Tach Problem on Evinrude

I know this has been covered a million times on here, and i did the search but sometimes problems can be unique.

boat- 1989 Larson Senza 165
engine- 1993 Rude 120hp

Problem- tach never worked properly before on previous engine 90hp... current condition - itermitten tach now (works but fails at WOT)

I checked all wires according to teleflex, it was set to 6.
the tach will work up to about 3 / 3500 grand - then at WOT it will fail.
if i slow it down the tach picks back up.
the tach does zero out when i turn the key to on position.

any idea's, what should i check first... does this sound like a ground problem to you, i have three black ground wires leading off the tach.

should i sand down / scuff terminals to get better conductivity?
should i follow ground wires to check actual grounds?
does this sounds more like a rectifier problem?

any help would be awesome! thanks fellow boaters!

Re: Tach Problem on Evinrude

Additional Information for Evinrude/Johnson engines:
1. If tachometer does not work properly on the Gray wire on
engines with water cooled voltage regulator, bypass the regulator
and attach to yellow/gray wire from the alternator to the rectifier.
2. On occasion, the engine's main ground terminal (from main
battery ground to the engine) has paint residue and does not
make a proper ground. The nut, washer, and all connections
should be removed, cleaned with a wire brush, and reassembled.
3. Some Evinrude/Johnson 48 (50), 88 (90) &112 (115) "SPL"
engines may require the addition of a 1/2 watt, approximately
2000 ohm resistor, connected between the tachometer's "Send"
& "Ground" terminals. If the pointer drops back to "0" as the
engine accelerates (around 3500 rpm), the resistor is needed. If
the problem still persists, the addition of a Bombardier Accessories
voltage regulator is required.

Re: Tach Problem on Evinrude

what I have learned above all else about owning an old boat: first step in trouble shooting electrical problem of *any* kind is to brighten the connections, by which I mean, take some sandpaper to 'em if ya have to, but make 'em shine.

yeah, I'd say you're headed in the right direction!