I have a 1980 which still has the R12 charge. Last summer the system
was not as cold as it should be so I put in a 12 0z can of R12. System
was then cold enough. I have used the system a bit since last summer
but not on really hot days so system seemed fine.
Today was hot so used it and it was not quite cold enough. So I added
another can of R12. When charging, the low side was between 35 and
40 pounds. After charging, it dropped to about 33 to 35 pounds. System
is now cold enough. ( I did not measure the high side pressure)

I ran the vent for a while with the temperature off to make sure the
heater valve was turning off. Air was about as warm as outside air.
So valve must be working.

Checked all hoses, connections, compressor and
condensor to see if I could see any refrigerant
oil indicating a leak. All looked dry.

Suggestions: Have I overcharged the system?
If there is a slow leak would it leak out over the winter, or does
the system have to be running for the freon to leak?
Should I have a professional try and find a leak? or is
it too small and would be a waste of money?

 

When the systemmis running, there is high pressure in the line from the compressor to the condensor and toward the accumulator. Once the refrigerant is past that point it becomes low pressure (30-35). When you shut it off the pressure equalizes. That is the hissing you hear after you shut it off. Both sides are egual at a pressure of about 120. That is a wide variable depending on air temperature. There is a specific temp/pressure relationship for each type of refrigerant. You must sevice the system running, at the low side and keep the engine running at about 2000rpm.

 

Although R12 systems will leak a tiny amount over time, a leak sufficient enough to require a continuous addition of refrigerant is probably not "normal".

As you repeatedly add refrigerant using non-professional equipment, there is a tendency to introduce air into the system via the hose connections. This "air" (and moisture) is a "contaminant" as far as the system is concerned. In order to return the system to proper functioning you have to first repair the leak if there is one, then evacuate the system to remove the contaminants (particularly the moisture), and then recharge it with fresh refrigerant.

I just did this with two R134 systems that had been converted using non-professional equipment, and now they both are producing twice as much cooling.
By using a good vacuum pump to evacuate the system, you can bring it to almost a perfect vacuum... this low pressure allows moisture to "boil" into a vapor, which the pump can then remove (increase pressure, increase the boiling pint... lower the pressure, lower the boiling point). This is why a vacuum source must be connected for about 20 minutes at full vacuum to get all the moisture out.
Excessive moisture is removed by blowing out the system with dry nitrogen.
So try not to let too much air/moisture in when adding refrigerant. Be conscious of possible introduction of air via your hose connections.

Moisture in the system can form ice which closes off openings in expansion valves and cap tubes. This prevents adequate cooling, and can also (along with air) produce acids & sludge which will cause system failure.

You can't remove moisture unless you pull a near perfect vacuum for at least 20 minutes with good equipment.... this is about 29.9.... or 250 to 1000 microns.

PS- I just started working on the A/C of an 80 yesterday. It lost vacuum about 10 minutes after I shut the pump down... so it has a leak that I have to locate.



[Modified by Tom454, 8:22 AM 7/22/2002]