63 A/C STV + Evaporator Question

[Steve G.]

Re: 63 A/C STV + Evaporator Question

I await your research and findings to be posted.

The classic auto air folks ran a test for me and their findings were that the schrader valve did open under pressure without depressing the pin.

It's not hard to do our own test. Try putting air into a tire with a 100 psi air hose without pushing the pin. I'll try it in the shop when I get there.

And the more I think about it, the more puzzling this becomes. Not only is the spring holding the schrader closed, the suction side pressure, which in normal operation doesn't drop below 20 psi is pushing on the bottom of the schrader holding it closed. Additionally, the surface area of the inside of the schrader, by nature of the seat arrangement, is larger than the pin side. So, a spring plus about 20 psi pushing against the larger surface of the schrader opposing only 25-28 pushing against the smaller surface area of the pin and exposed sealing surface and the schrader is supposed to open it?

And doesn't venting through this schrader defeat the purpose of the stv in maintaining evap pressure?

Seems to defy logic.

[Harry S.]

Re: 63 A/C STV + Evaporator Question

Ok, that is a high pressure pipe coming in so the pressure on a real hot day could be 200 plus.

[William H.]

Re: 63 A/C STV + Evaporator Question

Hello All,
Im reading My AC school book " Motors Air Conditioner Service manual" from1972 and the purpose of the STV valve is to keep the Evaporator from Iceing up when there is not much heat load on it. So it is designed to keep the Evaproator pressure at 29.5 psi or above. the bottom of the evaporator has low preasure liquid in it. This will ceartainly be at more than 29.5 lbs since it is a liquid. So it seems that his must be enough pressure differential to unseat the schreader valve. When I look in the book there are lots of different STV and POA valvs shown. Some of them have a schrader valve in this port and some do not. I would guess that the small size of this line and the fact that it has liquid freon or oil in it keeps it from being a problem as far as " Bypassing" the STV valve. This is really a great book you might want to look for a copy of it. Hope this helps.
Bill

[Harry S.]

Re: 63 A/C STV + Evaporator Question

Bill, thanks.

Yes, that is the same conclusion the three of who are working on the problem came to.