MACS Monthly Newsletter - 2021 - SEP1

Service Reports
MOBILE AIR CLIMATE SYSTEMS ASSOCIATION ©
By Mike Bailey, MACS Technical Correspondent
Things change. Sometimes for the better. While we have all
been trying to keep up with electronics, diagnostics, refrigerants
and equipment, the basic refrigeration system in light
vehicles has undergone a few fundamental changes.
Sometimes things go backwards. Orifice tubes are mostly
gone as manufacturers have back tracked to expansion
valves and receiver dryers for better performance.
Things have moved around. The receiver/dryer is now
commonly found incorporated into the condenser.
Condenser and evaporator design has been updated. Superheating
and subcooling refrigerant are common.
Things move automatically during operation that never
used to unless they were commanded by the driver (we are
looking at you, Mr. Recirculation Door!).
Components seem smaller, lighter, and more fragile.
Refrigerant charge weight has dropped dramatically. Even
the refrigerant itself has changed...again!
With all these changes most automotive systems still manage
to do a respectable job of keeping our customers cool
and the windows clear.
What hasn't been promoted with much fanfare is the addition
of the variable displacement compressor. This brilliant
piece of technology is not new and was initially developed for
a reason. See the MACS Service Report from April 2021 for
the whole story on mechanically controlled variable displacement
compressors and theory of operation.
The Refrigeration System
Let's divide the system into two parts. The air handling
ducts, doors, motors, wiring, and modules are one half. The
compressor, condenser, dryer, lines, expansion valve, evaporator
and refrigerant are all part of the REFRIGERATION system.
Add the compressor control valve (CCV) to the REFRIGERATION
SYSTEM and what we now have is a...VARIABLE
OUTPUT REFRIGERATION SYSTEM.
In theory, a variable output refrigeration system can
change and/or precisely adjust the BTU rating of the entire
system! When demand is low, the compressor is commanded
to move less refrigerant, therefore the refrigeration system
transfers less heat. When demand goes up, the compressor
MACS Service Reports is the official technical publication of the
Mobile Air Climate Systems Association, P.O. Box 88, Lansdale, PA
19446. The material published in MACS Service Reports expresses
the views of the contributors and not necessarily that of MACS. Every
attempt has been made to ensure
September 2021
1
Figure 1: Variable displacement compressor rear head
with manual control valve.
the accuracy of the content of MACS Service Reports. MACS, however,
will not be responsible for the accuracy of the information published
nor will MACS be liable in any way for injury, labor, parts or
other expenses resulting from the use of information appearing in
MACS Service Reports.
MACS Service Reports
September 2021
Electronically Controlled Variable Output Compressors
is commanded to move more refrigerant. As a result, the refrigeration
system moves more heat.
The service information provided from most manufacturers
using these systems is less than stellar. Have a look through
the theory and operation section sometime. There is not
much meat on that bone at all.
I receive calls from technicians for technical help almost
every day. Lately many callers have been asking for help with
diagnosing " weird " pressures, low pressure difference between
low/high gauge readings, poor cooling due to a lack of
air flow or just poor cooling in general.
On many occasions before calling, the technician has ruled
out the usual over/undercharge, air in the system and air
handling/duct/door issues. On the call, we cover some basics
and often we may end up back at the compressor.
Identifying A Belt Driven VDC
(Variable Displacement Compressor)
In the April 2021 issue, I described the visual traits associated
with mechanical VDCs. Tapered nose (for the most part),
a casting boss (bump) on the back head for the control valve,
the actual presence of the control valve and the fact that the
compressor does not cycle " OFF " when the heat load is low,
and the evaporator could freeze. See Figure 1.
Mike Bailey
MACS Monthly Newsletter - 2021

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