MACS Service Reports - 2016 - MAR1

Total Vehicle Climate and Thermal ManagementTM

March 2016

By Steve Schaeber, MACS Technical Editor
SENSORS AND SWITCHES: THERE IS A DIFFERENCE

Stepping Back in Time

Not every technician likes to jump into the middle of
someone else's job and take over, especially if that vehicle has come in from another shop that did a bunch of
work but never finished. After all, it's not easy to figure
out someone else's problem, especially if you have to put
something back together when you weren't the one who
took it apart in the first place. But sometimes it can be fun
to take on a challenge like this, and who knows? Maybe
you'll even come out looking like the hero when you get
it working!
When we have trouble figuring out why a system isn't
working the way it should, one of the first resources we
go to is another technician to see if they've run across
something similar in the past. With the millions of cars
and trucks out there, it's not possible for any one person
to know it all.
Many years ago the Cycling Clutch Orifice Tube
(CCOT) system was widely used in mobile A/C applications. Those of us who learned how to work on these air
conditioning systems back in the days when they were
quite popular were taught a few tricks of the trade that
helped make diagnosing problems quicker and easier.
But A/C systems have changed quite a bit in the last
25 years, and some of these shortcuts no longer apply,
although the legend of their use still lives on in shops
across the land.
We'll take a look at one trick in particular, what it was
used for, and why it may not apply to most of the systems
found in vehicles today. It's not a recommended service
practice that you'll find in service information, but it was
a widely used technique in shops because in many cases
it worked well and pointed you in the right direction.

Steve Schaeber

It's the summer of 1989, and it's been hot and humid
around town for the past few days. Your customer brings
their 1985 Chevrolet Monte Carlo with a 305-V8 into
your shop because the A/C isn't blowing cold. The first
thing you do is to verify their complaint by starting the
engine and turning on the A/C controls to MAX (Figure
1). Sure enough, that R-4 compressor doesn't kick in, but
the blower motor does, and seems to work at all speeds.
So you pull the car into your bay to look into the situation
a little further.
With the engine not running you opening the hood,
and start off by looking everything over; you check to
make sure the belt is on the compressor, the hoses are all
connected and not damaged, and that there are no obvious problems that would prevent the system from running. The clutch wires are present and connected, as well
as the wires going to the low side pressure switch. As
long as everything is there, connected and not showing
signs of damage or tampering, there's a good chance a
simple fix is in order.

Figure 1: GM used similar dash controls for more than 20 years on their
manual A/C systems.

ALSO INSIDE THIS ISSUE:
OVERHEATING... AT 10°F ................................................................................................................................................................ 8

MACS Service Reports is the official technical publication of the
Mobile Air Conditioning Society Worldwide, Inc., 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

March 2016

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 Worldwide 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

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