Tachometer goes crazy

[pat8839]

We had a glitch with my engine coming back to the dock yesterday-my RPM's were all over the place at one point. I was running around 30mph and the engine was around 3300 RPM but then my tachometer shot up to 4900 and back down, and kept going up and down like that. The engine never sounded different and the boat did not surge or change speed at all. The temperature never went up and oil pressure never changed either. I did put the boat in neutral to see if something looked wrong under the engine hatch but everything looked fine. Boat ran fine most of the way back with only minor fluctuations after that. The tach has been going up and down like that when I run it on the hose in the driveway but never near 5,000 rpms like today, and it has never done it on the water.

Any ideas? I did change the oil about 2 weeks ago but have not done anything else. I have not put in the new impeller this season yet...

I have a 2006 mercruiser 4.3L MPI

 

[Doernuth]

Re: Tachometer goes crazy

Your tach should be connected to the ignition coil. Check all the connections and make sure they are clean and making good contact.

If it is still "twitching" hook a shop tach up and see if it does the same thing. If not you may have a bad guage.

Also most of these tach's are universal, meaning there is a switch on it for 4, 6 or 8 cylinder engines. Make sure it is on the correct setting.

 

[pat8839]

Re: Tachometer goes crazy

We did get a good amount of rain yesterday when we were pulled up to the beach and there was a lot of water in the bilge which I pumped out before we left. Could that water possibly have corrupted the coil?

Thanks for the advice!

 

[solar7647]

Re: Tachometer goes crazy

If it's only the tach and the engine sounds as if it's not revving or changing rpm, could be a bad wire or tach. You would defiantly notice if it went from 3300 to 4900. I would check it with a meter to make sure the rpm are constant, then go for the wire. If you still have issues after verifying the wire is good then go to the tach, that way you don't needlessly spend a bunch of money on a new tach and it ends up being the wire.