The magnetic switch has two functions.
Motor ON/OFF
Pinion gear engagement and disengagement
Also, the magnetic switch operates in three steps when the starter is operated.
Pull-in
Hold-in
Return
If there is an open circuit in the pull-in coil, it cannot pull in the plunger, and it causes a symptom that the starter cannot be started (no operation sound of the magnetic switch).
If the main contact has poor contact, it is difficult for current to flow to the field coil and the armature coil, and the starter revolution slows down.
If there is an open circuit in the hold-in coil, it cannot hold in the plunger, and it could cause the plunger to repeatedly pop in and out.
<1>Pull-in
When tuning the ignition switch to START position, the battery current flows to the hold-in coil and the pull-in coil. The current then flows from the pull-in coil to the armature coil via the field coil, rotating the armature coil at low speed. Generation of the magnetomotive force in the hold-in coil and the pull-in coil magnetizes the pole core, and so the plunger of the magnetic switch is pulled in to the pole core. By this pull-in operation, the pinion gear is pushed out and engages with the ring gear, and the contact plate turns the main contact on.
The figure on the left below summarizes the flow of current in the pull-in step.
SERVICE HINT:
In order to maintain the voltage to actuate the magnetic switch, some models have a starter relay between the ignition switch and the magnetic switch.
<2>Hold-in
When the main contact is turned on, no current flows through the pull-in coil, and the field coil and the armature coil receive current directly from the battery. The armature coil then starts high-speed revolution, and the engine starts. At this time, the plunger is held in position only by the magnetic force exerted by the hold-in coil because no current flows through the pull-in coil.
The figure on the left below summarizes the flow of current in the hold-in step.
<3>Return
When the ignition switch is turned from START to ON, current flows from the main contact side to the hold-in coil via the pull-in coil. At this time, since the magnetic force set up by the pull-in coil and hold-in coil cancel each other out, they lose the power to maintain the plunger. Therefore, the plunger is pulled back by the return spring, and the main contact is turned off, stopping the starter revolution.
The figure on the left below summarizes the flow of current in the return step.