AC Compressor Issues

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Sep 2, 2012
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Blue Springs, MO
My Car
1971 Mach 1, Grabber Blue w/Argent stripes. Original 2V 351C Auto, Tilt, rear defog, Black Comfortweave Interior. Under restoration. Original colors, 4V 351C, 4-Speed, Spoilers, Magnums, Ram Air. All Ford parts.
I have the original York style compressor on my car. I vacuum tested before re-installing after restoration.

I have an R12 charge on the system, but it was not very cold, so I tried adding some more refrigerant last night. When adding I was monitoring the low side pressure and the sight glass. When monitoring the low side pressure it held around 25 to 30 PSI. Whenever I would open the valve to add freon, the pressure would quickly climb to the 70 to 80 psi range. If I shut the feed hose off, it would settle back down to 30 psi.

What am I doing wrong? Is this a sign of a bad head on the compressor? The AC is colder now, but even with the heat and humidity the suction line does not get frosty on the outside.

Is anyone out there seeing something similar? Should I switch compressors? I never like to see rising pressure on the suction side!



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If you don't have a set of AC gauges, you should buy a set. You're way over the normal low side operating pressure for the system. If you can read the high side, the chart below will help.

As Sheriff noted, you should be charging with the can upright, so the compressor only sees gas.

I very common problem with these systems is the heater water control valve is seized open, or the cable is not actuating the vacuum switch.

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Is your R12 container upright with the hose coming out the top (not inverted)? Are you opening the valve slowly?
Yes, I had the can upright this time to try to get gas only. Yes, I am opening the valve slowly.

Just went for a drive tonight to check it. Of course it got a lot cooler outside during dinner, so It wasn't much of a test. I added the sight glass video to my post.

To answer other questions, yes, I do have a gage set, I just hooked up low side last night trying to top the system off as it was barely cooling. Did I maybe get too much liquid in it? How do I tell? How do I correct.

Blower speed in the interior seems too low too. I need to figure out what is wrong there. It runs in all speeds, but high speed does not move the air I think it should. I would say it moves half the air of my newer cars, which seems wrong.

While here, how much R 12 does it take to fill a system? I have a line on some but at $ 50 a can don’t wanna buy more than I half to
It's normal to see the low side pressure increase while adding refrigerant, and as you described (depending on how far open the valve was), it should settle back down after you close the valve to your R12 supply. The high side liquid goes through an orifice (expansion valve, in the case of our cars) and drops in pressure across that valve. You're adding R12 on the low pressure side, so the R12 supply is increasing the pressure, temporarily. Speeding up the engine should minimize the pressure rise as it will allow the compressor to pull more on the suction side.

As for how much to add, the best way is to get a proper charge is to "weigh in the charge". The system has a specific amount of refrigerant it uses (unless some components were changed and were different volume that original). If you don't have a means to weigh it in, then watching the pressures and temperatures is the next best thing. When you brought the system back online, did you flush it and then add the prescribed amount of oil? That's another pitfall we used to see, on occasion (back when it was more common for DIY A/C charging)....too much oil in the system displacing refrigerant. Be sure to purge any air out of the lines on your gauge set before adding R12.

The gauge sets used by the HVAC folks will have temperature clamps where they'll watch pressure and temperature to determine where in the saturation chart the system is.
OK Team, I did a little more diagnosis today. Outside temp was 100 here in Kansas City, so I figure this would tell me the truth if I was chilling air.

While driving there was very little chill to the air coming out of the vents, so I went home and hooked up the gage set. I was reading about 10 PSI on the low pressure and bouncing between 150 and 200 on the High pressure side. The high pressure side service valve was very hot. I tried to add a little and it seemed to have little effect on the pressures. After adding a little more, the high pressure needle stated oscillating so fast you could barely see it. The vent temperature got no better. When I closed the service valves the static pressures at the gages were 10 PSI low side and 200 PSI High side.

I feel like the I have 1 of 3 problems.

1) A bad or sticking valve in the York compressor. That would be why the needle bounces so much.
2) Too much oil inn the compressor.(I tried to fill it exactly per the directions) Since the high pressure valve is on the bottom, I fill overfilling of oil could cause problems like this.
3) The little brass fitting just before the sight glass. I am unsure how that is supposed to work, but I remember when I was restoring the car, that is the one new part that I could not find. I could not get any air to flow through it. I feel this is working as it is just upstream or the sight glass, and the sight class shows moving fluid/foam when the AC is running.

Should I replace the compressor, or what do you guys suggest. I can find very little information on York Compressor failure modes. Everyone has tons of seal kits for them, but I can find nothing else.



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It's difficult to accurately diagnose AC function without seeing it.

Your readings indicate a low charge and the bubbles in the sight glass agree. It's not unusual for gauges to fluctuate when there is a low charge in the system.

How much oil did you put in the compressor? The York compressor calls for 10 ounces.

I don't think I'd replace the compressor just yet. I think you just need to charge the system until there are no bubbles showing in the sight glass. Your gauges should fluctuate very little when fully charged.
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I tried to get the freon to enter today and it would not take more in. Do I need to increase the RPM to get it to suck more in?
My AC diagnostics are rusty, to say the least, but I will say that it's normal for the high side to be hot...too hot to hold on to. You should be able to feel cold on the evaporator side of the expansion valve. Make sure the bulb at the end of the little copper tube on expansion valve is clamped to the outlet tube coming from the evaporator and should be covered with insulation or putty. I don't remember what the bouncing pressure indicates. Here's the very brief troubleshooting page out of the ford manual (in case you don't have it). 1690608181832.png
Here's the section of the '71 manual regarding the sight glass.

Have you verified that the heater valve in the hot water line is closing and stopping water flow? I blew up the compressor in my first car because it wasn't.

I think I may have the issue identified. I had evacuated the system and had a charge on it with engine #1. If you read my history you realize I lost that engine. So I set the service valves to isolate the compressor and save the charge when I pulled the engine. When I put it back in I did not pump the compressor back down before opening it to the system. So I probably need to replace the dryer tube, pump it down, and charge from a full vacuum.