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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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  • 1 Post By svHyLyte
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Old 09-07-2013
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bouncing tach

I have a Perkins motor on my Morgan and a year ago it started bouncing and I know it is not accurate any more. I checked connections at the back and all seems well. What could this be due to? The gauge reads then drops for a split second part way down but there is no audible change in motor rpms. This happens frequently but not constantly. Do I just replacethe ggauge or is it more involved?
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Old 09-07-2013
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Re: bouncing tach

Depends on your pickup/sensor/tach drive method.

Tachs are usually driven either by an alternator/regulator output or by pickups mounted at the flywheel.

Have you noticed your charging system falling off at the same time? That would point to a developing alternator problem. If the tach is not alternator driven then it's possible the speed sensor connections are failing, or the sensor has vibrated out enough to lose the required proximity to the teeth or slug that drives it.

I'd say a connection somewhere is the most likely culprit unless there are other alternator related symptoms.
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Old 09-07-2013
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Re: bouncing tach

You might try a temporary jumper wire directly from the tach source, either the alternator or sending unit from another place, to the gauge, after disconnecting the existing wires. If that does not solve the problem then either the sending unit or the gauge may be the problem.

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Old 09-07-2013
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Re: bouncing tach

Any chance the tach is mechanically driven? If so most likely a bad tach cable.
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Old 09-08-2013
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Re: bouncing tach

Tach is electrical. Will try a jumper cable and see what happens.
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Old 09-08-2013
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Re: bouncing tach

Jason--

A "bouncing tach" on a Perkins 4-108 is often caused by the alternator cycling on-off when one's batteries are fully charged. Try leaving your charger off for a day or so with a small load applied to to your batteries--lights, a refrigerator or whatever-- and then see if you still experience the same issue. We found this to be the case on our boat and now overcome it by simply applying some loads to the batteries when it happens.

FWIW...
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Re: bouncing tach

Definitely not overcharged batt. I am sure of that. Interesting to know that though
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Old 09-09-2013
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Re: bouncing tach

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Originally Posted by svHyLyte View Post
Jason--

A "bouncing tach" on a Perkins 4-108 is often caused by the alternator cycling on-off when one's batteries are fully charged. Try leaving your charger off for a day or so with a small load applied to to your batteries--lights, a refrigerator or whatever-- and then see if you still experience the same issue. We found this to be the case on our boat and now overcome it by simply applying some loads to the batteries when it happens.

FWIW...
How could that work - that would require a feed back circuit and none exists.
The voltage regulator is what does or doesn't send a charge to the batteries based on voltage/state of charge (depending on how sophisticated it is).

The tach is simply reading and counting RPM's either from the alt (which is still turning exactly the same whether or not it's making amps) or from the flywheel (ditto, still making amps).

I'll admit I've had a similar problem with RPM's bouncing around once I get past 2400 RPM, but that's a matter of calibrating the gauge, not the alternator (flywheel in my Yanmar's case).
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Re: bouncing tach

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Originally Posted by chucklesR View Post
How could that work - that would require a feed back circuit and none exists.
The voltage regulator is what does or doesn't send a charge to the batteries based on voltage/state of charge (depending on how sophisticated it is).

The tach is simply reading and counting RPM's either from the alt (which is still turning exactly the same whether or not it's making amps) or from the flywheel (ditto, still making amps).

I'll admit I've had a similar problem with RPM's bouncing around once I get past 2400 RPM, but that's a matter of calibrating the gauge, not the alternator (flywheel in my Yanmar's case).
As the problem was explained to me (as I am not an electrics person), both the internal regulator on the original Motorola stock alternator and most external regulators (ours is an AR5) on larger after market alternators (ours is a 120 amp) include a sense circuit that determines the static voltage in one's batteries. If they are fully charged, or nearly so, close enough to the tripping point at which the alternator is activated/deactivated, when the alternator is activated, with the resulting extra voltage the sense circuit "senses" that the batteries are fully charged and so, deactivates the alternator. The tachometer on the stock Perkins engine panel relies on an activated alternator. Hence if the alternator is cycling on/off/on, the tach bounces. We were instructed to simply activate some power demands to suck up the "extra" voltage--lights, fridge radar or whatever--and, by doing so, the "problem" was solved. Actually, at this point we've used our engine for so long I/We can pretty much tell our RPM's by the sound of the engine but I do like to be able to look down at see that she's steady at 2500 to 2700 RPM's from time-to-time, our usual cruising speed range when powering or motor sailing.

FWIW...
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Last edited by svHyLyte; 09-09-2013 at 07:08 PM. Reason: Correct typo
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