Testing Switches - Craftsman 28906 Professional Shop Manual

ELECTRICAL SYSTEM
16.
Voltage drop on a good circuit should be less
than 0.1 volts. A voltage drop reading on the
meter of greater than 0.2 volts indicates a fairly
substantial problem that demands attention.
As an example, if the tractor had a slow-turning
starter, the ground-side voltage drop measured
below 0.1 volts, and there was not a parasitic
load on the engine (e.g. PTO clutch that is not
fully disengaged), it would be logical for the tech-
nician to check voltage drop to the starter.
See Figure 7.51.
Figure 7.51
With the starter motor engaged, the voltage drop
reading here is nearly 0.6 volts, indicating a seri-
ous problem in the heavy-gauge circuit between
the starter and the battery.
NOTE: Checking voltage-drop at various points
along the circuit can help pin-point the problem.
Check voltage-drop at the output lug on the
starter solenoid:
If there is a significant difference, the problem
lies between the lug on the solenoid and the lug
on the starter.
If there is little change, the problem lies further
up-stream.
Check voltage drop at the input lug on the sole-
noid. If there is significant difference between
the reading here and the reading at the output
lug (greater than 0.10 volt), then the contacts
inside the solenoid may be burned. If there is lit-
tle change, the problem lies further up-stream,
between the battery and the solenoid.
Results may be cross-checked by testing volt-
age drop across the two posts of the starter sole-
noid while cranking the starter motor.
Testing switches
• Refer to the "Components" section of this chap-
ter that describes the function of the individual
switches to be tested.
Switches can be tested "hot" by looking for volt-
age at the appropriate posts. This is not defini-
tive, since the source of the voltage is not always
confirmed. Checking for voltage does not work
on switches that work by providing a ground
path to the magneto primary windings or a solid
state control device.
,
The most valid way to test switches is a continu-
ity test.
Understand the internal functions of the switch.
Key switches and PTO switches can be fairly
complex.
Isolate the switch from the rest of the circuit.
,
Test each pair of terminals for continuity in all
modes of switch operation: at-rest, and actu-
ated.
Many switches on Cub Cadet equipment are
typed by their at-rest state: Normally Open, Nor-
mally Closed, Common.
Normally Open (N.O.) contacts do not complete
a circuit when the switch is at-rest (plunger
extended). They close to complete a path
through the switch when the plunger is
depressed.
Normally Closed (N.C.) contacts complete a cir-
cuit when the switch is at-rest (plunger
extended). They open to break the path through
the switch when the plunger is depressed.
Some Cub cadet switches contain more than
one pair of contacts. The same switch housing
can contain normally open and normally closed
switch elements.
When testing a switch that contains more than
one set of contacts (elements), the male spade
terminals associated with Normally Closed con-
tacts will be stamped "N.C."
35