Actually Lumberjack, you are correct. By nature of the beast, 134a DOES operate at higher pressure than R12. But it is not enough to damage components, unless they were already about to go. But sometimes that is the case.

And that does account for the loss of efficency with 134a. It's normally a 5 to 10% loss. By changing switches and controls, you optimize the system for 134a, and it works a bit better. But it works fairly well without changing them. If you switch the restriction device (VIR, expanansion valve, orifice tube) you can gain some efficency there as well.

If you change the restriction device, and change the sensor/switches, you will lose almost nothing in efficency. There will still be a very slight loss due to different size of the evaporator and condenser used with 134a. They are slightly larger by design when the system is designed for 134a.

It is always recommended that you replace the dryer anytime you open the A/C system. But if the system does not have particle contamination, evacuating the system for a longer period of time will remove the gas and moisture contamination. Then you don't need to replace the dryer.

Ecavuating the system involves connection a vacuum pump and pulling all gasses and moisture out of the A/C system.
You know all the moisture is out when you stop pulling vacuum, and the vacuum does not raise more than about 2" of vacuum. If it rises to 0, there is a leak. But if it raises and then stops, there is moisture in the system. Water will boil at 40 deg F if in enough vacuum (pressure less than atmosphere). So it boils and evaporates, the vapor reduces the vacuum, when the vacuum drops enough to stop the evaporation process, the vacuum holds at that point. The more moisture, the more the vacuum is lost before it stops.

If you just replace the R12 and don't open the system, you won't know if there is particle contamination. SO you won't know. Too much will clog the filter in the dryer. You develop a restriction in the dryer, the system won't operate much, if at all. The dryer will be hot at the inlet and cold at the outlet, and you'll know it's clogged.

|UPDATED|5/19/2004 9:22:06 PM (AZT)|/UPDATED|

I have a early 77 corvette,AC system is open and i would like for it to work again.I want to convert over to R134. I have put on new hoses,and a compressor which is ready for R134. What else do i have to change. Do i have to put a new AC VIR? Or can i use something different then the old VIR,as I'm finding those are hard to find,and when i do they cost alot. Will i ruin my new comprssor if i try to use the old VIR.