Date: Fri Sep 25 00:36:46 1992
Subject: Re: How good is BMW A/C?
>While I'm still not convinced that losing a pound of freon a year is too
>horrendous, I just remembered a problem we had on my mother's Dodge. It
>couldn't keep its freon for more than a month. When I did a routine engine
>cleaning at the local car wash, the compressor hoses turned oily a few days
>later. Close inspection showed the oil seeping out along the full length of
>each hose, with slow tiny bubbles of freon appearing. Somehow the whole
>darn hose "failed". Maybe you could clean and dry the A/C plumbing, and
>look for something similar?
To answer a couple of questions at once, my 80 635CSI's A/C is marginal.
Plenty of cool but too little air flow. It will easily freeze if the
thermostat is turned up too high.
Performance testing the AC is easy and takes only a bit of
equipment. You'll need a suction side pressure gauge (available
for as little as $10) and a thermometer. While the system is
running at high idle, measure the outlet air temperature AT
the evaporator. Measure the suction pressure and note from the
pressure gauge scale what temperature this corresponds to. The
temperature of the evaporator should be about 10 degrees higher
than the temperature indicated by the pressure reading. This is
Example: The gauge reads 38 psi which is the vapor pressure of R-12
at 40 degrees. The outlet air is 50 degrees. There are 10 degrees
of superheat. If the pressure is LOWER, the superheat is too high
which means the evaporator is being starved for freon. If the system
is properly charged, the problem can be either a malfunctioning expansion
valve, the expansion valve bulb not in good contact with the suction line,
or a clogged liquid line strainer.
If this looks normal, you need to look at your high side pressure and
exit air temperature. The condenser pressure should correspond to a
temperature about 10 degrees higher than the outlet air temperature.
If the outlet air temperature is 120 degrees, the pressure should
correspond to the vapor pressure at 130 degrees. If the pressure is
much higher than this, you either have a blocked condenser or there
is air in the system.
If the temperature margins look OK but the evaporator is too hot (much
greater than 40 degrees) and/or the condenser temperature is too low
(should be around 110-130 degrees) AND the system has plenty of freon,
the compressor is the suspect or the system has too little
If the capacity is low, there is a partial solution. George Gobble
of Purdue (firstname.lastname@example.org) has developed a drop-in replacement
for R-12 that keeps the econazis happy. A byproduct of this mix
is it increases the heat capacity of a system by about 20%. He has
some special mixes that increases it even more.
A version of GHG-12 contains a leak-stopper called Cryro-Silane. This is
a water & air catalyzed silicone rubber monomer that polymerizes to a
tough silicone rubber upon exposure to air. It seeps into porus hoses
and similar problems and permanently stops the leaks. Cryro-Silane is
available separate from GHG-12, though the mix works very well.
The dryer must be removed and the system highly dried prior to use.
Since ordinary citizens won't be able to buy R-12 after November and
since the price of having service shops (who now have a monopoly)
install freon has skyrocketed, small leaks do matter.
BTW, if you don't want to fool with GHG-12 or pay the econazi tax on
R-12, a mix of 78% propane and 22% isobutane is a perfect drop-in
replacement for R-12.
Disclaimer: I've been heavily involved with the development of GHG-12.
It's good stuff.
Date: Mon Jul 6 11:58:36 1992
Subject: Re: A/C high/low pressures wrt temp @ humidity
> Anyone know what the correct A/C pressures are for my 83 528E?
>My A/C is undercharged (shutdown on Low Pressure switch), and I'd like
>to recharge it. I know that there are tables that show the correct
>high and low pressures with respect to outside air temp and humidity for
>most Air conditioners. Neither my BMW or Chilton books have such a
>guide. Chilton tells you to watch the bubble stream ("Turn on the bubble
>machine"), and wait for the high side to increase dramatically...
If you have a sight glass, don't worry about pressures other than to
verify proper operation. Simply run the engine at a high idle with
the AC full on and the doors open and charge until the bubbles disappear.
Add another ounce for an operating reserve and you'll be fine.
The pressures correspond to the vapor pressure of the freon at the ambient
temperatures in the evap and condenser. Your gauges should have a vapor
pressure scale on them. Look at the R-12 scale. The evaporator should
run a superheat of about 10 degrees. That is, the ambient temperature
should be about 10 degrees F higher than the indicated pressure. Ambient
is measured with a thermometer in the air outlet. This superheat ensures
all the freon is boiled in the evap and not in the return line or the
compressor. Condenser pressure will roughly correspond to the temperature
as measured with a thermometer directly behind it. It is typical for the
temperature to be 30-40 degrees above air ambient. The important thing
to watch for on the high side is excessive pressure above the vapor
pressure. Non-condensables (mainly air) collect in the high side and
raise the pressure (sum of partial pressures and all that stuff.)
Excessive pressure indicates the system needs to be evacuated and recharged.