Cub Cadet ISeries - Manual (Page 126)

ELECTRICAL SYSTEM
120
ELECTRICAL DIAGNOSIS
NOTE: Electrical diagnostic procedures and
tools are the same for all Cub Cadet and MTD
tractors. This section is written in a way to pro-
vide basic trouble shooting skills that can be
used on any tractor.
With a basic understanding of the behavior of elec-
tricity and the tools used to measure that behavior, a
technician can be about 80% effective at finding electri-
cal 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 summa-
rized 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 the following web site:
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
relatively simple electro-mechanical controls. Cus-
tomer 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
system 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 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.