I have a 1974 Coronet, factory air car, that I installed a new Sanden compressor.

I took it to a local shop that has been in business for decades to have it evacuated and charged. The guy said he put 2.5 pounds of Freon in it (apparently the factory spec) and the pressures were still very low. Pressures were 150/10 on a 92 degree day. He said he'd have preferred to see 275/45. He said the pressures indicated low refrigerant, but he was afraid to put any more Freon in it for fear of damaging something.

That was disappointing.

Have any of you who use a Sanden on an otherwise factory installation experienced this? Should I take it somewhere else, put more Freon in it myself, or do I have more complicated problems?

I have a manifold set, and I know just enough about A/C to be dangerous. I am capable of adding some Freon and watching the pressures as I do, if that is advised.

Thank you.

so apparently pressures are not the best choice to check charge.
I just tried to do mine based on pressures and put in 48 oz in. which everyone told me I was insane.
nothing died, but apparently way over charged.
I haven't had a chance to drain and refill yet but there are other calculations you should use based on temps.
here are a couple of sites I found to explain the better way to check.
most shops aren't going to take this kind of time, or even know that this is how they should do it.
just the machine tells them how much.
https://www.acservicetech.com/post/the-hvac-total-superheat-charging-method-explained#:~:text=Simply%20put%2C%20superheat%20is%20the,vapor%20refrigerant%20at%20the%20evaporator.
https://www.youtube.com/watch?v=pUYLmxOrfo0

The correct way to charge a system is by weight. Assuming you're using R-134 refrigerant in the R-12 system, the charge weight of the R-134 needs to be reduced 80-90% of what the book calls for in R-12. So, according to the charts below, your charge is correct.

Regardless of the pressures, has the discharge temperature at the registers been checked?

If you don't have a freon leak and still have a full charge, those pressures would indicate a faulty compressor. Not at all unusual on a rebuilt compressor if that is what you have.

If the stock system is spec'd for X lbs of R12, and the stock condenser is replaced with a modern high-efficiency condenser, does the R12 charge weight stay the same (all other components being stock)?

It's a brand new Sanden, out of warranty because I bought it over a year ago. Although a year old, this was the first attempt at using it.

I've been reading up on A/C systems (A/C is NOT my strong suit) and was coming to that conclusion myself.

I disagree on the pressures.
mine with 48 oz in it read 275 and 15.
it was blowing cold air. about 20 degrees diff to 30 degree diff between outside and cooled air.
nothing wrong with the compressor there.

but as said above you need to measure temps to verify.
if you have that kind of pressure diff and no cooling, then you might assume a compressor issue, but you can't just say that based on pressures.

If the stock system is spec'd for X lbs of R12, and the stock condenser is replaced with a modern high-efficiency condenser, does the R12 charge weight stay the same (all other components being stock)systemsote]



Once any of the lines, condenser,evaporator are changed, or any part that changes the internal volume of the system, the amount of refrigerant will be different from the sticker. I said in a previous thread that the sticker charge is your starting point not your finished charge amount for this reason. I've had different brands of receiver driers, accumulators, and hoses be all different sizes for the same application.
I charge by superheat on txv systems and by frost line on orifice tube systems. After leak check and before charging, wash the radiator, condenser, transmission cooler pack, make sure there is good airflow through the front of that car. It drives me nuts when someone brings me a car or truck to charge, and I find one of the coolers packed full of dirt.

I should have explained that with 2.5 pounds of Freon and pressures of 150/10 psi, the low pressure line under the hood was slightly higher than ambient temperature, and the high pressure one was extremely hot. No cooling whatsoever.

What compressor and what freon where used? I doubt you went with R12 but you should have posted it in your first post. Sanden makes both and if you used a R12 compressor with R134 the oil is not compatible. I would assume the guy who filled it used the correct freon.

I'm thinking restricted refrigerant flow. Does the drier have a sight glass to view the flow?

One of the problems in this case is that you've changed the compressor, and if that compressor is of a different compressor capacity by a considerable amount, the operating pressures will not align what to what you think they should be. If the compressor is way bigger, it will have lower suction and higher head once X amount of charge is added. If smaller it will have lower head and higher suction than what you would expect with x amount of charge. There are entire books written on the subject of refrigeration system balance. I guess the question is what Sanden model is it? IIRC the RV2 is 10.5 cid .
I am guessing you took off an rv2 which has an EPR built into, so with the Sanden did you put in a low pressure switch? For 134a, a 20 psi and 40 psi will work, this is your anti icing control, low pressure safety control, and your on off capacity control all in one.
Having a hot liquid line can be lack of airflow, but also not enough charge and no sub cooling, as well as way overcharged.. Put a thermometer on the suction line, add some gas in small increments, see if the superheat goes down and the suction pressure goes up. In 74 I would also guess that it has a Txv on it, it's hard to say what the superheat will be when fully charged, because so many of those all were mass produced and I find they range from 5 to 15 degrees superheat depending on the production tolerances and wear. Generally though when you stop seeing a change in superheat when you add gas, the charge is sufficient enough for the valve to gain control, It's a lot easier if your receiver as John mentions, has a sight glass, but the sight glass may only go clear under certain conditions, load, ambient temperature and engine RPM.
My old boss lived by the rule that if a vehicle cycles on the pressure switch with the a/c set on high, the hood closed and the engine running at highway rpm, something is wrong and the car will be coming back with a complaint.

Looks like you have a couple things could be happening. The lines getting hot ; Make sure you have good flow through the condensor. That can cause those lines to be warm and hot to the touch.
Also, someone mentioned the compressor; the compressor is not building up the pressure. At 92 degrees you low should be about 50 and high should be 250. You mentioned that 2.5 lbs of refrigerant was put in.
The system should be cooling good with that amount of refrigerant.

The compressor is 05-128-5, which is appears to be a Classic Auto Air part number. On the Classic Auto Air price list, it is described as "SD5 Rotary Compressor w/ Clutch 134a". It was part of a kit that I bought from Classic Auto Air, which included 440 mounting brackets, hoses and fittings, and a cut-out switch that shuts the compressor off if the low pressure line gets too cold.

The car had already been switched over to a Chinese Sanden knockoff when I received it. The bracket appeared to be home made, and the belt routing was wonky. That's why I bought the kit.

The condenser was already on the car, and I don't know anything about it, other than it is clearly aftermarket and not an original unit. I assumed it was added when the rotary compressor was added. I flushed it with some aerosol AC flush from NAPA. I can't say for sure whether it has a partial blockage, although I was able to blow the remaining flush out of it with compressed air.

Yes, it is charged with 2.5 pounds of R134.

Yes it does. A few minutes ago, I started the engine for the first time today and turned on the AC. For the first few minutes, I had bubbles that were obviously moving very fast. After a few minutes, I could see no flow. Either the bubbles disappeared and I couldn't see anything, or the flow stopped.

If you had a restriction in the line somewhere your high side pressure would steadily rise. You mentioned that you had a guages. I would attach the guage set and take a video of guage readings with the engine running and a/c on.
Post the video here.

Done. Video quality is horrible, but the pressures held steady for over 2 minutes after startup of the compressor.

The pressures aren't exactly what the shop told me. High is 125, low is a slight vacuum.

https://www.youtube.com/watch?v=vIBsXa7BDr0

You have below 0 on your low side. No suction at all. Do you have an expansion valve in the system? That might not be working. Also, low or no refrigerant will do that too.. The high side (red guage) is showing a low refrigerant charge.
Your system is not over charged. I did the see that high and low side pressures were equalized before you turned the system on which is fine. I did see that the low side (suction side) dropped immediately to 0. Could be a bad compressor.
It's not drawing enough suction.

It's for sure undercharged now, and it sounds like it has lea'. It also sounds like it has no low pressure switch in the circuit.

This set up has one pressure switch at the receiver dryer, which is fine. That pressure switch has a little white screw that is recessed in between the connector prongs that can be turned for more or less pressure required to activate the compressor.