COVER AS-HOUSING 6N9749 - Caterpillar

Illustration 16 g01073475
Solenoids combine the operation of a magnetic switch (relay) with the ability to perform a mechanical task (engage the drive). The starter solenoid produces a magnetic field that pulls the solenoid plunger and disc into the coil windings, which completes the starting system circuit. The solenoid is mounted on the starter motor so that linkage may be attached to the overrunning clutch drive in order to engage the drive.Solenoids contain two different windings for effective operation. When the ignition switch is turned to the start position, current from the battery flows through a pull-in winding and a hold-in winding. These windings contain many coils of wire and produce a strong magnetic field to pull the heavy plunger forward and engage the starter drive.When a plunger reaches the end of the travel through the solenoid, the plunger engages a contact disk that will operate like a relay and allow current to flow to the starter motor from the battery. This also serves to disconnect the series pull-in winding from the circuit and allows current to flow through a shunt hold-in winding only. Only the lighter magnetic field that is created by the hold-in winding is required to hold the plunger in position. This reduces the amount of control current that is required to eliminate heat buildup and provides more current for the starter motor.
Illustration 17 g01073476
When the ignition switch is closed, battery current flows in two directions. Current flows from the battery to the start switch and then through the pull-in winding, field winding, armature, brushes and to ground.The activation of the pull-in winding and the hold-in winding produces a magnetic force. The magnetic force pulls the plunger to the left, which moves the overrunning clutch and the pinion toward the flywheel ring gear.
Illustration 18 g01073477
When the plunger is pulled to the left, the solenoid contacts close. At this point the pinion begins to mesh with the flywheel ring gear and the pull-in winding is shorted. This causes current flow through the solenoid contacts to the field winding, armature, brushes and to ground. Current still flows through the hold-in winding to ground. The starting motor is energized. The pinion engages the flywheel ring gear. The engine begins to crank. At this time the plunger is kept in the pull-in position only by the magnetic force of the hold-in winding.
Illustration 19 g01073478
As soon as the engine starts, the flywheel ring gear turns the pinion faster than the starting motor is rotating. The overrunning clutch breaks the mechanical connection between the clutch and the starting motor. When the ignition switch is released, current flow through the hold-in winding and the pull-in winding is in the same direction, which causes the hold-in winding magnetic force to be reduced. The solenoid contacts are opened. The plunger and overrunning clutch are pulled back to the original position by the return spring force. The armature stops and the motor is OFF.Series-Parallel Systems
Machines with larger diesel engines require high power starters to provide adequate cranking speed for the