I just fixed the same problem with the tach on my 82300Sd and my buddies 78300SD (we learned the trick from threads on this web-site)
On the drivers side fenderwell there is conector about 1-2 inches tall. It's cylindrical like a minutre garbage can. Unscrew this(it should have a plastic leash that keeps the top from falling once unscrewed.) and you should 6 or 7 pin connecters. Pull the connector assembly out of the cap. Place a piece of foam, a cigarette butt, anything that is pliable that will force the connectors to make better conatact when you screw it back on.
You can also go to search on this web-site and type in tachometer. That's how we fixed ours.
Best of luck