A/C System line pressure?

[69 Rustang]

I am trying to get the A/C system operable again in my 73 Mustang. During my restoration of this car I have replaced all of the o-rings, the drier, and had the dash apart for a heater core replacement. When I bought the car 1 1/2 years ago the A/C system did function although barely blew cold. I had only tried it once and can't say that it was in good or great condition before because I really don't know. When I had it discharged 7 months ago it only had 0.5 pounds of freon (I estimate that it holds 2 lbs).

Yesterday I took it to have it charged at the shop that discharged the system. After keeping my car all day, they told me that there is a problem. They said the system cycled (compressor) on an off fine at first but then immediately went to cycling on an off immediately building up 300psi at the guage. They told me there was a "blockage in the system or lines" and that the compressor may be to blame as it could be worn out and pushing metal shavings into the lines. I had them discharge it and brought it home.

Today I disconnected the lines from the compressor and shot a little air through--air goes all the way through the system and my understanding is that air molecules are larger than R12 molecules so I think I would have found a blockage if there was a constant source of blockage. I did not do any testing or blow anything through the compressor. I did notice a small filter/screen in the head of the compressor (I think it is the suction side) but can't figure out how to get it out to check it.

The only things I can think could be the problem are:
1) The screen in the head is clogged (How to remove?) Is there another filter in the lines somewhere I missed?
2) The expansion valve may be at fault. From what I understand it controls pressure of the freon as it exits the evaporator. But I think it would take a while for that to become an issue. (How to test??)
3) The thermostatic switch is not functioning properly and is causing the system to cycle due to electrical malfunction (how to test???)
4) The compressor is shot and really is blowing metal through the system (any way to confirm????)
5) I just wasn't meant to stay cool in sunny Southern California. (no comments!)


Any help or answers to the above would be greatly appreciated.

 

[valley82]

O.k. -
The screen in the compressor - If it is in the line coming from the evaporator coil to the compressor (suction line), the side of the screen that you are looking at is the side that would be plugged (but that screen being pluged would cause the compressor to not pump as much refrigerant lowering the suction and discharge pressures) - if it is on the line going from the compressor to the condensor coil at the radiator (discharge line) then the screen would have to be removed to check the back side for debris but unless the head pressure cutout switch is on the compressor it would not see the higher pressure from the screen being pluged.

If your TXV has failed it may not be metering the refrigerant though the system properly (too much refrigerant moving through the system) which could cause the pressures to skyrocket( or not enough refrigerant through the system) which would look like the system was undercharged untill the shop added too much refrigerant and overloaded the compressor. (it meters the refrigerant at the inlet of the evaporator coil)

Remove the suction and dicharge lines from the compressor.
If you can pass air freely through the discharge line and it comes out freely through the suction line then I'd almost rule out blockage.

Although if the Filter/Dryer in the liquid line (between the condensor coil and the TXV has been open to the air for very long it should be replaced as the dryer portion will have pulled moisture out of the air and will not properly remove any moisture from the system.

The other thing that it could be is that the shop overcharged the system - too much refrigerant in the system will cause the pressures to skyrocket also - but the discharge and suction pressures would both be higher than normal.

If the compressor is not noisy and is still capable of driving the discharge pressure up to 300 psi, it is probably still in decent shape.

Kinda hard to diagnose over the internet, but hopefully this helps.

One other thing that could cause those conditons is that the shop did not evacuate all of the air out of the system before charging it. No air can be present in the system as it will become trapped i the condensor coil and reduce the amount of heat that the condensor coil can remove from the refrigerant - high discharge pressure (this would probably take 3 - 10 minutes to happen depending on how much air is in the system)

On that same note, if the Filter/dryer in the liquid line has been exposed to the environment for very long it could be saturated with moisture, when you start pushing refrigerant through the system any excess moisture will be pushed through the system and turn into ice at the expantion valve (TXV) causing it to stick in what ever position it is at. This will also take 5 - 30 minutes to occur - the system will run normally for a while then either act under or over charged.

I'm not bashing on your shop at all (probably an honest shop), but you would be amazed at how many hacks there are in the AC and Refrigeration industry,

 

[bnickel]

IIRC, the system in your car only takes 1.5lbs of freon, so it's likely they overchraged it. i also agree about the system being evacuated, if they didn't evacuate the system you'll have problems as well.