Tach problems

[NM Wolffman]

Help!! Need help troubleshooting a tach issue on my 1989 Evinrude 120 hp. Tach works fine on initial startup and for several minutes after startup. However, after the engine runs for 3-4 minutes, the tach goes to zero when the engine is reved up. When the engine is returned to idle, the tach again works fine. I have checked the connections under the dash (disconnected, cleaned and reconnected all the connectors). It has been acting up for a while (floats around while cruising around the lake), but has got worse. I never really worried too much because it was propped correctly and there was no worry of over-reving. I just moved from SC (sea-level) to NM 5000 foot elevation and I have to re-prop due to the altitude. I need an accurate tach to ensure I get the right prop.

Does this sound like a bad reg/rectifier? Is there an easy way to check in the driveway? Any suggestions/help will be greatly appreciated.

[RUSTY63]

Welcome to the BBC.

To check the Tach on a carb motor on the hose in the driveway...(THIS WILL NOT WORK ON A SYSTEM CHECK TACH...CARB, DI, OR E-TEC)

Remove the Gray and Black wires from the back of the tach, hook a DC volt meter lead..Red lead to Gray wire, Black lead to Black wire, start motor (with water of course) volt meter should read 9 volts at idle.

If not, there is another test but....It should be performed by a dealer.

[NM Wolffman]

Thanks for the reply. I will check it tomorrow. Maybe I'm getting ahead of myself, but if I get 9 volts at idle does this mean the reg/rec is good? Is the voltage supposed to change as the engine is accelerated?

[RUSTY63]

If you have 9 volts at idle the rectifier is doing its job.