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The starter motor is what sealed the fate of early electric cars. Ironically, electric cars inspired the starter motor. Early gas cars were grand and fast, but unpopular, since starting them usually involved one person in the driver's seat adjusting the spark and gas while someone standing in front of the car (with one arm honed to muscular perfection) spun the whole engine by hand with a crank. And there was always the chance of a backfire breaking said person's arm.

Yes, most early cars were electric because it was convenient and it worked. Ambulances were even electric. Then gas cars borrowed a trick from electric cars, namely the motor and battery. Someone bolted the motor to the engine in a way that it would rotate the flywheel at a suitable starting RPM. Use the battery to power the motor and provide spark, and presto, electric cars were forgotten for decades. Gas powered cars could now be started easily by anybody.

There are two types of modern starter motors I've seen:
Ones with attached solenoids and ones without.

Starters without attached solenoids usually only have one wire going to them, a thick high amp wire coming from the starter relay (and a ground, of course). They are geared so that when activiated the motor shaft will spin within the pinion which causes it extend long enough to contact the engine's flywheel and spin the engine. When the starter motor stops the still-spinning pinion spins back down the motor shaft away from the flywheel.
I've noticed these type to be a bit more reliable and safer to work with.

Starters with an attached solenoid look just like a normal starter but with a cylinder stuck to the side, near the pinion. These type have two wires, a live high amp wire from the battery and a smaller wire from the starter switch. The small wire activates the solenoid, which connects the high amp wire to the starter motor while also (usually, your starter may vary) extending the pinion along the starter motor's shaft into contact with the flywheel. This is great, but...

Make DAMN SURE you unhook the battery when changing this type of starter. Since there's no starter relay to turn off that hot wire, it is always live and will weld itself to the frame, oil pan, or whatever you touch it to.

Also, these types don't seem to wear as well. Especially on trucks and big engines that don't have much stuff around the starter to shield from water splashes and road muck. Since the solenoid is in charge of engaging the motor to the engine, there’s more moving parts to break and wear down.

I've replaced my solenoid-based starter twice in two years. Last time it decided it was going to keep trying to start, regardless of what position the key was in. If yours does this, TURN THE KEY OFF. The solenoid is probably stuck and the starter will burn out and/or eat up your flywheel. Then quickly unplug the battery. Try lightly hitting the solenoid with something. For my F-150 I stuck a long wooden dowel alongside the engine and tapped it through the hood. That will usually free up the solenoid (you will sometimes hear a 'clunk' sound when that happens), now you can start it and drive to the auto shop to get a new one. Sometimes you can get the solenoid by itself, but you'll probably be stuck buying a whole new starter.

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