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I've got an 83 Bronco with some issues. Although minor, it is very annoying to not have the tach working.
Sometimes it bounces around like a mindless maniac, sometimes it just flops all of the way over to 6 grand and just stays pegged, and often, it just lay there, uselessly.
I did a search on the topic, but couldn't find anything.
there is no cable for the tach. Check the electrical connections. If you pull the gauge cluster out, you will see an edge connector on the cicuit board. Try just removing and replacing this, maybe hit it with a little contact cleaner, and try the tach.
Mine did the same thing. I finally found a replacement in a junkyard. I tried cleaning the terminals, tracing the wiring, even poking around inside the tach, but it was just fried.