Tach rpm swinging erratically at idle

[Jim Cosgrove]

RPM gauge needle is now swinging wildly at idle. Sound of the engine W27 doesn’t change so I don’t think the engine’s  rpm’s are really changing. Needle settles right down when I boost the throttle. 

This is a brand new issue. I just changed the fuel filter, but I don’t see how that would be related because the engine sounds like it’s running fine. 

Any thoughts as to what’s going on?

Jim Cosgrove 

FATE 30U #343

Galesville, MD

[Pat Furr]

Check your alternator belt tension.  It may slip some at idle but tighten under throttle?  

Pat Furr

Bandolero, NS26C #133

Charlotte, VT

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[Thor Powell]

These are driven from the tach wire ,  check that it is not compromised. As well if it is an internally regulated alternator it could be a sign it has a bad diode.  Belt slippage could be a cause, about 1/4 defection , belts are cheap and easy to change.

What voltage are you seeing during the RPM swings.  Turn on an incandescent light and see what it does...

[Cedric Guthrie]

Loose ground wire from instrument panel ?

Safe Sailing

Cedric. Single Malt

26C # 207

Chester Nova Scotia

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On May 9, 2022, at 18:46, 'Jim Cosgrove' via INA Nonsuch Discussion Group <INA-Nonsuch-Di...@googlegroups.com> wrote:

RPM gauge needle is now swinging wildly at idle. Sound of the engine W27 doesn’t change so I don’t think the engine’s  rpm’s are really changing. Needle settles right down when I boost the throttle. 

[Jim Cosgrove]

Thanks for all the suggestions on the tach problems.  Belt seems tight and ok.  (I was kind of hoping that would be the problem as that would be the easiest fix.)  Still trying to trace wiring.

Jim Cosgrove

FATE 30U #343

Galesville, MD

[Cedric Guthrie]

Instead of tracing - just run any piece of wire from control panel to say an engine mount 

That will confirm or not loose connection/bad wire 

If that makes things works properly then run a new wire - does not do any harm to leave original wire in situ

Cedric. Single Malt 

26C # 207

Safe Saling

Chester Nova Scotia 

Sent from my iPhone

On May 13, 2022, at 14:50, 'Jim Cosgrove' via INA Nonsuch Discussion Group <INA-Nonsuch-Di...@googlegroups.com> wrote:

Thanks for all the suggestions on the tach problems.  Belt seems tight and ok.  (I was kind of hoping that would be the problem as that would be the easiest fix.)  Still trying to trace wiring.

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[Ron Schryver]

Hi Jim, is it possible that the engine is idling a tad too slow. The tach is an electronic tach and if the regulator trips out because the idle is too low the tach needle will drop to zero. If the idle speed is right on that threshold the needle will bounce. Once the engine speed picks up and the charging circuit is continually active the needle will settle down.

Hopefully it's that simple.

Ron

Ron & Diane Schryver

"Alpha Waves" 1987 NS30U #393

Georgian Bay Midland ON

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[Jim Cosgrove]

Hi Ron,

Thanks for the suggestion. I’m not sure that’s it but it’s worth looking into. The needle bounces around 900 RPM which is normal for my idle and engine sounds normal. 

I’ve tried attending two short videos to show the behavior of the needle. One at idle and the other with some throttle applied. 

Jim Cosgrove

FATE 30U #343

Galesville, MD

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[Joe Valinoti]

Looking at this I noticed that the volt meter is steady.  Since the tach signal is generated by the alternator, one would assume it’s a tach problem, not an alternator problem.  As someone mentioned, run a wire directly from the alternator to the tach and see if the problem goes away.  If not, I’d say you have a tach problem.  I happen to know someone who happens to have a spare tach for that panel.

Joe Valinoti
S/V iL Gatto NS30U #221
Sea Harbour YC
Oriental, NC USA

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[Jim Cosgrove]

Hi folks. Since I received a lot of helpful advice on this problem, I wanted to make sure I closed the loop on the issue. Turns out the problem was caused by corrosion on the connections. Once that was cleaned off, the tach is back to working normally. 

As I’ve said before, our Nonsuches are awesome but it’s this owners group that makes them truly outstanding. Many, many thanks for your help. 

Jim Cosgrove 

FATE 30U #343 

Galesville, MD 

[Bob Gehrman]

Thanks for following up!

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