MACS Service Reports - 2016 - FEB1

Total Vehicle Climate and Thermal ManagementTM

February 2016

By Paul Weissler, MACS Senior Technical Correspondent

There's no one right way to approach the diagnostics for tem, which takes a problem and provides a list of the most
every A/C electrical or electronic problem, but there gener- frequently replaced items according to vehicle mileage. It's a
ally are at least two pathways that can get you to the answer. nice reminder, but you still have to go through the diagnosOne is to start at the beginning and go through a system one tics to confirm the need (Figure 1).
step in sequence after the other. Most OE service information
Even where a pattern failure part is a frequent occurrence,
diagnostics use this method, and for good reason: they have there also may be many others, and those causes may be in
to cover all the bases for any problem in the specific area. a number of other areas that many technicians were able to
So you'll see pinpoint tests for every connection in a circuit, solve without calling a hotline or using the online guidance.
even if there's no indication that every test is likely to be ab- After a few years, when an improved part may already be
solutely necessary.
in production, the pattern failure reputation still may hold,
The other paths vary according to the issue. The most even if not justifiable. By then the actual causes may be in
popular one in the aftermarket is to play the percentages, a a number of other areas, but that many other technicians
subject we've discussed periodically in the past, particularly were able to find without playing percentages. If you spend
in connection with hotlines. That means that the hotline, or the extra money for a session with a hotline technician, he'll
an on-line diagnostic advisor in your aftermarket electronic usually run you through confirming diagnostics before he
information system, will take the details on the make and advises replacement of a pricey part.
model of vehicle, the symptoms, etc. and display the perStill another diagnostic method is to look at the factory secentages for all the fixes that solved the problem. Example: quence, and just try to get a fix by replacing the part at the
a new relay is a 95% fix; a new module is a
4% fix. A technician is likely to use the 95% fix
first and if it doesn't work, then the 4% module replacement. However, that percentages
approach, as we've explained, is just for the
problems that the call-in technicians couldn't
solve. It doesn't mean that 99% of the time
that these two fixes will solve the problem.
However, the 95% fix (or some percentage
above 50%, anyway) becomes what is known
as the pattern failure.
Figure 1: Example of a statistical analysis of reported repairs for a no-heat electrical problem.
Another example of the use of fix percentThis may be a convenient way to jog your thought processes, but you still have to go through
ages is the Common Replaced Parts chart prodiagnostics to confirm the cause.
vided by Mitchell's Pro Demand online sys-

ALSO INSIDE THIS ISSUE:
THE VEHICLE'S NOT THE SAME & THERE'S A NEW HVAC OPERATING SYSTEM ......................................................... 4
HOW MUCH OIL SHOULD THE SYSTEM HAVE?........................................................................................................................ 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

February 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.

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Mitchell Pro Demand

THE BEST APPROACH TO A/C ELECTRICAL & ELECTRONICS DIAGNOSTICS

Table of Contents for the Digital Edition of MACS Service Reports - 2016