63 A/C STV + Evaporator Question

[Harry S.]

[Harry S.]

Re: 63 A/C STV + Evaporator Question

I can understand why this would be hard to respond to.

Does anyone have an old bad evaporator? If yes, can you cut that small tube off and send it to me to run a test. I have my STV out of the car. I'll take care of shipping. I'll post the results after the test.

Thanks

[Steve G.]

Re: 63 A/C STV + Evaporator Question

I will look for some reference material today, but I believe the port with the schrader valve is the test port with no line connected to it. I can't imagine a reason for a schrader valve in a port with a line connected to it. You would use such an arrangement if the engineers intended the line to be removed without losing system charge. But in this case you would still lose charge out of the tube you disconnected.

[Harry S.]

Re: 63 A/C STV + Evaporator Question

Steve, as I understand the port is to transfer excess oil that has collected at the bottom of the evaporator. The transfer puts the oil back into the system. The transfer happens by built up pressure in the evaporator.

[Steve G.]

Re: 63 A/C STV + Evaporator Question

I'm sure that's right, but I'm sure it doesn't do it through that port. On a 69, which I believe is a POA system, there is a similar line arrangement. One from the evap and one from the expansion valve and a third nipple with a schrader valve that is capped and used only as a test port.

There is no logical reason for there to be a schrader valve in an oil return line. It is not a one way valve. It requires, as you are aware, a pin in the line connected to it (just like those on your gauge hoses) to open it. Once the line is connected it remains open. It will close again once the line is removed. There's no point in closing the valve after the oil return line is removed because refrigerant is escaping from the line you just took off.

[Harry S.]

Re: 63 A/C STV + Evaporator Question

I tend to agree with you. But the shop manual shows a schrader valve and all the STV and poa valves I have seen have one installed. I think you and I agree what the line is for. The question at the end of the day is, how does it physically function?

[Domenic T.]

Re: 63 A/C STV + Evaporator Question

Harry,
I have a 63 system on the shelf and will take a look today to see if there is any input I can give.

Dom

[Edward J.]

Re: 63 A/C STV + Evaporator Question

Harry that schrader valve may just be there to equalize the pressure in the system when the system is shut down or car is turned off ?

[Domenic T.]

Re: 63 A/C STV + Evaporator Question

Harry,
Not much help from me but my schrader valve is pointing downward on a tobe about 4 1/2 " long and has a cap on it. Just above it is the equalizer line. I looked in my 63 service manual and found nothing on A/C. My guess is that it is a service port. I am missing my 64 books that may cover servicing. My 65 book shows the valve and so far there is no name for the 4 1/2 " tube with the schrader valve and cap.
I will look a little further and if I get lucky I will post.

Dom

[Steve G.]

Re: 63 A/C STV + Evaporator Question

I tend to agree with you. But the shop manual shows a schrader valve and all the STV and poa valves I have seen have one installed. I think you and I agree what the line is for. The question at the end of the day is, how does it physically function?

Not sure what part you want to know about physical function, but an example of a schrader valve the valve in your tire valve stem and performs exactly the same function. It allows a hose to be connected and removed without losing the refrigerant. It is in the service port and the fitting on the hose of your refrigerant gauges has a pin in the centre that depresses the schrader valve when the fitting is tightened onto the port.

63-66CorvetteSTVAfter.jpg
The schrader valve is inside my crudely drawn green circle. That is the service port. The line from your evap goes to either that line shown attached or the port right below it with the blue plug in it.

If you're asking how a STV physically functions, I can't help with that. It's principle of operation is to regulate evap pressure by controlling suction from the compressor. It is used in systems where the comp does not cycle, but rather runs constantly.

I actually think that the line from the evap is more about sensing pressure than returning oil, but it's been a long time.

[Harry S.]

Re: 63 A/C STV + Evaporator Question

After some discussion yesterday it is not a service port. It is the oil bleed line from the evaporator. When oil collects at the bottom of the evaporator the pressure created by the running system forces by pressure the schrader valve to open thus allow that collected oil to get back into the system. The spring in the schrader valve keeps it closed and depressing the pin opens the valve. Except with the enormous pressure created in the a/c system, the pressure alone opens the schrader valve.

also, the POA functions the same way. Classic Autoair has a write up on their website.

mystery solved.

[Steve G.]

Re: 63 A/C STV + Evaporator Question

I'll have to look that up. That's a very different use of a schrader valve. The stv maintains a relatively low pressure of about 25-28 psi in the evap and the surface area of the pin is so tiny you would wonder how it can open it against the spring. Oil in the evap doesn't raise the pressure, so how does the schrader differentiate between the oil and the gas pressure? I know there is no schrader in the evap line on a POA, I just had one apart.

Very curious. I'm going to have to dig my old ac books up.

[Harry S.]

Re: 63 A/C STV + Evaporator Question

The schrader is essentially a check-valve with a pin and spring installed. As I understood yesterday the spring is not strong enough to hold the valve closed under the high side pressure coming in from the evaporator. It just opens like any check-valve.

[Steve G.]

Re: 63 A/C STV + Evaporator Question

The schrader is essentially a check-valve with a pin and spring installed. As I understood yesterday the spring is not strong enough to hold the valve closed under the high side pressure coming in from the evaporator. It just opens like any check-valve.

There is no high side pressure in the evap. High side pressure stops at the expansion valve. There is only ever 25 to 28 psi in the evap. Divide that pressure down by the square area of the head of the pin and you have a very tiny push on the head of the pin. Also, how does the pin differentiate from the gas pressure and oil pressure?

Schrader valves are never used as pressure regulating devices. Typical relieve valves use a ball and spring arrangement or a spool where there is a larger surface area for the pressure to act upon. I've only ever seen schrader valves in situations where there is a physical contact to the pin to open the valve.

[Harry S.]

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.