1986 300E

My aux fan is not coming on @ 100+ degrees (AC not working right now, so should not be a factor).
Used to kick in @ 92 degrees (cool harness), but does not come on now at all (with or without cool harness).

Jumped the terminals at the temp sensor (zero resistance)- Fan came on fine.

Blindly assumed it *MUST* be the temp sensor itself (yes, I have a resistance meter...), so I pre-emptively replaced it.... and no good!

My new theory is that whatever component measures the resistance value must be faulty.

Now which component would that be? Is this functionality part of a multi-function relay?

Other ideas?

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1986 300E 5-Speed 240k mi.