Tach Inop, '96 200hp merc. Troubleshooting tips?

[Captain Starbuck]

The tachometer on my 1996 Merc 200/140 doesn't work correctly. When the engine first starts, the tach needle moves around in about the range you'd expect at idle. Once leaving idle rpm on the engine, the needle drops to zero. I've replaced the connections on the tach with no help. Can anyone tell me what wires wires on the tach should read? I know one has +12v, one has neg 12v, and the others are mystery to me.

Thanks.

[Boat Doc']

The tach runs off the voltage regulater. Your motor probably has two. Try unplugging the gray wire from one regulater and plugging into the other regulater. This will normally fix the problem. You should also check amperage output to the battery. You may only have 20 amps instead of 40. Each regulater is responsible for 20 amps and one of them drives the tach thru the gray wire. To answer your question black is ground, purple is 12 volts with key on and gray is sense wire. At the lowest voltage setting on a meter you should see a linear change in voltage as the throttle is increased.

[Draggin' Bait]

When I started having tach problems on my 1996 40 hp Merc it was the rectifier on the engine. Tach would wander all over for awhile, then just drop to zero. Larry

[Captain Starbuck]

Thanks for the info. I can run my engine out of water, but is there any way to plug certain inputs into the tach with the engine not running to check the tach itself?

Thanks,

[Boat Doc']

Engine needs to be running

[Captain Starbuck]

Thanks Doc. I'll give it a try.

[doans]

Maybe the rectifier is not functioning!?