1970 AC System

[Juliet P.]

I recently restored the factory air in my '70 convertible. Actually, I disassembled the system, sent the parts out for restoration and then dropped the whole thing off at Tony's Corvette Shop and asked them to get it working again. They recommended & used a new blended refrigerant called "Auto-Frost" which contains a mixture of several components, mimics the R-12 cycle much closer that the R-134 and takes about 75% coolant capacity in weight. The car now blows 48* air out the vents on a 90* day. What a treat!

The problem comes during prolonged usage with the car on MAX (recirculate) and the coldest setting of AC. During the National Corvette Caravan, with 95*+ days and something over 90% humidity, the AC stopped working. When we stopped and I opening up the hood, the cold pipes and hoses had over 1" of frost buildup on them. The thing was so cold it froze up! (I'm not complaining!) Once I realized what happened, I just turned the right dial down about 1/4 off the Coldest setting and it didn't happen again. My question is whether this was a typical thing when the cars were new, in high humidity areas? Or is it possible that there is something else going on?

Thanks, ~Juliet




1970 Corvette Registry

[Tom Freeman]

Re: 1970 AC System

Juliet,
Sounds as if the capacity may be off a bit. But I am no expert in Auto-Frost, but I am willing to bet that someone here will be

By the way, Auto-Frost is 51% HCFC-22. HCFC-22 is the one that requires the use of barrier hoses. Hope they remembered to changed out the hoses with the approprate ones. ( http://www.epa.gov/spdpublc/snap/ref...ubs.html#note5 )

tom...




EPA: Alternatives

[Juliet P.]

Re: 1970 AC System

Thanks for the info. Yes, they (unfortunately) replaced the hoses. My original ones were DOA. Fortunately they had a local shop crimp on new hose and re-used the original fittings. The crimp isn't as pretty as it was, and it definately does not look like the original crimp - an anticipated disappointment. But at least the muffler etc are original. I figure earning 25 points in ops for a working system is better than loosing a few points for hose crip config in the Mech section. My restored original compressor looks great and the orig restored evaporator w/ the sight glass is downright georgous though.

It sounds like looking for good used hoses (which *was* my plan) isn't an option with this coolant. Drats. ~Juliet




1970 Corvette Registry

[Harmon C.]

Re: 1970 AC System

I have had my 73 A/C stop after 150-200 miles due to the frost and just shut it off a few minutes and it goes to working again.I now plan on it happening just to see how it is without A/C. I have an old rebuilt freon system so the autofrost may not be the problem. If this happened when they were new I don't know. Lyle

[Larry Barickman]

Re: 1970 AC System

Juliet.. The system is slightly undercharged. The temperature/pressure relationship on the low side too low causing the temp to be below the frost point so the ice builds on the evaporator. At highway speed the pressure is lower than in city because of the airflow over the condenser. I don't know what pressure it should be with Auto frost but that's the problem. Also that was a great used car dealers trick... Let out some refrigerant so the car has ICE COLD air. Hope you get it fixed.

[Juliet P.]

Re: 1970 AC System

Hi Larry, Long time no speak. That explanation makes perfect sense. I'll call Tony and see if I can get a bit more AutoFrost added to the system. BTW, the clutch is working just dandy on the rebuilt compressor. It's about time huh? Thanks! ~Juliet




1970 Corvette Registry

[Jim T.]

Re: 1970 AC System

Juliet my ole 70 is still blowing cold air on R12 with its almost original system, replaced the dryer in 1988, and I haven't experienced the freeze up that you mentioned that I know of since the car was new.

[Pat Bush]

Re: 1970 AC System

Hi Juliet -

Glad you got her working. You will need to have the shop "feel" their way through the charge. You are a little low on freon as stated by a couple of other folks. As usual, Corvettes have special compressors that are not quite as high as others in compression so a normal charge of 75% might not be quite right to get things working properly. Notwithstanding that, blends do have slightly different pressure differentials that will make a difference in how your expansion and POA valves deal with the freon in-out bound. They may need to put a little more freon in to meet bump pressures to where the original R-12 components react correctly.

Hope this helps -

Pat