Yanmar Alternator Problem

[Peter]

A question for the electrical Gurus on the forum:
My Yanmar 2GMF has a 30 amp alternator with internal regulator.
Measured output voltage when engine is cold : 14.6 volts
Measured output when engine is warm: 12.4 volts
These measurements were taken right off the alternator terminals.
Any ideas where I should start looking for the problem?

[One Way David]

In the automotive arena, 12-14 volts is spec. Varies with engine RPM. Wouldn't marine applications be the same?

[bcooke]

when the engine is warm does it idle lower?

12.5 is too low for a 12v system. The alternator needs to be at a higher voltage (pressure) than the battery in order to push the electricity back into the battery and charge it. 13.5v is normally considered the minimum charging voltage in a 12v system.

First guess is that with the engine warm it is not turning fast enough for the alternator to get its juices flowing. Its a common problem. Alternators like to turn fast. I don't know your particular application. It is possible the Yanmar engineers simply gave up on trying to make charging voltage at minimum idle speed and called 12v good enough. It won't hurt anything but you won't be charging any batteries either.

Second guess is that you could also be looking at the early stages of alternator failure.

[Tim]

What was the state of your batteries' charge at the time of your alternator voltage test?

[Quetzalsailor]

We have a Yanmar GM30YEU, but it has a fancy Balmar regulator and 100 amp alternator. Voltage varies with engine RPM, from marginally too low at idle - 800ish RPM, to useful at, say, 1100 or so. But of course, no higher voltage with higher RPM; that's part of what the voltage regulator does. If you have an ammeter, you can watch the rate of the charge. The voltage showing on your voltmeters will be 14ish if there's enough RPM to generate it.

Tim's no doubt right, though I've not observed it. If the batts were flat and the RPM is marginally low, I can believe that a flatter battery would pull the voltage down. You can certainly hear diminished RPM or more engine effort as electrical loads are applied. I cannot imagine why the voltage varies with engine or, presumably, alternator temperature.

[Ryan]

My first question would be the same as Tim's, mainly how discharged were the batteries? Is it possible that the engine ran long enough to top up the batteries, and the regulator fell into maintenance mode?

If it is indeed related to alternator temperature, you may have a bad voltage regulator.

[Peter]

Thanks for all the replies.
The reason I noticed the problem is because the Volt meter started reading low while under power.

To answer all the questions:
All readings were taken at 2400 RPM
Even at idle the Volt meter has always (previously) registered around 14 Volts.
My batteries are always charged up as I have two solar panels permanently mounted.

I too suspect an emerging alternator failure. Some part must be ok when cold, but fails when it gets warm.
I've gone over all wiring, plugs, and grounds and found nothing amiss. I guesss it's time for a trip to the shop!

[Peter]

Searching on Google I found this snippet:
"When an alternator doesn't have a separate ground terminal, the negative connection is made between the alternator housing and the engine block. This connection must then be treated the same as any other electrical connection. "

Aha!
The problem first surfaced in September on a three day outing. Before I left I gave the engine a washdown so it would smell sweet for the journey. Maybe, just maybe, when it warms up there is some resistence-causing corrosion between the cast iron block and the aluminum alternator housing. Time for a ride to the marina :-)

[Peter]

Just to close this thread ... Problem solved with a nice new shiny 40 amp replacement. Later testing on the old unit finally revealed an intermittent break in the rotor windings, something the repair guy had not encountered before. He found me a good used rotor to replace the defective one, and now I have yet another spare part to carry around :-)