Tach falls off at high rpm

[Times A Wastin’]

I’m running 2001 6bta 330hp in my 4087 Bayliner. Anything over about 2000 rpm, my starboard tach on the upper helm drops off erratically and as the RPM increases the tack drops all the way to zero. I checked my tack in the cabin and it seems to work fine which tells me the sender and the connections to the engine are OK?
Another clue is after about 16 hours on the anchor with the house batteries providing a little lower voltage I noticed both starboard and port tachs were falling off at even lower RPM.
Would I be correct in thinking either I have a poor ground or a poor power supply to the upper helm?
Any suggestions where I look next would be greatly appreciated.

Topic

[Clark Leighs]

I realize that there may be more to this as the lowers seem to be ok and just the uppers goof off but I’ll add it regardless.

Just my $0,02 .
I am not who figured this out.

If you have a sensor that reads the flywheel teeth then the sensor face can collect rust particles. THose particles can stick. When they stick they interfere with the tiny current pulse the teeth generate as they whiz by the sensor face. At lower revs maybe the sensor can still read the pulses but as revs increase and the engine heats maybe the sensor can no longer read the pulses.

Usually fixable by finding the sensor and removing and cleaning the face.
Reinstall by winding in untill the sensor touches the flywheel teeth lightly.
Then back the sensor out about 1/2 turn for clearance and set the lock nut.

1 user thanked author for this post.

Tony Athens

[Times A Wastin’]

Thanks for your input Bob. I’ll be sure to check the upper helm harness connector. Great suggestion.

[Bob Odell]

I had a similar problem many years ago and traced it to the harness connectors that separate the flybridge from the boat harness. They are multi-pin AMP connectors. They had become wet and corroded over the years. Basically, the signal level at the tachometers was significantly reduced at the upper RPM ranges.

In another instance, the problem was at the back of the tachometer itself.

The higher the RPM, the higher the frequency of the magnetic pickup signal. The higher the frequency, the easier it is to attenuate.

[Times A Wastin’]

I agree Tony, all 3 systems should be separate. Thanks for the link and diagram. I’ll can work with that to continue my search for the problem.
Regards,,

[Tony Athens]

Why would the house batteries be tied into the engines? 1st thing I do is LEARN your DC Power distribution system so you can properly troubleshoot the boats electrical…………………………..IMO, all 3 systems should be separate and only share a common gnd.

[Author]

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