Your memory is fine.

Someone told you the wrong thing, and you remembered it.

I just try to fight truth decay (as Joe calls it), so as not to have wrong information propagate more than necessary on the interwebs, the best wrong-information magnifier in history.

And, yes, I agree with everything else you said.

- Eric

 

Im pretty sure its not the expansion valve that needs to be recalibrated but the poa valve.

 

The POA valve is on the low pressure side of the system. The expansion valve is on the high pressure side and controls the flow of refrigerant into the evaporator. In the newer systems the expansion valve was replaced by the orifice tube or valve. On a factory system they may both need to be recalibrated, but it is the exp. valve that absolutely must be calibrated for the refrigerant used. Aftermarket systems don't have a POA valve.

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Ask any company selling expansion valves if they sell r12 exp valves and 134a exp. valves. Don't think you'll find any. Ask classic auto air if its the exp valve or the poa valve that needs to be recalibrated for 134a. When I worked there it was always the poa and never the exp. And they are considered the authority on Factory auto a/c.

 

And if that was the case, why has there never been anything mentioned about recalibrating the exp valve on the forums until now? Now, recalibrating the poa has many discussions throughout the forums... And come to think of it, the newer systems do use an exp valve. Take a look at the rear units on, say, a chevy Tahoe for example.

 

Let's consider this from a functional perspective.

... And let me preface this by saying that I am NOT an A/C expert, so if anyone has corrections, they are welcome.

Without going into every detail of the principles behind refrigeration, the compressor is pulling refrigerant (hopefully in a gaseous state) from the evaporator, so that it can compress it to the volume of a liquid, so that it will throw off heat and become a liquid as it passes through the condenser, so that it can be sprayed into the evaporator by the expansion valve, where it will vaporize, picking up heat as it does so, and carrying it away (same way that an old fashioned "swamp cooler" does by evaporating water, same as the way that a can of freon gets cold when charging your A/C system).

In the Frigidaire system, the expansion valve is controlled (in a negative feedback loop) through a capillary tube by a temperature sensor that is attached to the outlet, so that if the outlet starts getting too close to freezing, the valve will start to close, reducing the amount of refrigerant sprayed in, and thus reducing the amount of cooling (if allowed to drop below freezing, the evaporator will ice up, blocking all airflow, and no longer cooling the air).

In the Frigidaire system, the amount of suction that the expansion valve sees is controlled by the POA valve. If there is too much suction, more refrigerant will be pulled through the valve than is supposed to, and its ability to provide the right amount of refrigerant for the temperature conditions will be compromised. Too much suction (such as when driving at high RPMs) will make the refrigerant expand and evaporate too quickly, and may also pull some liquid straight through the evaporator without even having a chance to evaporate and absorb heat.
Similarly, too little suction will fail to clear the expanded gas from the evaporator, leading to reduced evaporation and reduced cooling.

Having said all of that, it would seem as though the expansion valve, with its built-in negative feedback system, is already set to meter the amount of refrigerant that will provide the maximum cooling without approaching freezing, regardless of what is passing through it, while the POA valve would need to be adjusted to take the most complete advantage of the very specific vapor pressure of each individual type of refrigerant (293 kPA at freezing for R134 vs 300kPa for R12).

- Eric

 

It's been 25 years since I wrenched at the A/C shop so I may have forgotten a few things, but not that much. When both R12 and R134a systems were available side by side for the same vehicle they definitely did have specific expansion valves. Here is a manufacturer's version of the debate.

Here is a link to the article I quoted it from. Highlight added by me! The "smaller molecule" debate has been cleared up by MDchanic, but the myth lives on.

http://www.rodandcustommagazine.com/...ntage_air.html

 

Its funny how classic auto air wasn't involved in this article...Well I done. CJSDAD FTW. To all those guys that got there exp valves reconditioned from classic from 04 to 06, Im sorry. I didn't recalibrate it for 134a. Please call classic and tell them so they can recalibrate it for you.

 

It appears that this enjoyable 'spirited' discussion has run it's course. I went with the R-12. The compressor, drier and a leaky hose. The hose needed to be remade including new fittings. I had just enough R-12 left over from a batch that I bought in the late '80s when it was announced that production of R-12 would cease. I paid the then rising cost of 4 bucks an 8oz can. I could have replaced the parts myself. 8 years ago I did so for my '71 Buick but I screwed up something and had to take it into a shop. This time I left it to a professional. At 76 I am slowly realizing that there are some things I should not do anymore. Problem with that is I haven't gathered a list of service places that I can trust. At least doing most all of my own mechanical work for 60 years I can see when I am being screwed......except for AC. My AC for many of my early cars was 4 - 5 windows down at 65 mph....or more.


Now to find someone to put my new overhauled 455 into my Buick convertible.




Thanks Guys.
Neil