TM 5-3805-291-23-1
THEORY OF OPERATION - CONTINUED
0003 00
ENGINE - CONTINUED
(1)
Starting Solenoid. Starting solenoid is an electromagnetic switch which closes high-current starting motor
circuit with a low-current start switch circuit, and engages starter motor pinion with ring gear. Solenoid has
windings (one or two sets) around a hollow cylinder. A spring-loaded plunger rests inside cylinder.
Plunger can move forward and backward. When start switch is closed and electricity is sent through wind-
ings, a magnetic field is created which pulls plunger forward in cylinder. This moves shift lever to engage
pinion drive gear with ring gear. Front end of plunger then makes contact across battery and motor termi-
nals of solenoid. Next, starting motor begins to turn engine flywheel.
(2)
Windings.
(a) When start switch is opened, current no longer flows through windings. Spring now pushes plunger
back to original position while moving pinion gear away from flywheel. When two sets of solenoid
windings are used, they are called "hold-in" and "pull-in" windings, respectively. Both sets of wind-
ings take an equal number of turns around cylinder, but pull-in winding uses a wire with a larger diam-
eter which produces a greater magnetic field.
(b) When start switch is closed, part of current flows from battery through hold-in windings. Rest of cur-
rent flows through pull-in windings to motor terminal. Current then flows through motor to ground.
Solenoid is fully activated when connection across battery and motor terminal is complete. When
solenoid is fully activated, current is shut off through pull-in windings. At this point, only smaller
hold-in windings are in operation. Hold-in windings operate as long as required to start engine. Sole-
noid will draw less current from battery and heat generated by solenoid will be kept at an acceptable
level.
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