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freezing low side hose in stv system

doug#4 on Sat March 26, 2016 12:37 PM User is offlineView users profile

Year: 69
Make: RR
Model: Shadow
Engine Size: 6.23
Refrigerant Type: r12
Ambient Temp: 68
Pressure Low: 34
Pressure High: n/a
Country of Origin: United States

1960's GM STV system freezing up all of a sudden. Please help!

My car has a low side hose that is freezing up from the outlet of the stv valve to the inlet suction side of the compressor. No freezing of evap or line between evap and stv.

A/C vent temps in dashboard: 50 degrees F (on lowest temp stv setting 34 psi)

Ambient temp: 65 degrees F ambient. They used to be 36-37 degrees at the vents, even in very hot weather.

This car has NO high side test port.
Very few bubbles in the sight glass which is clear most of the time.
The freon has stayed in the car for 6 years without leaking. (Used nylog on o-ring fittings.)

The stv is an old style fridgidaire unit like gm used in the years 1965-1966, except instead of a vacuum modulator for user adjustable temperature variation, it uses an electric servo motor. It has 4 positions, roughly corresponding to 32psi, 40 psi 45 psi and 60 psi, which is the design, to allow for different outlet temps for the user in the car. There is no freezing in the line on the 3 highest settings. Only on the lowest. This is the setting I need to cool the car!

My problem is with the coldest setting, which meant to be adjusted to provide a 31-32 psi gauge reading according to the service manual. I have tried adjusting the valve so that it regulates to 34 psi instead of 31-32, and it still freezes up the line between the stv and the compressor, and also raises the vent outlet temps to 55 degrees. The evap does not freeze, nor does the line from the evap to the stv. This system had a brand new Apco Air (new not rebuilt) compressor, stv, and txv (usa built). These parts were installed 6 years ago, and have served well every summer since then, although the car has only driven about 4000 miles and never had trouble till now.

I don't understand why we have freezing even at 34 psi. Could there be a problem with the valve such that the low side test port is seeing the pressure on the side where the evap is, but somehow blocking flow to the compressor? The evap is not freezing. Another thing, the evap should be much colder if it is really 34psi. When the system was rebuilt 6 years ago, the stv was set to spec, which was 31-32 psi, which provided 37 degree vent temps. I don't understand why we have 50 degree vent temps at this same setting today?

Regarding the lack of a high side test port, I may have to hook a thermocouple to the high side to get temp and estimate pressure from that using a t/p chart. IS that ok, or is that not an accurate way to get a pressure reading?

I am thinking of hooking thermocouple to both pipes, suction and discharge, at various points along the lines. Also gonna try adjusting the stv and see how high it has to be adjusted to stop the freezing. But I don't see either of these solving the trouble. Perhaps a new stv is in order?

I also have another question about a/c theory. My question is what is to stop this line from freezing in a normal working system? In other words, what stops the compressor from sucking the heck out of that line BETWEEN the stv and the compressor? Even if the valve works right, I understand how it regulates the evaporator pressure to say, 33psi, but what can regulate the line between the stv valve and the compressor? (Perhaps I don't understand the fluid dynamics properly...I mean, maybe if the evap pressure is set to 33, then the line between the stv and the compressor has to be 33 also? Because the gas is pressing forth through the valve in that line, all the way to the compressor? is that it?

Thanks for any help.

I have tools/equipment/supplies to fix system and continue r12 operation. This car used to freeze me right out of the car in the hottest weather even in standing still traffic.

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