Merc rectifier/voltage regulator

I have a 96 40HP (4cyl) Merc outboard. Last year (08), about month and a half into the season, the rectifier burned out. Easy fix (15 minutes to install a new one), but not cheap ($160). Before it went, I noticed that the needle on my tach was jumping all over the place, especially at high rpms. When I replaced the rectifier, the tach worked perfectly. Seemed to be related to the rectifier problem, but I could be wrong.

Before putting the boat in the water last season, I installed a dual battery system. I have to wonder if that also had something to do with it.

Now, after a little over a year, my tach is starting to act up again, needle jumping wildly, but only at or near full throttle (w/ engine in gear). I hope it's not an indication that my 1 year old rectifier is on its way out.

Any thoughts? Do Merc outboards of this year/timeframe have history of electrical problems/bad rectifiers? Could it be related to the dual battery system (which works perfectly, btw)? Do I need a different rectifier, if there is even one available? Appreciate any feedback. Thx

Re: Merc rectifier/voltage regulator

The tach uses the alternator for a sender that's why it jumped around. With duel battery setups if you turn the switch with the engine running on the cheaper switches it opens the alternator circuit momentarily and can burn the rectifier out with the voltage surge.