MACS Monthly Newsletter - 2017 - JAN8

Dave Hobbs

ground?
If you
knew (or guessed)
that the diode would
forward bias a short
around the relay's
coil you would be
Figure 14: Relays have transitioned correct.
Can you
from using diodes to resistors for
spike suppression. There are minor guess what would
technical considerations to consider happen next? Blown
one relay over another beyond the
terminal position and coil resis- fuse? That would be
tance. Relays still using a diode nice. Blown relay dihave to be chosen based on the schematic shown above. The positive ode? Maybe. Damcircuit in the vehicle can NOT be aged module? Murconnected to the anode. A diode can
Law would
forward bias a low current fly back phy's
voltage just fine. It can not forward seem to present that
bias a larger current . Damage to
the relay, a flow fuse, etc. are the as a distinct possibilleast of your worries.
ity!
A final word of
caution regarding jumping or activating
modules or relays in general; some modules
use transistorized outputs that are current
limited and reverse polarity protected from
well-meaning techs who love to use jumper
wires and Power Probes. Other modules get
one sniff of reverse polarity or over current
and the magic smoke gets released. Use EXTREME caution when attempting to activate
a driver circuit by back probing a module
with power or ground. I personally like to
disconnect the modules and back probe harnesses with a fused ammeter or Power Probe.
As I discovered and reported in a recent MSR
covering working with complex schematics,

Figure 15: Replacement Relay Nightmare
Would you guess a short circuit is about to happen as soon as the module
tries to run the cooling fan with this relay from the tool box odds and ends
drawer? If the fuse and diode hold out - will the module survive?

electronic controls are getting more complex
and less robust. There is so much more electronic content that can be damaged compared
to vehicles built a few years ago that would
have simply blown a fuse if you probed the
wrong polarity into the wrong circuit. On
newer vehicles, probing the wrong pin on a
B.E.C. or other module, installing a 'known
good relay' that contains a diode in an application that it's not intended for can have
some very disastrous results. My final advice to all my fellow MACS members (and
the one I see in the mirror each morning) is
continue to learn all you can about whatever
you are working on (especially electronics)
and always pay great attention to schematics
because Ohm's Law is never wrong! ■

MACS Service Reports is published monthly by the Mobile Air Conditioning Society
Worldwide. It is distributed to members of MACS Worldwide 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 Worldwide reserves the right to choose and edit all submissions.

Editors:
Elvis Hoffpauir, Steve Schaeber
Production Designer:
Laina Forcey
Manager of Service Training:
Steve Schaeber
Mobile Air Conditioning Society Worldwide
P.O. Box 88, Lansdale, PA 19446
Phone: (215) 631-7020 * Fax: (215) 631-7017
Email: membership@macsw.org * Website: www.macsw.org

Unless otherwise noted, all photos/art by author.

January 2017

MACS Service Reports

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