Some of the confusion stems from the fact that R12 has the same
temperature and pressure on the low side (that is, in degrees F, 32
psi = 32 degrees). One of the reasons R12 was used in automobiles was
that it's simple to see what the evaporator temperature is.
R134a doesn't work like this. My gauge set has a temperature scale on
it beside the pressure scale so it's easy to see what the evaporator
temperature is.
On systems with expansion valves, you will see quite low pressures
when the valve is closed, but they will be higher when the valve is
wide open. The pressure may not show the actual evaporator
temperature as a result.
On CCOT systems, it's even more complicated as the evaporator can
contain liquid freon at times, and excess liquid freon collects in the
accumulator (between evaporator and compressor) since there is a fixed
orifice. GM typically uses CCOT systems, MB has tyipcally used a
receiver/dryer and expansion valve, but I don't know what they use
today.
With a receiver/dryer, it's easy to tell if there is adequate freon,
as you won't see bubbles in the sight glass during full load
operation. No sight glass on CCOT systems, you have to use the
pressure.
Low pressure with boiling sounds in the evaporator is a new one for
me, but I'd try adding freon first since it's cheap and easy. A bad
compressor usually results in high low side pressure and no cooling --
and no heat in the condenser.
Peter
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