The solenoid is basically a great big electromagnetic switch. When you hit the
key, the solenoid does two things. It connects the battery directly to the
starter (acting like a relay) and it throws the small gear on the starter down
the shaft to where it can engage the ring gear on the flywheel.
Sometimes the lever/gear can jam and not want to move, or the gear will jam
against the flywheel (usually when the teeth get worn). That's why whacking it
with a hammer can help (if that's the problem).
Gordon Glasgow
Renton, WA
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