RE: Q: A/C low line pressure #s?

["Rodney Moore" <m6bigdog@xxxxxxxx>]

Tom,
I wish it was that easy myself.
All though your low side pressure read normal for the engine rpm change from
idle.

I assume you have an R-12 system?  R-134a has different pressures @
temperature than R-12.

The low side pressure can range from a low of 24 psig with 20% humidity @ 70
deg. F outside air temp.  to a high of 72 psig with 70% humidity @ 110 deg.
F outside air temp.  The evaporator discharge temp would range from 44 deg.
F minimum to 91 deg. F with 70% humidity @ 110 deg. F outside air temp.
These pressures would be a an engine rpm of 2k or more.

In general, if there are no bubbles in the receiver/drier sight glass when
the evaporator is wide open(doors open on a warm/hot day blower on max.)
your system will have a sufficient charge.

There are several reasons for the low side pressure variation.  Compressor
(engine) speed, air flow over the condenser (coils in front of radiator) and
expansion valve fluctuations.

I would also be interested in your high side pressure to complete the check.

The best check is to get an A/C R-12 performance chart and check the low
pressure, high pressure, evaporator discharge temp in reference to the
outside air temp and humidity.

Rod...



ate: Mon, 28 Aug 2000 16:22:28 -0400
From: Tom.Murphy@domain.elided
Subject: Q: A/C low line pressure #s?

Hello BMW Experts!       I may be getting near to having a real working A/C
unit again!  I am getting some pretty cool air (~ 55 degrees, driving), but
don't know the correct pressure which would optimize the system.  The car
is an '87 325is, with an aftermarket Seltec compressor.
     My low pressure line is at ~ 35-40 lb. at idle.  When the engine is
revved, the pressure goes down to ~ 26-28 lb.  At what RPM should I be
measuring the pressure, and how many lbs should it then be?
     I'll give a more complete write-up when the job is actually done.

                              TIA! Murf  '87 325is


- ---
Rodney Moore
BMW CCA #86830
Golden Gate Chapter
m6bigdog@domain.elided

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