If I understand correctly, the tach reading is high, but steady, not a wandering needle? If so, my guess would be the dip switches getting corroded. The dip switches on the tach are set for the number of poles in the alternator. When I switched to a high output alternator, which had a different number of poles, the tach read high, until I changed the correct dip switch setting. The trim pot is only for fine adjustment and would only be good at one particular rpm. It's possible that there's some corrosion in there.

The dip switch is under a rubber cover on the back of the tach. Maybe note the current setting, then flip the switches around a bit or better yet, spray some DeOxit or similar contact cleaner in there. Somewhere in the technical section there's a document that shows the different types and settings, but as long as you put it back the way it is, it should be OK.

If the needle was jumping around, I'd suspect a loose wire.

Is your charging system working normally? On some alternator set-ups a field wire or other alternator failure can throw the tach way out (mine goes low in that scenario).

I had the EXACT same problem when I first put the boat in last year. Because I saw some fogging in the tach, I came to the same conclusion as you did relative to point corrosion. I spun the potentiometer 90 degrees clockwise, 90 degrees counterclockwise, and then back to it's original location. Everything was back to normal after that.

Good luck!

Joe Welna

On my tach, the trim pot is accessed via a small hole; you'll need a small screwdriver (jewelers screwdriver). They cleverly placed the hole under the metal bracket that holds the tach in place; when I had mine off, I drilled a hole in it so I don't have to take the tach off to make the adjustment. Note there were a few different tach used, so yours may be different than my picture.

It's the potentiometer that calibrates the tach to the engine RPM. Might be a good idea to have an optical tach handy so that if a few twists on the pot does fix the problem you can calibrate it accurately to the engine.

Sam,
It would also be a good idea to twist back and forth the A-B-C-D selector switch on the back of the tach, if yours has one. That is another spot that can develop corrosion, and needs to be "exercised".

I'm not the membership person, but it looks to me like your C36IA Membership (capital M) is valid until Feb 17 2013.

You can buy handheld optical tachs on eBay for $15. I bought one and a heat gun from a guy on there for $30 all in. Both very handy to have on a boat.

did you try rotating the A, B, C, D selector as Tom Soko mentioned? My tach has the same 4 settings, but you select them via dip switches, very small switches arranged horizontally under a black cover. You can see the black cover in the pic I posted earlier. If those selections are corroded and therefore incorrect, the tach would read way off and you most likely could not correct it with the trim pot.

I'm sure by this point you've also tried swearing at it. That's usually one of my first techniques; doesn't necessarily work but makes me feel better:)

Sam,
As a replacement for the drop of red laquer, raid the Admiral's make-up kit, and use a drop or two of fingernail polish. It's basically the same stuff. You can use fire engine red if you want, or go with something more "subtle" !!!

I realize this is an old thread but the same thing happened to us yesterday. Motoring in dead calm at about 2600 rpm-6.5 knots. When I glanced at the instruments, rpm's had jumped to above 3000. More throttle gave me close to 4000 and idle ran around 1200.

I figured I should not take a chance so we ran slow for the rest of the day.

We have a 375, so I wasn't sure that moving the tachometer selector back and forth would apply. Turns out it did the trick. Only 4 years old in freshwater but I guess corosion does not care.

Always finding good information in the forum.