Merc 850 CDI Erratic Tach

[SeaKaye12]

Hi,

Today I took the boat out and immediately noiticed that my tach was reading appx 1000 RPM high.

At WOT I was getting appx 6500; usually it is 5500.

The engine is running the same. I doubt that it really is revving any higher. I think that the tach is erratic.

After a while it reverted back to "normal"....

Any idea as to what is causing this? Hoepfully it's not indicative of some impending black box/trigger/stator failure...

Thanks for reading, Chuck

 

[corm]

Re: Merc 850 CDI Erratic Tach

Hi,

I would think that it would be your tack thinking about dieing.
It runs off the alternator and they rarely go bad. Your rectifier may
be thinking about failing but the rest of the system should be ok.
If the tack quits then I would move the tach wire to the other terminal with the yellow wire on it. If it works you rectifier is bad if not it is probably the tach. I would test the rectifier before replacing the tach to be sure.