MACS Monthly Newsletter - 2022 - JAN3

where service ports are located. Often, they end up being
placed where they are just because of being in the most
accessible places on vehicles.
To address the second part of the question about, " Is
there ever a reason to have more than one of each " , that
would be a resounding YES! That's because multiple service
ports would provide the capability to check pressures
in multiple places and easily find restrictions. So,
with unlimited resources and unlimited funds, I'd create
a " service port wish list " which would involve a high side
service port at the inlet and outlet of every major component
and hose/line on the high side and also a service
port at the inlet and outlet of each major component and
line/hose on the low side. On a typical expansion valve
system, this would involve 6 service ports on the high side
and 3 on the low side. See Figure 2. On a typical orifice
tube system, it would involve 4 service ports on the high
side and 6 on the low side. See Figure 3. This would only
occur on a prototype system such as used for testing in a
wind tunnel or for training and discussion purposes and
never make it to production vehicles, but I'm sure many
of you have encountered a tough diagnostic situation and
wished that you could have placed some service ports in
some of the locations shown.
Now let's take one of these systems and simulate a restriction
and see what the effect might be on pressure
readings at different points. But before going further we
need the proper information on the type of system we're
dealing with; the type of compressor it uses and the ambient
temperature and humidity, just as we'd be doing if we
were working on it in a shop. Then we need to establish
what the pressures are with the system operating properly
and the vent temperature. That information is as follows:
System Type: Single Expansion Valve System
Compressor Type: Fixed Displacement Piston
Compressor
Ambient Temperature: 90°F
Humidity: 70%
High Side Pressure: 210 psi (at a slow idle)
Low Side Pressure: 35 psi (at a slow idle)
Center Vent Temperature: 48°F
A popular service port arrangement on a
TXV system would be the following:
High Side Service Port Location: On the liquid line
about 2 inches downstream of the drier.
Low Side Service Port Location: On the suction line
about 2 inches from the evaporator outlet.
So, we'll start by showing an illustration with the ports
in those locations and the pressure readings with everything
operating properly. See Figure 4.
Figure 4: Properly functioning Expansion Valve System.
The next step is to create a partial restriction in the
Figure 3: Accumulator/Orifice Tube System Service
Port " Wish List " .
January 2022
3
upper portion of the condenser. This affects condenser
efficiency dramatically and the combination of the reduced
condenser efficiency and the compressor pumping
against the restriction is going to cause the pressure
at the compressor to increase significantly and the low
side pressure will also increase as a result. However, the
pressure at the original service port location is going to
drop significantly because that service port is located
downstream of where the restriction is. With the high side
MACS Service Reports
Richard Hawkins
Richard Hawkins
MACS Monthly Newsletter - 2022

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