MACS Monthly Newsletter - 2022 - JAN2

to the compressor and is there just one section of rubber
hose or maybe two?
Technician: The rubber hose is about 3 feet long and
there is only the one section.
Me: So, to make sure that I have the information correct;
the high side service port is located on the condenser end
of the discharge line and there is about 2 feet of rubber
hose between it and the compressor and there is about a
two inch piece of metal line between the service port and
the condenser.
Technician: Yes.
Me: OK, and the low side port is located on the block
valve end of the suction line with about two inches of metal
line between it and the block valve and there is about
3 feet of rubber hose between it and the compressor and
there is only the one section.
Technician: Yes, that is correct. But why does it all of
that matter?
Me: Because you could have a restriction in one or
more of those hoses or one or more of the other components
and what you're seeing on your gauges could be
quite different from what the ACTUAL pressures are at the
compressor. The further a service port is located from the
compressor and the more components that are located
between it and the compressor, the more likely it is for
that to occur.
Technician: Hmm. I hadn't thought about that before. It
makes sense.
Before continuing on, I need to mention that MACS
members are the cream of the crop when it comes to A/C
diagnostics and most will understand all of this, but they
like other shops have to hire new technicians from time to
time, so this MSR is more geared toward the less experienced
technicians on the staff.
Near the beginning of the webinar, I was asked the following
question: If you were an A/C engineer at one of
the OEMs and you had unlimited resources (and of course
unlimited funds to do this), where is the ideal location to
install service ports on an A/C system, and is there ever
a reason to have more than one (of each)? To address the
first part of the question: My preference is as close to the
compressor as possible. See Figure 1. That's because I
want to know for SURE what the compressor head pressure
is and also want to know for SURE the level the compressor
is pulling the low side down to, and taking readings
right at the compressor is the only way you can be
sure.
NOTE: Even with being connected to service ports right
at the compressor, there is always the possibility of a problem
with a Schrader valve, hose connector, gauge or hose
connected to the gauge. So, for that reason, always make
sure that gauge movement corresponds to temperature
changes of metal lines right at the compressor.
As indicated in the conversation with the technician, the
further away service ports are located from the compressor,
the more likely you are to see a pressure reading that
January 2022
2
Figure 2: Expansion Valve System Service Port " Wish List " .
MACS Service Reports
Figure 1: Having the service ports located right at the
compressor, such as on this manifold, ensures being able
to get the actual pressures the compressor is producing
(unless there is a problem with a Schrader valve, hose
connector, gauge or hose connected to the gauge).
is quite different from what it is at the compressor. When
that occurs, it is the result of restrictions. Consider an expansion
valve system with a restricted condenser and the
high side service port located downstream of the drier.
Because of the restriction in the condenser, you could be
seeing a high side pressure on your gauge which might
appear to be below normal, but the actual pressure at the
compressor is highly elevated. Couple that with a low side
pressure reading which is higher than normal (because
the compressor is trying to pump against the elevated
high side pressure) and the conclusion is that the compressor
is weak and it ends up being replaced unnecessarily.
There is no standard that exists in the industry on
Richard Hawkins
Steve Schaeber
MACS Monthly Newsletter - 2022

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