There are two coils in the solenoid, a pull in winding and a hold in winding. The pull in winding is connected between the S terminal and the solenoid output post which is connected to the starter motor. The hold in winding is connected between the S terminal and the solenoid case. Since the starter motor has very little resistance, when the S terminal is energized both coils become electromagnets and draw the plunger into the solenoid. When the plunger has moved in far enough to make the connection of the switch the pull in winding now has battery voltage on both ends and no longer serves as a magnet. The hold in winding continues to keep the plunger in the solenoid counteracting the spring pressure.
In the case of this starter the plunger is keeping the solenoid switch contacts closed after the power is cut off to the S terminal. Since the pull in winding is grounded through the starter motor, as long as the switch in the solenoid is closed the pull in winding will back feed to the S terminal and there will be voltage there. Since both windings share a common connection at the S terminal the voltage being back fed through the pull in winding now also goes though the hold in winding to ground. Total voltage drop across both windings will be equal to battery voltage. Since the S terminal is now close to the center of the complete circuit the voltage at that terminal will be somewhere around half of the battery voltage.
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Today's Featured Article - Talk of the Town: Repairin a Gas Tank - by Staff. This interesting discussion is from the Tool Talk Discussion Forum. Remember that safety is your first priority - make sure you know what you are doing before attempting a potentially dangerous activity!
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