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Old 07-05-2002, 06:29 PM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
R-12 AC pressure readings - need freon or not?

I first checked high and low side with engine off -- pressure reading at both was just under 100psi.

Pressure at low side after running AC for awhile was about 33psi -- high side read 190psi.

Are these readings low, high, or just about right? Sight glass shows moving milk.

If low, approximately how much freon will it take to get a full charge?

TIA, as always!


'91 300E, 214K miles

Last edited by pmizell; 07-05-2002 at 07:44 PM.
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Old 07-05-2002, 08:23 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The only thing that can be said for pressures are the pressure of the liguid/gas condition at a particular temp.

There are tables that will give this value for different gases. To give an example: say I have a 30lb cylinder with 10 pounds of R12 inside. If you go to the table and look at sea level with a 90 deg day the pressure will be exactly xxx lbs. Over 100. If you have the same cyl with 2 lbs of R12 with the same temp/altitude the pressure will be the same. If one keeps removing and let stabilize the refrigerant, there will be a point at which the liquid boils off maintaining that pressure untill no liquid exists in the container. At this point the pressure starts to drop and continues in proportion to the gas remaining in the system.

So, a proper charged system will have xxxlbs at rest pressures equalized low and high side. A system with half a charge will have exactly the same pressure because at any temp the equilibrium between liquid and gas is a constant.

As to the dynamic situation, your pressures are generally all right. The low side is a little high if you did your performance test properly at 2000 rpms. Your hi side pressures are probably low but have more to do with the heat exchanging properties of the test. Was there a good fan on the condenser? How long had the car ran in one place? The tell tale is of course the sight glass on the R12 system. With your pressures and the visible foam in the glass I would say you could probably use any where from 0.5 to 1.0 lbs. The proper way to know is to recover the refrigerant and weigh out the proper fill.

As another example of the nature of pressures in refrigeration systems. Two cylinders of refrigerant are setting on the floor and are not labeled. If one were to know that one was R12 and the other R134a the chart and a guage and a thermometer would tell us which was which.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 07-05-2002, 11:08 PM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
Thank you Steve for the reply. I just finished conducting the pressure test correctly at 2000 RPM's. Here's what I measured:

low side went to about -5psi
high side settled in at 200psi after about 10 mins of running

Center vent was blowing about 57 deg F; ambient outside here was 81F.

Interestingly, while attaching high side hose, I lost some precious elixir -- enough that the low aux fans would not kick in at all. I suppose this answers my question ... I need at least some freon added.

My earlier test had aux fans on continuously, with car in shade and about ambient temp of 95.

I'm gonna add some now ... should I be watching sight glass while charging or the high side pressure? I have 12oz cans (4 total). I would imagine it needs at least 1/2 lb, maybe a full lb.


'91 300E, 214K miles
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