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Charging light up on instrument panel


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I noticed the charge light is illuminated on my yanmar 3ym instrument panel, though the volt/amp meter is still showing current going to batteries, I had the alternator rebuilt about 30 hrs ago, anybody know what might be the cause

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How bright?  A failed exciter diode (inside hte alternator) would cause the light to illuminate, but not a full brightness.  The alternator will still charge, just to to spec output.

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Two small things to check first.
Any chance the belt could be slipping? I know you will likely hear a slipping belt, but there can be times when it slips and it does not squeal. Which brings me to the next thing to check. A slipping belt means that under high load, the ALT may not be spinning fast enough to produce full output. The same thing happens if the engine is idling too low. Does the light go out if you raise the Revs?
High Load will be if the batteries are very low in charge state when you begin Charging. So also check that the Batteries are in OK condition. If they are not accepting charge, or are taking a very high current from the ALT, then the ALT light might also light up.

Failing that, go with the great advice above.

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Batteries were  at 90%, and belt tension is good, couple of days before the tachometer threw a wobbly briefly, I turned motor off and restarted and it came right again, maybe a sign off a bad connection somewhere, 

thanks for all advice, I will do some checks when next on boat

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Poor electrical connections tend to be the most common cause of such problems. Internal issues with an ALT is less likely, especially on a newly rebuilt unit. Not impossible, but the least likely.
Do you know how to use a multi meter in both Voltage and Ohm's mode??
There could be a clue to a problem in what you have decsribed with the Tacho and turning engine off and on again. The ignistion Key may have a faulty/dirty/worn connection internally. That internal switch is sending a "sense" voltage to the ALT. If that switch has become "noisy" as in a poor connection, the ALT will be getting a poor and no constant voltage. That can result in the Tacho giving a non consistant reading. If you have a Voltage or current metre on the switch panel, you may see those readings vary in a constantly up and down way as well.
Also check the power to the Ign switch and from ign switch to ALT.
If you don't have monitoring metres on the switch panel, then use the Multi meter to view the voltage into the battery. It should remain a calm reading of around 13.8V upto 14.4V, depending on the setup.

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