12 - 41
NOTE: Check that the connections to the switches are
secure and that a switch is being activated properly
before performing electrical test on switches. (Safety
switches on speed selector and clutch levers may be
out of adjustment and not activating.)
IMPORTANT: When checking switches, remove them
from their respective circuit by disconnecting the wires
from the switch at the connector(s). Damage could
result to the meter or machine components if switches
are left in.
Normally Open Switch
To test a normally open switch (key, headlight, safety,
or seat) connect the ohmmeter across the switch
terminals. Meter should indicate open circuit (infinite
resistance). Activate the switch. The ohmmeter should
read up scale to zero resistance (Close Circuit). This
indicates the switch is operating properly. Also check
from each terminal to the switch case (if case is metal).
reading should show infinite resistance indicating no
short to ground.
Variation from test results described indicates a
defective switch.
Normally Closed Switch
To test a normally closed switch connect the ohmmeter
across the switch terminals. Meter should indicate a
closed circuit (zero resistance). Activate the switch and
the meter should move to open circuit (infinite
resistance). Check from each terminal to ground
(switch case). Meter should show open circuit (infinite
resistance).
Variation from test results described indicates a
defective switch.
Ignition Switch
NOTE: Refer to the wiring diagram of the unit involved
to determine switch functions and test using the meth-
ods described.
The ignition switch incorporates a number of functions,
although not all functions are used on all equipment.
The switch has three positions: OFF, RUN, and a
momentary contact START position. Use an ohmmeter
to check the continuity of the switch in each position.
OFF Position - Should be continuity between contacts
G and M. These connections ground the engine
magneto and stop the engine in the OFF position.
RUN Position - Should be continuity between contacts
B and A. These connections supply power to the rest of
the wiring harness. Connections G and M open to each
other.
START Position - Hold switch in START position while
testing. There should be continuity between contacts
S1 and S2. These connections apply power to close
the solenoid contacts and operate the starter motor.
In addition to the above test, place the switch in the run
position and check between each contact and ground
(metal case) to be sure no terminals are grounded. If
the switch is operating properly, there will be no
continuity between contacts other that those described.
12.5 SOLENOID AND RELAYS
Solenoid and relays are both magnetically operated
devices. Both devices operate on the principle that
passing a current of electricity through a coil of wire will
create a magnetic field strong enough to attract a piece
of iron or steel. Each device uses this principle in a
slightly different manner.
Relay - A basic relay consists of a coil of wire wound
around a soft iron (magnetic) core. When current is
passed through the coil, the core is magnetized and
pulls down on a magnetic lever. The lever in turn is
attached to several switch contacts which open or
close other electrical circuits. In this fashion, a small
current can control one or more larger electrical
currents and actuate several other devices. In most
cases a relay contact moves only a fraction of an inch
and the magnetic pull is small.
Solenoid - A basic solenoid consists of a coil of wire
wound around a hollow tube. A magnetic core slides
inside the tube. When current is passed through the
coil, the core is pulled into the solenoid with
considerable force. With proper design, a solenoid can
exert considerable force over a distance of several
inches. A solenoid can therefore, pull a lever, close a
heavy contact, or perform other jobs that require a
straight line pull.
If a relay or solenoid fails to operate, the cause may be
either electrical or mechanical.
To check electrically, connect a voltmeter across the
coil of the device and activate the circuit that operates
the relay or solenoid. If the meter indicates no voltage
is applied, the cause is in the control circuit.
If the meter indicates proper voltage across the coil but
the device does not function, remove the power,
disconnect the wiring and check the continuity of the
coil with an ohmmeter. The meter should indicate
resistance, in the order of 3 to 5 ohms, if the coil is
intact. A high resistance indicates an open coil and a
defective device.
There are also a number of mechanical problems that
may cause the problem.
The starter solenoid in the Gravely equipment is a
sealed unit used to actuate the starter motor on the
engines. These solenoids may have three or four
connections. The two large connections carry high
current to operate the starter motor. The small
connections are connected to the coil and carry the
control current.
