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functioning) or the device is open and is therefore
defective. Use an ohmmeter to check for continuity.
9.7 DIODES AND RECTIFIERS
Diodes are solid state, semiconductor devices. They
contain no moving parts and conduct current better in
one direction than the other. They are electrical "check
valves" and permit current flow in one direction, but not
in 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.