The starter in a vehicle, be it car or truck, operates from the power on board. Most automobiles use a 12-volt system, and so the starter requires 12 volts in order to function properly. There are two rather large wires attached to the battery of all vehicles. One is a negative, or ground wire, and usually travels directly to the engine block where it is connected with a large bolt. The other wire is the positive, or power wire. That wire travels down and is attached to the positive terminal of the vehicle's starter. Wires from the ignition switch inside the passenger compartment also run to the starter so that it knows when to turn and when to idle. Otherwise, the starter would be running all the time and would burn out quickly.
Once the ignition key is turned to the "start" position and power reaches the starter, it activates a solenoid inside the starter which then pushes on the drive gear. When starters go bad, the culprit is many times the solenoid. Like anything else with moving parts, they wear out over time and require replacement. The same solenoid idea can be found in electric door locks. When they receive power, they expand and force the locking mechanism either in or out.
The starter drive gear is basically a thick metal rod with a gear attached to the end of it. In a resting state, the drive gear is almost all the way inside the starter. When power is applied and the solenoid activates, the drive gear extends toward the flywheel. The flywheel has corresponding teeth which match the gear drive on the starter mechanism so that when it comes out, the flywheel turns the engine and allows it to start. If the gears on the end of the drive become rounded or mangled, the result will be a loud noise and intermittent or no turning of the flywheel. That can happen when the gear drive no longer comes out all the way due to a bad solenoid, or if there is a "flat" spot on the flywheel with no teeth.
Before working on the starter of any vehicle, disconnect the battery completely so that the large hot wire connected to the battery won't arc on anything and damage the electrical system. Also, starters can be very heavy, so be careful when removing it that you are out of the way. If the starter comes out and drops, it can cause injury. Also, be sure that all the bolts are seated completely when putting a new starter in. If the starter is allowed to walk around, the gear drive may not fully engage and can become damaged.
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