From: mark@wdcwdc.sps.mot.com (Mark Shaw)
Subject: Re: A/C manifold gauges question
Organization: Motorola Western MCU Design Center, Chandler Arizona
Date: Thu, 10 Sep 1992 17:42:24 GMT

|> (Jurek Rakoczynski) writes:
|> > (Mark Shaw)writes:
|> > If the indicated R12 temperature is less, you are undercharged.  If the indicated R12
|> > temperature is more, you are overcharged.  Rather simple and you don't have to deal  with
|> > those higher running pressures.  This method is not precise (doesn't actually tell how much
|> > freon to add/subtract) but is the next best thing to watching a sight glass for bubbles.
|> 
|> The R12 pressure (or is temperature the correct word?) in a
|> non-operating system should be the same no matter what the total
|> charge is, as long as there is still liquid R12 in the system.  For
|> example, the pressure in a propane tank will read the same at a given
|> temperature no matter how much propane is actually in the tank. 
|> That's why you can't used static pressure as a charge indicator.
|> 
|> BTW, The vehicle is a `83 Chevy Suburban, CCOT A/C w/o the rear A/C
|> evaporator.  As for my problem, if I adjust my readings from memory
|> to the correct scale, at ~105F, I was reading ~300psi high side, and
|> ~55psi low side.  I could not get the low side below ~55psi without
|> the system being undercharged; the evaporator inlet and accumulator
|> outlet were warm.  This is even after I replaced the fan thermal
|> clutch with a solid adapter from NAPA.

Both you and Norb Brotz have a good point about a saturated R12 mixture would give faulty
static pressure readings. Now I have to figure out why the pressure goes up after I charge
the system?  Might have to do with my static quick charge method which does not require the
engine to be running.  Boy! I hate running that engine and checking pressures at those
elevated levels (makes for messy and environmentally bad removal of hoses).

As for your specific problem, I didn't make the connection that you are also in AZ.  The
general rule of thumb pressures that James Swonger gave would work well in Florida where
the ambient temperature is lower.  You will indeed have trouble getting your inlet pressure
down to 20-30 psi if you're checking it during a 100+ degree day!   With an outlet pressure
of 300 psi for an inlet pressure of 55 psi you are pulling a 245 psi pressure differential
which pretty good for your compressor.   

One way to get your outlet pressure down (and inlet pressure down) is to spray water into the
radiator/condensor.  This should bring both down.  If you then have a 220 psi outlet with
a 20 psi inlet, everything is OK.  If you spray water into the radiator you get water mist 
everywhere.  Be sure your fittings are dry before trying to connect up a freon can to avoid
getting moisture into the system.

If you had the factory (published by Helms) manual it would give you the pressures and
outlet temperatures to test for.  I will check my Celebrity manual tonight and see what it
has -- I'm getting too old to remember everything.  

Sounds like your system is working fine.  Trying checking it in the early morning.  The other
rule of thumb I use is that the maximum temperature differential should be about 40-50 degrees.
I test this with a thermometer in the evaporator outlet with the AC on HIGH fan, using outside
air, temperature control on full cold, windows open, and engine running at about 2000 RPM.
The evaporator outlet temperature should be 40-50 degrees cooler than the outside air.  If the
outside air temperature is below 90 the evaporator temperature will "bottom out" at about
35-40 degrees and the AC clutch will start cycling. 

regards, 
Mark
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