Bouncy tachometer

[Peregrine1983]

Hi all,

After a long day underway on our Pearson 424 - a mix of great sailing at 7+ knots in 25+ knots of wind, plus and few hours of on and off motoring in choppy conditions, the tachometer for my Westerbeke 58 started bouncing around between 0 rpm and 2500.

The engine hummed along at 2000 rpm the entire time. I'm positive it never deviated. I can tell by ear when it's at 2000rpm as that's my usual cruise speed and we do 6.4kts at that rpm in flat water.

When I did eventually throttle back to like 500 rpm, the tach seemed to "re-engage" suddenly pegging to the correct rpm all the way back up to 1500rpm, at which point it started bouncing again.

Restarting seems to cure the bouncy tach issue, at least temporarily.

Any thoughts on what is causing this? Do I need a new tach or is this a sensor issue?

Thanks in advance.

[PDScott]

Not sure but it may be that your tach is driven by your alternator. If the batteries are topped right up ie motoring at cruising speed,then the alternator will shut off and you tach will drop out. If you are running lights or other equipment under way the alt will turn on and off trying to keep the batteries full. This happened to a friend that has a large solar array which kept the batteries full. Hope this might be something to check out first. Peter

[Suijin]

Another and probably more likely scenario is a loose lead, probably at the alternator where you have a ring terminal on a post. Check for tightness, corrosion etc. it’s also possible for the post on the alt to be loose.

[Peregrine1983]

Thanks PDScott and Suijin. My gut is leaning towards a bad/loose connection as Suijin said. It seemed to bounce more when we'd get jostled by a wave. I'll check out the connection at the alternator.

[captain465]

Ihave a Westerbeke M-100 in my boat and that happens whenever the alternator belt starts to get loose. Check for belt tension (should have about a 1/4" deflection in the longest section of belt run.

[Peregrine1983]

Thanks captain465! Another thing to check.

[GeoBay]

I have a westerbeke 21 and the tachometer does the same thing. Also noticed that when the hour meter clicks at each minute the needle jumps at the same time. I have checked all the connections and test out fine???

[dfelsent]

I had an old Audi 100. Bouncy speedometer! 0 to 60 once or twice a second.
Bad connection on the circuit board. I second all the bad connection ideas here.