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72 corvette AC problems on Sat June 30, 2012 9:23 AM User is offline

Year: 72
Make: Chevy
Model: Corvette
Engine Size: 350
Refrigerant Type: 134
Country of Origin: United States

I recently purchased a fully restored 72 Corvette Coupe with a "new - original" factory air that had been converted to 134. The owner lived in the mountains of NC, I live in Houston. I don't think the system was ever used much. So once I got the car in Texas, I started using the AC.....well it doesn't work.

1st inspection showed that we were not getting proper air-flow cooling through the radiator due to bottom shroud was missing. This was causing the system to overheat and purge freon.

After this was fixed, I saw an immediate improvement in the unit......except for when I started driving. At idle the system seems to work correctly. During testing we would take the idle up to 2500 RPM and still worked great.
My local service shop has done all of the testing with all of the proper equiment and they are telling me the pressures are spot on.

So upon leaving the auto shop, as soon as I get the car running say above 2200 rpm, the system starts to lose it's cooling capability. I can feel the system surging on and off. I feel this surging through the engine draw down when the AC runs vs when it's not running. This surging feels like an on/off cycling about every 10 seconds. If I come to a stop (at normal idle), the system starts cooling properly again.

Any thougths?

iceman2555 on Sat June 30, 2012 4:34 PM User is offlineView users profile

First, I do not understand the need to operate or test an AC system @2200 - 2500 rpm. Seems a bit excessive. Why would the system operate at 2500 in the shop and then fail to operate at 2200 when driving. What events are different other than the engine RPM. If not mistaken this system utilizes a POA/TXV for system controls? Has this been changed? If not, then what is the cause of the 'cycling'. This is a constant run system and should not be 'cycling'. Has a system protection device, HPCO, or other high pressure regulator been installed in the system?. If these additional parts have been installed, and it sounds as if a HPCO has been added to the system, then determine the cause of the excessive pressures during driving conditions. Check the engine cooling system very through, the fan clutch, air flow over and thru the condenser/radiator. All of these could contribute to the possible condition experienced.
Corvettes are not good candidates for retro fitting, the excessive heat maintained by the engine compartment, the heat thru the fire wall/floor boards (problem with 12 also) all contribute to possible cooling performance concerns.
Just finished a friends '77 and it works very well. He decided to convert to a true orifice tube system, so the entire system was changed to accommodate this. A new PFHE condenser was added, a 1980 L-82 OE electric fan assembly, new radiator, fan clutch. The compressor was changed to a Sanden SD7H15, this is a driver and not one that would have to remain 'pure' for show points. Hoses were made to match the system requirements. The system was recharged with 134a and the system works very well. To increase cabin cooling performance, we removed the carpet and installed 'Dyna-Mat' to reduce outside heat intrusion. This reduced the amount of heat thru the floorboard/firewall drastically. This of course took hours of preparation and labor, plus copious amounts of cash. But the system works and works very well.
One of the largest aspects of a successful AC repair is 'short-cuts'. Over time it has been proven that this one aspect is the largest contributor to loss of cooling performance and premature failure of parts. Take your all components function as designed...and your system should function.

Good luck!

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

Edited: Sat June 30, 2012 at 4:35 PM by iceman2555

mk378 on Sat June 30, 2012 4:50 PM User is offline

There would be no HPCO on the stock system, they just pop the relief valve. Has it been charged by weight, or just guesstimation? Did whoever converted remove the original mineral oil? If there is too much oil you get over pressures similar to overcharging with refrigerant.

Edited: Sat June 30, 2012 at 4:51 PM by mk378

bohica2xo on Sun July 01, 2012 2:15 AM User is offline

The term "converted" covers a lot of ground.

A proper 134a conversion is supposed to have an HPCO switch added to prevent blowing the HPRV.

The original compressor on that car would have been an A6, feeding a TXV & controlled by a POA valve. It was not designed to cycle, and had no means to do so.

All sorts of abortions have been used to convert systems like that. If the POA was eliminated during conversion, it will have a cycling switch. It may have both a cycling switch and an HPCO - which means that cycling is possible from either a low evaporator pressure or a high condensor pressure. All kinds of variables.

So please tell us more about the system. What is there, what has been changed & what is missing.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

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