Coldforged.org tells us a story of his recent car troubles, which inspired this short and generalized post you’re now reading.

Why won’t a car start?
(Please keep in mind my explanation is inherently simplified)

Sometimes, the battery has lost its charge for a variety of reasons.

Sometimes the wiring of various parts of the car’s systems leads to increased resistance of current flow. Terminals sometimes corrode or work themselves loose.

Sometimes, the ignition switch fully or partly wears out. Most ignition switches are multi-pole, and when they work properly, they energize multiple circuits simultaneously. One of these circuits is the starting system.

Sometimes, the starter relay malfunctions. Usually, this doesn’t allow the passage of high-amperage battery current to the starter. Sometimes the relay’s contacts remain together after the ignition key is returned to the ‘run’ position. I’ve replaced a number of starter relays, they fail often.

Sometimes the starter itself wears out. Right now I forget the specialized name of the mechanism in the starter that extends the starter gear into the flywheel. When the ignition key is turned to the ’start’ position, this energizes the starter relay allowing battery current to flow to the starter, turning it. The unnamed starter gear extends into the flywheel’s ring teeth to start turning the engine’s crankshaft. When the ignition key is returned to the run position, the relay is supposed to close and the starter gear is supposed to retract an inch or two, disengaging it from the teeth of the flywheel, as well as to stop turning.

Sometimes, when the internal starter mechanism that extends and retracts the starter gear from the flywheel wears out or becomes corroded, or the relay malfunctions, after the key is returned to the ‘run’ position because the engine has started, the starter gear remains engaged in the flywheel. In this condition, the engine is turning the starter beyond its normal speed, because the car’s engine is forcibly turning it. You don’t want to drive a car with the starter gear still engaged in the flywheel. It’s not designed to turn that fast, there’s too much stress on its bearings. A non-normal sound is usually audible. If you hear this, turn off the engine immediately!

Initially, ColdForged’s starter relay likely had pitted contacts, or some similar malfunction that wouldn’t allow current to pass from the battery to the starter. Perhaps wiring terminals were loose or some wires had too much resistance. It’s really not possible to say precisely without seeing the car during the initial malfunction. Complicating the issue was the intermittent nature of the car’s starter problem, sometimes during troubleshooting, everything works fine. A mechanic’s experience of what normally wears out in the starter system comes in handy.

When ColdForged drove his/her car to the parts house, I think the starter gear was extended into the flywheel, this literally killed the starter, and likely caused all the smoke he/she saw and smelled.

(I don’t think or read about ‘this stuff’ as much as I used to, my terminology is passing from memory)