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double post sorry freezing low side hose in stv system

doug#4 on Sat March 26, 2016 12:38 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.

Edited: Sat March 26, 2016 at 12:40 PM by doug#4

bohica2xo on Sat March 26, 2016 12:56 PM User is offline

The suction line between the STV & the compressor can ice up in some operating conditions - completely normal. The STV controls Evaporator pressure to prevent freezing. The evaporator pressure should be held as low as practical - just above freezing at the lowest fan setting for maximum cooling capacity.

The Rolls is a parts car, assembled from whatever they could find that year. They do indeed have a high side port someplace. The last two I worked on needed 10+ feet of hoses on the service manifold, with one port in the trunk and the other under the hood.

You say the sight glass is ok, so it has enough refrigerant. Use your thermocouples on the condenser inlet & outlet & see how well it is working.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Edited: Sat March 26, 2016 at 12:58 PM by bohica2xo

mk378 on Sat March 26, 2016 3:04 PM User is offline

When you replaced the TXV, make sure the sensor bulb is properly attached to the side of the evaporator outlet line and insulated from external heat.

You want to run the evaporator in the high 20s psi range for coldest air at the vents. Like Bohica said, freezing after the STV can be normal because further expansion occurs in the valve, causing further cooling of the already cold gas leaving the evaporator. This is harmless as long as the system is not letting liquid reach the compressor. That is the job of the TXV, it limits the flow through the evaporator so complete evaporation always occurs.

doug#4 on Thu March 31, 2016 12:20 PM User is offlineView users profile

OK thanks guys,

I have a couple more questions for you.

You guys say this freezing is ok? Am I in any danger of damaging this compressor with the hose freezing? I mean could it stop the flow of freon and/or oil inside the hose?

Also, is there any possible way that an evap with a pressure of 30 psi could be 50 degrees F at the suction line? I though the freon T/P chart ensured that it had to be much cooler than that?

The suction line never froze before though, and we had 6 years of good service from this unit before this problem started, so I know it was done right. No work was done on the system since then. I am just wondering what could have changed. The car was stored during each winter including this last winter. But the freon seems not to have leaked at all since the sight glass is clear and if it was leaking, it wouldn't have stayed in for 6 years.

I ordered a new STV valve rebuild kit, including a new piston and seal, thinking that the valve might be malfunctioning at the lowest setting. Any time I have ever had a freezing suction line before, I was able to fix it with a stv valve or poa valve. I also thought I'd get a new hi side hose made for it with a port for service. I am tempted to change the txv too, since they are so cheap, but only if I can find an old stock one which is made in usa, like the one I put in 6 years ago.

If this can somehow be corrected without opening the system that would be great, but, it used to produce 37 degree vent temps when the stv was set at 30-32 psi. Now we have 50 degrees or more vent temps at same stv setting even on cool days. I don't understand how this is possible. I thought if it was at 30-32 psi, it had to be cold because of the laws of physics and the nature of the pure r12 freon which is inside. Maybe the stv is somehow denying the low pressure to the evap but allowing it at the spot in the valve where the test port is located. In my experience, the only time I ever got 50 degree vent temps was when the evap pressure was way high. So I though maybe use a thermocouple to test evap temp. or a thermal imaging camera on the evap coils. I will do as you suggested and test with thermocouple at all points in system. Maybe I can just use the thermal imager to make it faster. I don't know if it would be as accurate though especially since you have to set the coefficient for measurement.

If anyone is interested in how this system differs from typical cars of the era I have described it below...thanks for your help

As far as a parts car, the a/c system on this car is made with mostly gm/harrison/fridgidaire parts including the txv, stv, dryer, and a6 compressor. The evaporator and condensor are RR custom made parts with very large evaporator in a metal case with two large blower fans from Smiths of England, which allow for silent blower operation with good cfm, and full speed operation is like a leaf blower worth of air out of the vents. So, you can freeze meat in the car in the summer. Another great thing is that the a/c system is in the middle of the dashboard, where they cut a hole in the firewall 18" wide and 10" high to put the evaporator right in that hole. This is much better than gm cars where the evaporator is shoehorned in sideways, next to the hot exhaust manifold on one side of the car, in a plastic case. So this RR has best a/c of any car I've ever seen. It performs better than new cars, and it's miles ahead of most 1960's cars. Most '60's cars have very cold air but not enough of it to cool the cabin quickly on a very hot day, and usually not large enough vents. So whatever they did with that a/c system in this car was a good job. In the service manual, they state that car mechanics should not service the system, and to call a refrigeration engineer. I thought that was a good attitude and perhaps they used same attitude in the design. The lack of high side service port is a peculiarity which was updated within a couple years after this car was made. Perhaps they assumed that the refrigeration engineer would add one if he needed it. I guess some home systems don't have service ports either.

Edited: Thu March 31, 2016 at 12:23 PM by doug#4

bohica2xo on Sat April 02, 2016 1:48 AM User is offline

When the STV is operating it is possible to have a vacuum at the compressor inlet port under certain conditions. Perfectly normal. The reason an A6 has an oil sump and a positive displacement pump is for conditions like that.

If there is truly no high side port you should install one. There is simply no way to diagnose a system without high side pressures. Has the compressor discharge line been replaced? It was usually on the side of the muffler on the discharge line.

Your sight glass says you have liquid, so the charge level is not the big issue. You may have a condensor issue, but without a high side reading it is only guesswork.

Does your STV look like this:

"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

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