MACS Monthly Newsletter - 2021 - APR1

Service Reports
MOBILE AIR CLIMATE SYSTEMS ASSOCIATION ©

April 2021

By Mike Bailey, MACS Technical Correspondent

Understanding the Compressor
We have all been there.
A vehicle comes into your bay and a compressor is diagnosed. It may still
turn...or not. Maybe the driver hub freewheels with no resistance, perhaps it
is noisy, or it could be locked up solid. Sell the job, a flush may be performed,
then a reman or new compressor and the obligatory dryer are installed along
with a recharge. Post-repair testing and gauge readings confirm the system
is again healthy. Send it.
A week or two later the same vehicle is back in your service bay and
the compressor is dead again. What Happened? How can this scenario be
avoided?
There are many factors to consider for a compressor to survive in the
challenging underhood environment. Tiny engine bays have necessitated
smaller compressors now sometimes mounted down near the crank pulley,
catalytic converters attached to the exhaust manifold, cooling system(s), and
the A/C itself shedding heat from the cooling module, wildly varying RPM,
filter/dryers, and a much smaller refrigerant charge have to be taken into
consideration. If these variables stay within design limits, the compressor
will do its thing and the customer will be happy.
Technicians can reduce the chance of a compressor " comeback " by keeping all these factors in mind during inspection and service. It seems like a
lot to remember, however understanding how the things live in this environment will help technicians work toward most of these points becoming
routine checks that are unconsciously performed during service.
The down-low
That noise on system start-up again. Turn the system on, hear the " click' of
the clutch and wait for the cool air. Instead, a " growling " or possibly a clutch/
belt squeal is heard from the compressor before things quiet down and get
cool. Compressor slugging is a real issue. On many vehicles we can not
" service it out " during repair. It may be an inherent issue within the system
design.
" Slugging " is the compressor clearing liquid from its cylinders on start-up,
after sitting, before it can start compressing vapour as designed. A compressor is not a liquid pump, and liquids generally can not be compressed. The
liquids in the compressor causing slugging are refrigerant and lubrication
oil.
Consider what happens during slugging.

There is more bad news. Oil and refrigerant mix. Droplets of liquid refrigerant will be mixed in with the oil. Pushing this oil/refrigerant out of the cylinder causes a pressure spike. Rising pressure increases temperature. When
the temperature rises, the liquid refrigerant may boil and expand (system
high side pressure has not built up yet), which intensifies the " growling " . If the
liquid in the compressor is mostly refrigerant, the same noise will be heard
but it can be more pronounced.
Will slugging damage a compressor? Yes. The rapid expansion of liquid
refrigerant to vapour can overstress the reed plate and physically damage
the reed valve. If the compressor is engaged and brought up to a high RPM
almost instantly (consider turning the system on with the engine at high
RPM) the internal stresses of trying to compress the oil and the flash-boiling
of refrigerant droplets can break piston rings and reed valves. Depending

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

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.

April 2021

Mike Bailey / Four Seasons

Refrigerant oil will be expelled out of the cylinder by the piston moving
toward the reed plate. The liquid oil squeezes past the reed, into the discharge port and then out toward the condenser. The oil does not cause a
" hydraulic lock " as the reed valve is easily pushed open by the exiting oil,
but the volume of oil may not be able to be expelled quickly enough. Cylinder pressure then spikes, and the compressor may be damaged.

Figure 1: Slugging can damage reed valves. A " Deslugging " module will cycle the compressor rapidly on start
up. Rapid cycling on start up allows the safe pump-out
of any liquid refrigerant or oil which has accumulated
after sitting for a specific amount of time.

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