Craftsman 28906 Professional Shop Manual page 21

CRAFTSMAN
Electronics
The outdoor power equipment has historically had rela-
tively simple electro-mechanical controls. Customer
expectations and regulatory demands continue to drive
change in the industry, while electronic controls have
become relatively inexpensive.
In many cases, electronic controls can simplify a sys-
tem that would otherwise be very complex. Instead of
creating a huge mass of switches and relays that are
tied together by spaghetti-like wiring harness, sensors
(switches) in an electronic system send signals to a
processor. These input signals are processed by a
control module that produces outputs.
Outputs can include power to run an electric PTO
clutch, a trigger signal to a starter solenoid, or the
grounding of a magneto to turn-off an engine if an
unsafe condition exists.
Most electronic devices are quite dependable, but they
are vulnerable to things that simple electrical devices
are not bothered by. Examples include:
EMh Electro-Magnetic Interference is created by
electronic "noise". This noise is created by igni-
tion systems in general with non-resistor spark
plugs being especially "noisy". Alternators, and
even power passing through wires can also gen-
erate EMI. Countermeasures against EMI
include metal shielding (take a look at the igni-
tion system on a fiberglass-bodied Corvette),
and filtering devices built into vulnerable compo-
nents. Something as simple as putting non-
resistor spark plugs in a machine with electronic
controls can disable the controls.
Voltage Spikes: A dramatic increase in voltage
will damage many electronic devices. Such
spikes may be caused when jumper cables are
disconnected or a voltage regulator fails. Some
early automotive systems could even be dam-
aged by personal discharge of static electricity.
Most are better protected now.
Low Voltage: Many electronic devices simply
stop working if system voltage falls below a
given threshold. If a 12 volt system is run at 11
volts with a failing alternator, electronic controls
may stop working.
Bad Grounds: Bad grounds can reduce the
effective system voltage, create resistance and
heat, and send false signals. This is the single
most common breeding ground of electronic
gremlins.
Heat and Vibration: Heat and vibration are
hard on most mechanical devices. The same is
true of electronics.
Moisture: Moisture causes a nasty combination
of corrosion and shorts. Corroded connections
and wires create resistance that results in low
voltage and ground issue. Many electronic com-
ponents are "potted" or encased in a sealant that
protects them from moisture. They are still vul-
nerable to bad inputs caused by corroded exter-
nal connections and damaged switches.
Improper Tools: Some test lights can over-
load electronic circuits.
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