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I think that is an 'idle up' switch. Increases the revs a bit, like a replacement for the older style knob where you could set your own revs when cold.

I replaced mine in an hour, and I'm no grease monkey.

Jack the tail end up and check if it is open or LSD rear end. Spin one wheel. If the other side spins the same way LSD, if the wheel spins the opposite direction open diff. Can be a lottery which one...

They are known for being 'smoky' on start up, particularly in cold weather. Just unburnt fuel from engine management mucking about. Check the vacuum hoses for the brakes as they do perish.