the tach on my 1973 Johnson 65hp stopped working. tried a new tach, still didn't work. Service guy said I need a new rectifier. Motor seems to run pretty good with the "bad" one still on.
Rectifiers can be tested. There are four diodes in there. All four are tested in both the forward and reverse direction (pass current in one direction only) All four have to pass. That's 8 tests. 4 forward and 4 reverse.