One end is connected down at the front of the engine, right side, near outer fan blade. Unscrew that knurled end. Attach a cordless drill and see if tach needle still bounces around.
If needle bounces, remove end at tach and pull the cable out of the housing. Clean with a rag and lube with some white lithium or similar lubricant. Then reconnect to tach and try tuning with drill again.
If it still bounces I would spritz a small amount of silicone lubrication into tach. DO NOT OVER SPRAY.......Let tach set face down for a couple of days.
After tach has set if it still bounces when attached to drill or engine you need tach.
I just did this process and installed the tach day before yesterday. It WORKS!!
A new tach is $110. That bulb that lights up the tach, not the oil or charge light, cost $10. Most expensive 12volt, 2 watt bulb I've ever seen!!
You can loosen either or both ends and not damage the tach.. It's a cable in a housing and attached at both ends with a threaded fitting.