A typical solenoid like you describe uses the two large terminals to provide a path from the battery to the starter motor with the disk in the solenoid acting as the switch.
The two smaller terminals, one provides a means to energize the switch and the other provides temporary power out of the solenoid. The terminal that has the wire connected inside is normally connected to two electrical coils in the solenoid. One coil is grounded to the solenoid case and the other is connected to the large terminal that goes to the starter motor. The Pull In coils is the one that is connected to the large terminal. The Hold In coil is connected to the case ground. When you apply power to the small terminal both coils are energized and pull the solenoid plunger to engage the drive in the ring gear and then also push the contact disk to close the circuit from the battery cable terminal to the starter motor terminal and also the temporary power terminal. Once the circuit is closed between the battery and the starter motor the pull in coil has the same voltage on both ends of the coil and no longer functions as an electromagnet. The solenoid plunger is now held engaged only by the hold in coil. When power is removed from the small terminal the electromagnetism drops off in the hold in coil. As the power is removed the collapsing of the lines of force in the winding create a small bit of current in the coil. This current is now applied to the pull in winding as the circuit between the battery terminal and starter motor terminal is open and the pull in coil is once again grounded through the starter motor. The magnetic lines of force in the pull in coil are opposite those of the hold in coil and cancel each other out insuring there is no longer any magnetic pull to keep the solenoid plunger from returning to the original position.
The other small terminal that provides temporary power while the solenoid switch is closed is normally uses for gasoline engines to provide battery voltage to bypass an ignition resistor. On diesel engines it is sometimes used to allow an electrical ether assist to be used only when cranking the engine. On propane engines it is sometimes used to energize a primer valve while cranking.
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Today's Featured Article - Listening to Your Tractor - by Curtis Von Fange. Years ago there was a TV show about a talking car. Unless you are from another planet, physically or otherwise, I don’t think our internal combustion buddies will talk and tell us their problems. But, on the other hand, there is a secret language that our mechanical companions readily do speak. It is an interesting form of communication that involves all the senses of the listener. In this series we are going to investigate and learn the basic rudimentary skills of understanding this lingo.
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