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9 -36
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.
To check the solenoid, disconnect the cables to the
starter motor, turn the ignition switch to the start
position, and listen for the solenoid to snap inside
contacts closed.
If no snap is heard, check across the coils with a
voltmeter. The voltage should read 12 volts with the
ignition switch in the start position. If no voltage
appears, the defect is in the start circuit.
If the voltage is correct, turn off the power and check
continuity of the coil with an ohmmeter. If the coil is
open, the solenoid is defective and must be replaced.
If the coil has the proper voltage applied, and the
continuity check indicates the coil is intact, the solenoid
plunger is stuck or the contacts are welded shut and
the solenoid must be replaced.
If the solenoid snaps shut, but the start does not
operate, check across the large contacts with an
ohmmeter. If there is no continuity when the solenoid
snaps shut, the contacts are defective and the solenoid
must be replaced.
9.7 FUSES
Fuses are connected in electrical circuits to protect the
circuits from damage due to overload or short circuits.
Fuses are a "weak link" in the circuit. They contain a
metal link designed to melt when a certain current
value is exceeded thus opening or disconnecting the
wiring. Once a fuse blows or melts it must be discarded
and replaced with a new fuse of the same value.
Since the function of the fuse is to protect the circuit,
NEVER attempt to defect the protective device by
bridging or replacing with a device of a higher current
rating.
Electrical testing of these devices is simple. Since the
device either conducts current (and is therefore
functioning) or the device is open and is therefore
defective. Use an ohmmeter to check for continuity.
9.8 DIODES AND RECTIFIERS
Diodes are solid state, semiconductor devices. They
contain no moving parts and conduct current better in
one direction than the other.
Diodes allow current to flow through one circuit without
"backing up" into another. In engine alternator circuits,
a diode is used to convert current which flows back and
forth (AC) in a circuit to current which flows only in one
direction (DC). A device which converts alternating
current to a direct current is called a RECTIFIER. A
diode is one type of rectifier.
To check a diode, isolate if from the circuit by
disconnecting one end. With a multitester set on the
lowest ohms scale setting, measure the resistance in
one direction, reverse the test leads, and measure in
the other direction. Readings should be high in one
direction and low in the other. (If the readings are low in
both directions, the diode is shorted, and if the
readings are high, the diode is open.) If the readings
are the same in both directions, the diode is defective
and must be replaced.
IMPORTANT:
Diodes are marked to indicate polarity (a
band on one end, an arrow on the side, or they fit on a
holder only one way.
Rectifiers
A battery is charged through the use of an alternator
located in or on the engine. A charging circuit contains
a rectifier because alternators produce alternating
current (AC) and batteries require direct current (DC)
for charging.
The rectifier may be built into the engine or it may be
an external part. It may also contain a regulator to
prevent overcharging the battery. (Servicing of
rectifiers built into the engine should be done by an
approved engine manufacturer’s service center. Such a
service center has access to the information and parts
required to test and repair or replace engine
components, including rectifiers and regulators.)
Units that contain both a rectifier and regulator are
tested in a working circuit to make sure the regulator
portion of the device is operating.