MACS Monthly Newsletter - 2022 - SEP8

Then he replaced the bad relay with a new one and
took the vehicle for a test drive and the system cycled
like it was supposed to. Going a step further, he removed
the cover from the relay and could see that the
points inside were severely burned.
We know the system was not cycling properly and
the bad clutch relay was the reason for the compressor
running continuously. Now it is time to focus on the
title of the article: " Why did the compressor lock up? "
The short answer to this is: because the compressor
was running continuously when it was supposed to be
cycling.
When telling this story in clinics and on tech calls, I
have encountered some people that just could not believe
that could cause compressor lockups. One of the
most common responses has been along the lines of:
" Compressors can literally run for hundreds of thousands
of miles, they don't need the rest they get from
cycling off and on. "
That is correct, compressors can run for hundreds of
thousands of miles, but that is on " problem free " systems.
Also, indeed they do not need the rest they get
from cycling. The purpose of the cycling is not to rest
the compressor, but to prevent the pressure in the low
side of the system from getting too low resulting in the
evaporator freezing up.
The clutch cycling switch on this vehicle is designed
to cycle the clutch off at about 25 psi. With the clutch
relay sticking, the low side pressure will drop below 25
psi and ice will start forming on the fins of the evaporator.
With the compressor continuing to run, ice will
continue to build up on the fins and the air flow through
the fins can be completely stopped. This reduces the
heat load and causes the low side pressure to continue
to drop.
As the pressure drops the evaporator temperature
also drops. The evaporator temperature gets so cold
that the oil in the evaporator starts to thicken up and
the amount of oil the refrigerant is carrying to the compressor
is decreased. The colder the evaporator gets,
the less oil flow there is. If the evaporator gets cold
enough, oil flow can completely stop.
If the technician who installed the compressors for
the limo company had tried to drain the compressors
that locked up, he would have found that little to no
oil would have drained from them and that would have
been a big " red flag " that a problem existed.
Whenever a compressor fails, there is a reason for it.
If the cause of the failures is not determined and corrected,
the failures will continue as was the case here.
This is called a root cause failure analysis and is an
important part of the job. v
MACS Service Reports is published monthly by the Mobile Air Climate
Systems Association. It is distributed to members of MACS and is
intended for the educational use of members of the automotive air
conditioning service and repair industry. Suggestions for articles will
be considered for publication, however, MACS reserves the right to
choose and edit all submissions.
Unless otherwise noted, all photos/art by author.
September 2022
8
Editor:
Production Designer:
Manager of Service Training:
Steve Schaeber
Laina Forcey
Steve Schaeber
Mobile Air Climate Systems Association
P.O. Box 88, Lansdale, PA 19446
Phone: (215) 631-7020 * Fax: (215) 631-7017
Email: membership@macsmobileairclimate.org
Website: www.macsmobileairclimate.org
MACS Service Reports
https://macsmobileairclimate.org/events/category/2023-annual-training-event/ http://www.macsmobileairclimate.org
MACS Monthly Newsletter - 2022

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