Automotive Air Conditioning Information Forum (Archives)

air-in on Tue August 10, 2010 9:14 PM

Year: 1978
Make: Chevrolet
Model: Corvette
Engine Size: 350
Refrigerant Type: Retrofit R134
Ambient Temp: 108 F !
Pressure Low: 30 psig
Pressure High: 230 psig
Country of Origin: United States

Ok here it goes. Appreciate any help or suggestions:
Friend's 78 Corvette, excellent shape 25,000 miles. AC compressor was making noise and the AC was not blowing cold. With the gauges the pressures seem low and as i tried to add refrigerant the compressor began to make more noise and seem to be possibly self destructing. So we stopped and decided at that point to replace and retrofit to R134.

Work and Parts replaced:
-New Compressor- not remanufactured.
-new drier
-new orifice tube
-system flushed in preparation for R134 retrofit. Did not see alot of shaving or signs of compressor debris.
-PAG 100 Oil
-Sealed system, leak checked and vacuumed for 45 minutes.
-Researched as best i could and came up with R12 charge amount of 48 ounces?????---NOT SURE IF THIS CORRECT- anybody know for sure?
-Figured i would charge to 90% of r12 capacity (.90 x 48 oz = 43 ounces of R134) and monitor pressures.
-Began charge in garage and as expected in Phoenix 108 F temps the water temp quickly rose to about 210 F (not abnormal). Continued to charge using a scale and the high side pressure began to rise very quickly.
-Was able to get about 34 oz of R134 in before i became uncomfortable with the high side pressure- approx 330-350 psig.
-The low side pressure seemed a little high also- approx 50-60 psig if i remember correctly.
-Figured since it was so hot i should get some ram-air and monitor on the highway. Luckily was able to keep the gauges hook up and they reached the interior so i could monitor from the passenger seat as we drove. The duct temps were initially at 80-85 F.
-As we continued a few miles at 65 mph the pressures came down and stabilized at 230 psig for high and 30 psig for low sides. Problem is that the duct temps never got below about 70-75F.
-Figured ok--- maybe we need to continue to persue the target charge of 43 ounces of r134. Was able to get a couple of ounces before the high side began to skyrocket and of course everything was 'hotter than the hubs of hell' (:
-So we aborted and call it a nite.
Give all of that do you guys have any suggestions? I tried to be methodical but am very discouraged. Not sure about charge amount. Second guessing on the decision to convert to R134. Second guessing the whole decision to take on this car.
i am not quite sure where to go from here. I have recieved alot of help from this forum utilizing the search function. I have not been able to locate any previous post that apply.

Thanks Guys,
Aaron
aaronhamel66@gmail.com

iceman2555 on Wed August 11, 2010 1:48 PM

Return the system to R12. Corvettes are notorious for lack of AC performance. Never could understand why one vehicle would work well...and the next one in the shop would make one wish to pull your hair out. They were bad enough in the 'pre cat' days.....adding the cat under the floor board just seemed to make it worse.
Corvettes are true bottom breathers.....they need to be moving down the highway to maintain good air flow. Insure that the fan clutch is truly operational....all all other engine cooling systems are working as they should.
If the decision is made to stay with 134a.....located a more efficient condenser.....prefer a Parallel Flow Hi Efficiency unit......this will aid in some of the encountered pressure problems. But still prefer r12 in this system.
If additional cooling is needed, in the '80 model, L-82 engine, the factory vehicles had an aux cooling fan mounted in the shroud, in front of the factory engine fan. This was a very good arrangement. Had an '80/L-82 with a blower and this fan arrangement really helped in the SFL summers. AC worked ok....but what the heck...spent most of the time with the tops out anyway. Still used the ac to keep your feet cool......the floor boards could get quite warm.
All in all this vehicle will function better with 12.....JMHO!!!

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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

Dave in Texas on Wed August 11, 2010 2:00 PM

I don't know what the 134 prices are in AZ, but here a can goes for $16+ at AutoZone. Look on Phoenix CL.....R12 goes for $18 and up.

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At 2, I went home with a 10. At 10 I woke up with a 2 !!

air-in on Wed August 11, 2010 2:40 PM

Thanks Iceman. I suspected the decision to convert. if I go back to R12 is the PAG-100 compatible with R11? also, I am still not entirely sure of the proper R12 charge amount?
Thanks,
Aaron

mk378 on Wed August 11, 2010 4:14 PM

You'll need to remove all of the PAG. PAG plus R-12 becomes sludge.

iceman2555 on Wed August 11, 2010 5:58 PM

Personal preference is for mineral lubricants...some disagree....just a personal preference.
Checking on the spec's...should have some info in the AM......GM may have some old manuals that offer this info....GM??????????????

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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

Dave in Texas on Wed August 11, 2010 8:05 PM

AC Manual shows the 1979-'82 Vettes using 48oz R12.

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At 2, I went home with a 10. At 10 I woke up with a 2 !!

padgett on Wed August 11, 2010 8:29 PM

Just a thought from an amateur but my 92 TranSport was retrofitted to 134 years ago and has always worked well in central Florida. Recently the "puller" (main) electric fan went out. Pressures went through the roof and the duct air was only about 10F under ambient. Pressure off looked right.

Replaced the fan motor and everything went back to normal.

The Stingray was always hard to cool. Back in the day we used to prop the back of the hood up and the heat would just come boiling out.

In 78 I believe they were still using a mechanical clutched fan. When they fail, it looks and feels like air is moving but not nearly as much as needed. A good fan when hot will make a distinctive howl at 2000 rpm.

What you report sounds like a low charge coupled with not enough air flow through the condenser. At home when I test an a/c I have a big squirrel cage fan used for a central home a/c that is put in front of the car. 220v but was free. It makes a significant difference.

This would also explain why some conversions worked and others did not - it is dependant on that clutched fan for operation at city speeds or idle and is not obvious when it fails.

Just a thought.


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Many Carpets

iceman2555 on Thu August 12, 2010 10:43 AM

Located an old manual last nite....as DaveinTexas says....48 oz....be sure to allow for the amount necessary to precharge your manifold/hoses.
Also missed the PAG 100...for some unknown reason....probably all the 'freon' sniffed over the years....my simple mind interrupted this as a POE....for sure...flush and remove the PAG....the chlorine in 12 will destroy PAGs.

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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