Diagnostic Techniques; Electronics - Cub Cadet RZT-S Professional Shop Manual

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RZT-S

Diagnostic Techniques

NOTE: Electrical diagnostic procedures and tools are the same for all Cub Cadet and MTD mowers. This sec-
tion is written in a way to provide basic trouble shooting skills that can be used on any mower.
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.
Although 80% effectiveness is not bad, the remaining 20% of the diagnoses are the really difficult ones that can
devour the same amount of time as the easier 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 Current. 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 the tools and techniques for diagnosing electrical problems on outdoor
power equipment.

Electronics

Outdoor power equipment has historically had relatively simple electromechanical controls. Customer expecta-
tions and regulatory demands has driven 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 creat-
ing 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 pro-
duces 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 electric "noise". This noise is created by ignition systems
in general with non-resistor spark plugs being especially "noisy". Alternators, and even power passing
through wires can also generate EMI. Countermeasures against EMI include metal shielding such as in
the ignition 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 sys-
tems could even be damaged by personal discharge of static electricity. Most are better protected now.
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