you buy to try and determine the type. Refill compressor with correct new oil and reinstall. Takes 6 oz.

Blow out the evaporator with nitrogen or dry shop air. Nitrogen preferred, but not everyone has this available. I do not think you would need to flush this item............but you could if you wanted, and you had good success with the condenser.

Rebuild/repair the STV and reinstall.

Install the OLD receiver drier at this point.

While you are at it, check the inlet to the TXV valve (expansion valve) to make sure the inlet screen is free of debris.

Now pull a deep vacuum on the system for a few hours to remove moisture and see if the vacuum holds and you have no leaks. Break vacuum. Then pressure up the system to about 50 psig with some cheap refrigerant (R134a) and check for leaks with soap solution and your new leak detector. Check especially in the cabin for an evaporator leak, check the compressor for a front seal leak, and check all surrounding engine areas and all connections for any other leaks. Depressure and repair as needed. Do a final pressure check to ensure no leaks.

Then vent down the system, replace the drier with the new one, add remaining oil ( 11 oz - 6 oz you added to compressor = 5 oz remaining to be added) into the inlet of the condenser. Seal things up and pull a deep vacuum for a few hours to again remove any moisture and air.

Now you have a clean system, with a new receiver/drier, correct type and quantity of oil, and all new o-rings (and hoses??). You are ready to add your refrigerant. Add first can with engine off. Add remaining cans with engine running and AC on at 2000 RPM and blower on High speed.

If any doubt about the compressor seal, fix it when doing all this other work. Or send it to Dom to do. You do not want compressor seal issues at this time if you can avoid it.

As I said many times, you really need to know how to do this AC work yourself and have the needed tools if your car is an AC car. Doing this work now will help educate you on your AC system and how to keep it running in the future. I can assist as needed. Let me know.

Larry

63 340HP

Your leak allowed air to enter the system, but the limited operation is unlikely to contaminate the oil or form acids in the system. An oil flush and new dryer are always safe, but with the failures as described the vacuum will boil out any moisture from the existing oil.

Original STV seals were designed for mineral oil and R12. Make sure the new STV seals are compatible with the Synthetic POE oil needed for R134a (because you don't know the current oil type). Modern R134a/POE seals are backward compatible with R12/mineral oil, but the original R12 seals are not forward compatible with R134a/POE oil.

When charging with R12, only add refrigerant until the site glass bubbles disappear. Test it is fully charged at max AC, by cooling off the condenser with water and verifying no bubbles return to the site glass (if they do, add a little R12). Overcharging the system results in excess refrigerant in the condenser, reducing the internal surface area available to shed heat and condense the refrigerant gas, and the excess reduces the effective heat transfer capacity (cooling capacity is better when a little short on R12, better than with excess R12).

R134a cannot be charged with a site glass, and must be weighed in, because some of the component gas compounds have lower partial pressures and will flash bubbles at a proper charge (R12 is not a mixture of compounds and has consistent phase change properties).

SW Vette

Larry,

Thank you for the comprehensive reply. I was really hoping to hear from you

I've realized that what you've said many times is absolutely true; anybody who owns one of these A/C cars needs to become knowledgeable of the components and their functions, proficient in the service procedures, and equipped with all the specialty tools required. Otherwise you are at the mercy of dubious characters whose expertise is questionable at best. The time and money invested will be repaid many times over with the satisfaction of knowing how to make a vintage R12 A/C system blow cold and work reliably. I'm only at the beginning of that journey.

Just wondering, if I use my old drier, won't there be old residual oil in it as well? Should I leave it connected to the condenser and flush them together as a unit?

I do have nitrogen to blow through after flushing.

68hemi

With all of the suggestions of flushing and purging with dry air or nitrogen you need to vacuum evacuate the system for 24 hours. Moister in these systems is measured in PPM. I would NEVER put an old dryer after all of this as it likely has reached its limit of usefulness.
I worked for a company that began putting after market A/C in cars in 1957. They were hanging Frig A King under dash units in new Cadillacs that the factory system did not work well enough in AZ. This company also did mid an low temp truck cooling units as well a home and industrial cooling. I learned a lot from the man that started the business, Guy Isley in Mesa, AZ.

SW Vette

63 340HP,

Thanks for the response - you're another of the knowledgeable folks I was hoping for input from.

If I read it correctly, you're suggesting a complete flush and drier replacement may not be necessary? My fear is that, without knowing for sure if there was ever any R134 PAG oil in the system, but seeing old black o-rings and original style Schrader valves, I have no way of knowing what type of oil - or how much - is currently in there. I understand that any o-rings I remove should be replaced with the backward-compatible green ones. If I didn't flush the system I'd need to use the ester-based oil in case there's still some PAG oil there. My preference would be to use R12 mineral oil as I understand it's less hygroscopic than the alternative. In that case I need to be sure there's no traces of PAG oil remaining, thus a thorough drain, flush, and refill.

I'd like to reuse my old drier, but it seems foolhardy to leave a questionable component in an otherwise fresh system.

Randy G.

I can't add to anything said here except to say I had forum member Dom (Dominic) rebuild the original A/C compressor on my '67 then I had the A/C shop convert everything back to R12. They claimed the drier was fine. Now, on a 95+ degree day, the A/C blows very cold and it's a pleasure to drive with no issues.

Good luck in your pusuit to fix it correctly.

63 340HP

The concern is the unknown oil. PAG oil is R134a specific, and not a good lubricant with R12 (it does not suspend well in R12). POE oil is what most new compressors are delivered with, because POE (ester based) oil is compatible with R134a & R12 (as Powershift mentioned), but absorbs moisture quicker (as you note, more hygroscopic). Mineral oil is preferred with R12, as it is more widely compatible with old seals and new seals and more miscible in R12.

A drain & flush with POE oil, and fill with mineral oil would be more than adequate, with a good long term vacuum test.

A deep vacuum pulls the moisture out of all internal parts of the system, including any residual moisture in the desiccant dryer. The concern is that the desiccant does not easily diffuse, or lose, moisture. To be assured of a dry system with an old desiccant dryer it needs to hold the vacuum for a long period to diffuse the moisture and assure it's dry (it's not ideal for a quick shop turnaround). The system will lose vacuum as moisture is boiled out of the desiccant, because water vapor demands a greater displacement volume than liquid water trapped in the desiccant. If the vacuum does not change overnight, 12 hours or so, it's dried out.

A new receiver dryer also has physical particulate filters that are clean, and acid neutralizers, safeguards on a new installation. If the system showed no concerns of debris, moisture, or acid, then these benefits of a new dryer are just insurance.

dplotkin

After reading the aforementioned posts and based on my experience with 5 factory air equipped mid century classics I offer the following sincere, heartfelt advice:

Refrigeration systems are not hit or miss propositions. They must be tight, full of oil, properly charged and free of air and moisture, the compressor must pump, and the regulation (hot gas bypass, STV, POA, clutch cycling) must work correctly. If you are unsure about any of this, my best advice is to start over. At a minimum the compressor should come off and its oil contents drained and quantity recorded. Same with the dryer. drain it, record it, and replace (or have new desiccant installed) it. Flush the system and put it back together with new oil in the quantity removed. An original or even rebuilt STV will leak and/or hang up. There is a reason STV was only around from 62-64...65 in some applications and gave way to pilot orifice absolute (POA). Its because STV sucks.

The Harrison STV/A6 system will cool really well on R134 if the STV is gutted and allowed to run wide open with a t-stat cycling the clutch. I have that system on a 63 Impala and a 62 Bonneville same as your Corvette except for details. If you have an STV, pull it out and install the kit. I find the cycling bandwidth on the T-stat in the kit too wide. So I jumped it out and cycle the compressor myself every 15 minutes or so to keep the evaporator clear of frost.

Dan

Powershift

Drier must be replaced if you open the system for major work. PERIOD.

I recommended using the old drier just to allow you to get the system back together and pressure tested. When you achieve that, replace it with new before final vacuum is pulled. That is what I recommended above.

The drier is a can with a dip tube and a desiccant bag. The desiccant can hold about 1 tablespoon or so of water. After that it is exhausted and no longer effective. With time the zeolite desiccant can begin to wear and produce dust. The bag can also come apart with age. Both are bad news, and the fine particles will plug off the TXV (expansion valve) screen. Do not try and reuse it without having it completely rebuilt with new desiccant.

In the old days (1960's) I knew guys that would remove the old drier, cap off the in and out connections back to the system, and then bake the drier in an oven at 300 F to dry up the desiccant. Then they would reuse the drier. But if they were wrong, R12 was less than$1 per can. Now days, the risk is too great in my opinion to try and reuse the drier and hope for the best. Dan and 64 340hp and Randy and 68hemi all have great suggestions for you............and they are not in opposition to anything I have told you so far............ so you can read thru all our posts and pick the best path for yourself. We will help you as needed or as you request. These guys have excellent suggestions for you.

PS: Just a bit of 1960's trivia for AC auto systems: The same guys that baked their driers also recharged their car AC system without pulling a vacuum. The simply blow enough R12 thru the entire system to the atmosphere to displace all the air that was there. True stories. Again, R12 was very cheap............and we had access to it at the chemical plant for FREE.

Larry

EDIT: The COMMERCIAL AC guys (not us Corvette AC amateurs ) use a MICRON Gage to measure vacuum on the system to see if all air and moisture has been removed. They pull down to 500 microns or below total pressure (29.98 IN HG vacuum) which will ensure 1. no leaks 2. no air 3. no moisture. I have one of these gages, but almost no one uses it for automotive work. But it can be very effective if you can achieve the 500 or less magic number.

SW Vette

Thanks, Larry. To be clear, I was inquiring as to whether the old drier should be flushed along with the condenser prior to pressure testing, not whether it could be flushed and then reused. Obviously I'd like to keep the new drier fresh and free of old oil or any other contaminants circulating around during the flush.

I am still unsure of the exact steps or best procedure to follow in flushing the system....my research continues

Powershift

I would simply isolate the condenser and flush and dry it. If you feel good (lucky ) about how it went, you can isolate and flush and dry the evaporator. But the condenser would in my opinion be the one to do first. You can blow out the hoses and evaporator with 50 psig or so nitrogen and call it good. DO NOT try to flush the entire system as a complete system. Do it by individual component............and only do what you feel necessary.

Larry

SW Vette

It's time for an update to this topic. First, a big thank you to all who responded. As Larry suggested, I took all the information on board, did some research on the interwebs, and chose a path forward that seemed the most appropriate for my set of circumstances.

I must say, anyone who thinks that the original factory A/C system is marginal or inadequate has probably never experienced the performance of a properly sorted OEM setup. Once everything is set and adjusted to factory specs you could hang meat in the cockpit, which may sound obscene, but you get the idea

I decided to keep the STV as original. After tearing it down and closely examining all the components, I saw no significant wear on any of the moving parts. The adjusting screw, however, was frozen and there was some crusty buildup in and around some of the orifices. It looked like the valve had not been functioning properly for quite awhile. Then I looked more closely at the diaphragm which at first appeared ok, and found an area that let daylight through:



Sure enough, it had been leaking.

Some might say aha! That's why you should update and eliminate the leaky sumb!tch! But I'm the stubborn type, and tend to believe that if the rebuild is done by the book, there's no reason this STV shouldn't last another 10 years under the mild conditions it's likely to see as a garaged and pampered show pony.

That said, having done the job once - remove the valve, fully rebuild and reinstall the unit - then sleeping on it and deciding there was one step I was unsure if I'd done 100% correctly, so once again removing, totally rebuilding, and reinstalling the valve, I will admit there are numerous ways for the job to go wrong and end up with either a non-functional STV or a rapid failure. The 1965 supplement to the factory shop manual has an excellent step by step procedure that needs to be followed exactly if there's to be any chance of a good result. I can see why many folks would rather not take that chance and prefer to simply do the update instead. Time will tell

To summarize, I followed Powershift's advice pretty much to the letter (thanks again Larry ), blowing everything out with nitrogen, making sure the TXV was free and clean, replacing all o-rings, removing, draining and refilling compressor with mineral oil, replacing the drier with a unit from pacecarJeff, pulling a deep vacuum, and recharging the system with the liquid gold until no bubbles appeared in the sight glass. I followed all the test procedures as outlined in the factory manual, readjusted the STV exactly to spec, and voila! Now, if I aim the vent at my face and turn the blower up to high, icicles will form in my nose

I have no way of knowing how long it will remain this good, but for now, I'd say it was well worth the effort for the feeling of satisfaction from achieving maximum refrigeration. Something like making a fire from rubbing two sticks together Id imagine ​​​​​​​

68hemi

Glad to hear you got it done with good results. One other thing people should be aware with factory A/C cars is that there is a high speed blower relay in this system that if it is bad they will not blow as much volume as they should.

SW Vette

And also, the blower fan fuse needs to be at least 20 amps (some references even call for 30), not the 10 amp fuse used for a non-A/C application. A 10A fuse will fail every time #3 fan speed is selected with A/C on.

dplotkin

Good for you. I have five 60s cars with Factory air systems. These systems have significant over capacity, were designed to deal with leaking air from outside and Sun load through a lot of glass.

When they are working properly they can cool a small home, seriously, they probably have almost a ton and a half of cooling capacity.

the weak Link in your system will be that STV valve, if you're lucky and it holds up great.

Dan