Usually only the tachometer needs to be changed out, but not always. You can install the new tachometer and recheck vs. a digital tachometer. In the back of your manual there is a test procedure for testing the sender, check it out. Also check your electrical harness, where it connects to the gages, make sure all connections are clean and corrosion free, dielectric grease works great, also check electrical harness connectors from the panel to the main harness and from the main harness to where it connects near the port aft engine mount; if the connectors are bad, cut them out butt splice and heat shrink all.
If all your connectors are corrosion free, pack them with dielectric grease and plug them back together, use wire ties through the wires pulling both connector half’s towards each other, helps keep the moisture out and keeps them from pulling apart.
You will find most engine manufacturers do not warranty the instrument panel, guess they figure there are ten ways from Sunday to screw them up upon installation; just in case your are wondering, none of them are water proof, water resistant is a stretch…
When I first started working for Yamnar in 1993, I replaced a mechanic who just completed a twin re-power on a 60’ catamaran, customer called two weeks later complaining of faulty instrument panels, Gage lens fogging up. I went to examine the panels, while there I met the vessel project manager, young fellow, proud of his charge. I mentioned for a boat in the throws of a full refit, the boat looked nice and clean, he remarked, " Yes, I wash it every week and wax it at least once a month." I asked if he covered the steering station before hosing it down, he said, “no, I soap up everything with a boat brush and hose it down real good, no need to cover it up, its Marine.” Needless to say, $800 later had new instrument panels and a better understanding of water and electricity.
Not to be out done, a month later the same skipper called me, Hey Marty, can’t get these new Yanmars to stop running when I hit the kill button… I replied, did you wash the engines,…Err…Yep, your shorted out the stop relays.