Electrical Diagnosis - Cub Cadet 6X4 Shop Handbook

ELECTRICAL DIAGNOSIS

With a basic understanding of the behavior of electricity
and the tools used to measure that behavior, a techni-
cian can be about 80% effective at finding electrical
problems.
80% effective is not bad, but the remaining 20% of the
diagnoses are the really difficult ones that can devour
the same amount of time as the easy 80%.
Experience plays a big part in successfully diagnosing
the really difficult electrical problems. Experience
leads to greater understanding.
Two German Physicists, working independently during
the late 18th and early 19th centuries summarized
what they had figured-out about electricity into some
basic laws that can help a technician understand how a
system works or why it does not work. Their names
were Gustav Kirchhoff and Georg Ohm, and their laws
are named for them.
There are basically three things that a technician is
likely to test in trying to identify an electrical problem:
Volts, Resistance, and Flow. To help technicians
understand the behavior of electricity, this section
begins with an explanation of:
• Basic electrical values.
• Ohm's law.
• Kirchhoff's current law.
• Kirchhoff's voltage law.
• How the system is wired together.
NOTE: A graphic explanation of Kirchhoff's laws
can be found at:
http://online.cctt.org/physicslab/content/phyapb/
lessonnotes/DCcircuits/lessonKirchoff.asp
The section then continues by explaining handy tools
and techniques for diagnosing electrical problems on
outdoor power equipment.
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:
• EMI: 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 ignition system on a
fiberglass-bodied Corvette), and filtering devices
built into vulnerable components. 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 components are "potted" or
encased in a sealant that protects them from
moisture. They are still vulnerable to bad inputs
caused by corroded external connections and
damaged switches.
• Improper Tools: Some test lights can over-
load electronic circuits.
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Chapter 9 - Electrical