Tach, Alternator odd trouble.... - SailNet Community
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post #1 of 7 Old 08-27-2013 Thread Starter
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Tach, Alternator odd trouble....

1986 Young Sun with an 80 HP Ford Lehman Diesel.

Earlier this winter, my alternator finally died. A new one has been installed along with a regulator. Its been solid until a heater hose clamp failure that dripped coolant on it while it was running and hot. An alternator shop claims shes still in good shape but: ISSUE: My tachometer now fluctuates and is off and on far more regularly. And when off, stays off for a while. (Usually first hour after starting engine.) ALSO, it does not charge batteries when the tach is at zero or fluctuating (I have checked batteries for input voltage and found no charge coming from alt).

Thoughts? Ideas on where to start? Possible issue(s)? I probably will call a mechanic but am looking to learn as much as possible to address myself when possible.

Thanks!


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post #2 of 7 Old 08-27-2013
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Re: Tach, Alternator odd trouble....

Sounds like an alternator driven tach, and the symptoms look like worn brushes, or some other issue with the brush connections. Have you got a smart regulator, or is the tach driven direct from the internally regulated alternator?

The symptoms match failing brushes, but you'd think the alternator shop would have seen that...
dried antifreeze insulating the slip rings??

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post #3 of 7 Old 08-27-2013
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Re: Tach, Alternator odd trouble....

The tach signal comes from the alternator but there are things that could be wrong other than the alternator itself.

Some systems use 12V supplied via the ignition switch accessory position to provide the excite voltage. So a faulty ignition switch, or wiring between that and the alternator, could do the trick. Old ignition switches can develop bad contact, I replaced mine last year because it would only start the engine intermittently.

What do you mean "My tachometer now fluctuates and is off and on FAR MORE REGULARLY"? So it had this problem before you changed the alternator and it got worse? That really points to wiring.

Do you have an ignition light?
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Last edited by MarkSF; 08-27-2013 at 02:49 PM.
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post #4 of 7 Old 08-28-2013 Thread Starter
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Re: Tach, Alternator odd trouble....

Thank you MarkSF and Faster.

I've had the boat 7 years this past July. On rare occasions in the past (maybe2 or 3 times a year at most), while motoring (again the motor RPM was consistent and running smooth) the tach would jump to zero, maybe stay there for a moment or two, jump back up to a higher RPM reading or maybe a lower reading or both but would quickly go back to normal after a few moments.

Since I put in the new alternator and had my hose clamp fail, the tach has done the same thing as above, but only for much longer periods (up to an hour). And I also finally noticed that when the tach was screwey, it also wasn't charging the house or engine batteries.

Yesterday - Wednesday Aug 27th. I replaced the original ignition button and key. I put in a brand new key switch (like a car). I'm positive all wires are tight and connected. If this doesn't fix it, I'll assume the shop missed the stuck brushes.

~Scott


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post #5 of 7 Old 08-28-2013
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Re: Tach, Alternator odd trouble....

Quote:
Originally Posted by Rasputin View Post
Thank you MarkSF and Faster.

I've had the boat 7 years this past July. On rare occasions in the past (maybe2 or 3 times a year at most), while motoring (again the motor RPM was consistent and running smooth) the tach would jump to zero, maybe stay there for a moment or two, jump back up to a higher RPM reading or maybe a lower reading or both but would quickly go back to normal after a few moments.

Since I put in the new alternator and had my hose clamp fail, the tach has done the same thing as above, but only for much longer periods (up to an hour). And I also finally noticed that when the tach was screwey, it also wasn't charging the house or engine batteries.

Yesterday - Wednesday Aug 27th. I replaced the original ignition button and key. I put in a brand new key switch (like a car). I'm positive all wires are tight and connected. If this doesn't fix it, I'll assume the shop missed the stuck brushes.

~Scott
Where the tach was acting up before you changed the alternator, the wiring to the exciter terminal may be the cause?

If the new ignition switch doesn't solve the problem, you might try running a wire, through a switch, if you want, directly from the battery supply to the exciter terminal of the alternator? If that doesn't work then the contacts in the alternator may be the problem.

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Re: Tach, Alternator odd trouble....

Quote:
Originally Posted by Rasputin View Post
Thank you MarkSF and Faster.

I've had the boat 7 years this past July. On rare occasions in the past (maybe2 or 3 times a year at most), while motoring (again the motor RPM was consistent and running smooth) the tach would jump to zero, maybe stay there for a moment or two, jump back up to a higher RPM reading or maybe a lower reading or both but would quickly go back to normal after a few moments.
This can be normal behavior on an a alt with a sloppy regulator when the battery bank gets full. When the bank gets full if the regulator allows it to over-shoot its voltage limit, this can happen, it will cut the field power entirely thus shutting down the alt and tach. When the voltage drops it comes back on...

Quote:
Originally Posted by Rasputin View Post
Since I put in the new alternator and had my hose clamp fail, the tach has done the same thing as above, but only for much longer periods (up to an hour). And I also finally noticed that when the tach was screwey, it also wasn't charging the house or engine batteries.
This would not be normal unless you have an additional charging source such as solar, generator or wind on-board that have a higher regulation set point than the alternator regulator....

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post #7 of 7 Old 08-29-2013 Thread Starter
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Re: Tach, Alternator odd trouble....

To Maine Sail: I would notice that the alternator wasn't charging after being on a mooring all weekend. The house batteries would be down to 12.2 or so and the starter battery would be at ~ 12.5/6. My point being, once the engine is on and we're motoring away from the mooring, the alternator shuold be doing some charging... and it isn't as long as the tach was going wacky.

I only have 3 sources of charge. Shore power, Alternator and a Honda 2000 Generator (which is terriffic). No wind or solar as of yet.

Huge thanks all. I'll be on the water this long weekend and see what transpires.


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