Nicola

Greetings,
This system has the original STV set up with R12.
System has been low on freon for quite some time but the compressor will cycle.
Low side pressure is 100 psi plus or minus.
High side is about 150.
System blows cool but not cold.
Pressures should be Low 30 and high side about 300.

90 degree day.

Any ideas???

Mike Geary

any bubbles visible in the sight glass?

Nicola

Yes a few....
When we charged the system, not enough pressure in the cylinder for the freon to flow.
Guess we could have squeezed a bit more freon with A/C off.
Pressures are so out of whack did not bother at this point.
What are you thinking?
Nic

midyear

expansion valve may be stuck open

Powershift

A couple of things for starters:

1. If this is an original system, the compressor is always on, and does not cycle. If yours cycles, something is changed.

2. If the discharge pressure hits 300 psig and you live in Canada, you have other issues. Maybe in Nevada in the summer on days........

High low side and low high side pressures indicate either:

1. STV valve is stuck closed (most likely cause)

2. Compressor is on its last legs (probably not).

Not certain how best to proceed at this point. When I had my 1967 POA valve stick closed I was able to coax it open by running a small pressurized can of R12 oil slowly thru the valve. I created enough turbulence and lubrication to get the POA to respond. I would check around the STV for any loose hoses or other issues and also check the compressor for unusual noise or heat buildup. You may be able to run the system for awhile and the STV will come around. Otherwise you are probably looking at pulling it and having it repaired.

Mike G may have some other ideas to try.

Larry

Powershift

Also, some 1966 cars may have Schrader valves on both the suction and discharge block at the compressor rather than just one on the discharge (like 1967 cars). Which configuration do you have?? If you have one on the suction at the compressor we can better evaluate the STV operation rather than guess.

Let me know.

Larry

Nicola

Thanks for your time.
There is a shrader valve on the outboard hose on the compressor and one at the STV valve.

Powershift

Is their ONE or TWO Schrader valves directly at the back of the compressor??

I need one valve on the suction line at the compressor and one at the STV valve to do a better job of evaluating STV performance/sticking. Is that what you are saying, or is there only ONE Schrader valve on the back of the compressor, and it is on the discharge line (the side with the muffler)??

Larry

Nicola

Hi Larry,
Can't remember but only one valve at the outboard line on the compressor.
Sorry car is at the shop can't remember right now but can check tomorrow.

Nicola

There is a pic of the engine bay in my profile.
Don't know if that helps at this point.

Powershift

Nick:

I just went out to my car and looked. Outboard side on my 1967 car (and your 1966 car as well) is the discharge hose. That is what I thought. We need one on the suction line at the compressor as well to be able to check the pressure drop across the STV valve.

So I guess we will just have to guess on the STV operation.

Larry

EDIT: If the STV valve is running stuck closed, the valve should have frost and possible icing when the AC compressor is running. You can check this.

Nicola

Hi Larry,
Anyway to possible free up the STV valve?

Powershift

Re-read my Post #5.

The STV (unlike POA) is adjustable without removing it from the system. I would suggest getting and reading the 1966 Chevrolet Service Manual regarding STV operation and adjustments. There may be a few things you can try. I am not that knowledgeable regarding STV versus the later POA, so I can't provide you any more specific recommendations regarding adjustments. The service manual may help. If I had one (66 manual) I would try and provide you the section/page numbers.

Time, or a few taps with a small hammer, may help it to free up. You can also try what I did with my POA with the pressurized oil. Just be very careful when doing this to not slug the compressor with liquid oil. A little bit at a time.

Although it is probably the STV that is at fault, the compressor could still be an issue. You need to see if the STV frosts or ices when in operation, indicating a significant internal restriction when the refrigerant expands across the internal orifice/valve trim.

If we had the additional suction line Schrader valve we could confirm it one way or the other, but Chevrolet didn't give us one. It would have made things easier for everyone that has these AC cars. I believe that the 63-64-65-early 66 AC cars had this extra Schrader. Late-66 and 67 cars did not.

Larry

Nicola

Hi Larry
So if the valve does not freeze up the issue here is the compressor?
How do you check for a faulty compressor?
Nic

Powershift

Frost up or ice on the outside of the valve.............not necessarily "freeze up".

No good way that I know.....unless we have that extra Schrader connection. If you feel that it is bad, then pull it and have it rebuilt. Dom (C2 forum member from California) is the best in the business for this work.

Hopefully it is not the A6 compressor. But if needed, I can provide Dom's contact info.

Until then, I would concentrate on the STV.

Larry

Mike Geary

I may have missed this from earlier posts, but..

?Did this system, with these components (A6, STV, TXV) work correctly before?

As Midyear stated, an expansion valve (TXV) that is stuck open will result in high suction and head pressures, since it wouldn't be doing its function of dropping the liquid pressure going into the evaporator.

I would think that the TXV is more likely to be stuck than the STV.

Your head pressure at 90* should be closer to 220psi.

Powershift

Mike:

Glad you decided to "join in" on this. It is always good to get multiple perspectives.....especially from someone with your AC experience.

I would think that if the problem was the TXV (expansion valve) that the cabin conditioned air would be very cold, not just cool like Nicola says....since max refrigerant flow would always be occurring. I would also think the compressor head pressure would be higher than 150 psig on a 90 degree day.

Any suggestions to try and narrow the problem down for Nicola?? Things he can try?? If TXV is stuck open, the only thing I know is to pull it out and test it. A lot of folks have had problems with the TXV reproduction replacement.

Larry

Mike Geary

Larry:

I'm going by memory here, but the TXV orifice needs to be closed down to significantly reduce the volume of R12 entering the evaporator. This restriction reduces the pressure, and results in a mixture of LP vapor and LP R12 which boils off in the evaporator --absorbing heat from the load in the process.

So with the TXV stuck open, Nicola is not getting the necessary "boil-off" in the evaporator. Marginal cooling and high suction pressure seem to fit the scenario.

Just my shadetree HVAC thoughts.

MIke

Powershift

An EVERCO Troubleshootng Guide I have for auto AC systems indicates that a failed open TXV will have both HI suction and HI discharge pressures, with warm cabin air and a clear sight glass. Although 100 psig suction pressure is definitely high, I don't think 150 psig discharge pressure on a 90 degree day fits this definition. But you and MIDYEAR may be right.

For a stuck closed STV valve, the guide indicates suction pressure will be high, discharge pressure will be low, cabin air will be warm/slightly cool, and the drier sight glass will be clear. I felt the 150 psig discharge pressure would be considered LO for a 90 degree day, where, as you said, it should be over 200 psig. This guide also indicates a compressor problem for these same conditions (HI suction and LO discharge). When my POA valve was sticking/stuck closed, I saw around 60 psig on the suction side and maybe 190 psig on the discharge. This was on a warm Louisiana day.

I think Nick needs to answer your question in Post #2 regarding any bubbles in the drier sight glass when the AC system is running. I'm wondering if he could of had a system that was VERY VERY low on refrigerant and he introduced a lot of air during attempts to re-charge.

I guess the next move is his if he still needs help. There are a number of questions to be answered before we can go any further.

Larry

Nicola

Hello gents,
System was low on freon for quite some time.
I never really used the a/c on the car but I did run it from time to time.
Had an issue with vacume valve so I replaced it with "Chinese junk" as the brass valve was not to be had. I was able to get my hands on some reclaimed R12 in a cylinder.
So everything regarding operation is working fine except for cold A/C.
Before top up the the system was blowing cool with bubbles in the sight glass.
Tried to charges system but was stuck for time.
Charged the system to the point where no able to get anymore freon in the system due to 90 low side pressure and 150 on the high side.
Sight glass still gas bubbles.
Tried tapping the STV with a hammer to hopefully get the flow working properly so we could finish charging the system.
Made sure the hoses were purged so no air got in the system.
Ambient temp was 85 to 90 with high humidity.
Don't think I have forgotten anything...
I could actually put the r12 cylinder in hot water to increase tank pressure to force more R12.
Don't know if that would straighten things out?
I read about STV adjustment.
I know these compressors always leak at the front seal over time.
System did blow ice cubes years ago.
I will await the verdict here.
Thanks again to all.