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Old 26-04-2022, 08:12   #1
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Alternator showing 28-29 volts open circuit

We have a challenge here at Jack's Boatyard.

Yesterday, a boat that has been under construction for some 30 years, was launched for the first time. (The original builder did a fabulous job, but was meticulous and VERY slow - then died. She sat for quite a number of years before being bought and the construction restarted).

She's got a four-banger Perkins diesel in her. I don't know the exact model or year.

When they started her up it showed no charge on the ammeter and 12.6 volts on the new batteries. Clearly the alternator wasn't charging.

When I came aboard the engine was running. I stuck my voltmeter on the B+ terminal of the alternator and it showed 28 - 29 volts to ground. Interesting.

I traced the wiring. The cable from the B+ led to a 75 amp circuit breaker, then to the batteries (the ammeter was driven by a shunt in the ground cable).

The 75 amp breaker was blown.

We shut down the engine. I tried to reset the breaker but it wouldn't. I suspect it is toast.

My question is: would a 12v alternator show 28 - 29 volts open circuit? Or do we have a situation where we have a 24 alternator on a boat that is otherwise completely wired for 12 volts? I'm reluctant to just replace the breaker and try again, or bypass the breaker, for fear of doing more damage.

Thanks all.
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Old 26-04-2022, 09:23   #2
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Re: Alternator showing 28-29 volts open circuit

I suspect diodes in alt are wrong. The breaker saved the boat.
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Old 26-04-2022, 09:33   #3
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Re: Alternator showing 28-29 volts open circuit

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I suspect diodes in alt are wrong. The breaker saved the boat.

I didn't think about the diodes being the problem - but I DID think that maybe the breaker had stopped some really bad things from happening - which is why I didn't try bypassing it.

Thanks for your thoughts.
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Old 26-04-2022, 09:42   #4
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Re: Alternator showing 28-29 volts open circuit

Consider removing the alternator, bring it to an alternator shop and see what it is really doing?
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Old 26-04-2022, 09:46   #5
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Re: Alternator showing 28-29 volts open circuit

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Consider removing the alternator, bring it to an alternator shop and see what it is really doing?

That was exactly my next thought. Thanks.
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Old 26-04-2022, 19:05   #6
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Re: Alternator showing 28-29 volts open circuit

Quote:
Originally Posted by Scorpius View Post
We have a challenge here at Jack's Boatyard.

Yesterday, a boat that has been under construction for some 30 years, was launched for the first time. (The original builder did a fabulous job, but was meticulous and VERY slow - then died. She sat for quite a number of years before being bought and the construction restarted).

She's got a four-banger Perkins diesel in her. I don't know the exact model or year.

When they started her up it showed no charge on the ammeter and 12.6 volts on the new batteries. Clearly the alternator wasn't charging.

When I came aboard the engine was running. I stuck my voltmeter on the B+ terminal of the alternator and it showed 28 - 29 volts to ground. Interesting.

I traced the wiring. The cable from the B+ led to a 75 amp circuit breaker, then to the batteries (the ammeter was driven by a shunt in the ground cable).

The 75 amp breaker was blown.

We shut down the engine. I tried to reset the breaker but it wouldn't. I suspect it is toast.

My question is: would a 12v alternator show 28 - 29 volts open circuit? Or do we have a situation where we have a 24 alternator on a boat that is otherwise completely wired for 12 volts? I'm reluctant to just replace the breaker and try again, or bypass the breaker, for fear of doing more damage.

Thanks all.
IMHO.. Alternator's circuit should never be open when engine is running.For that reason there should not be any breakers or fuses in the circuit.
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Old 26-04-2022, 19:05   #7
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Re: Alternator showing 28-29 volts open circuit

Alternators usually show a higher than nameplate voltage when not under load. That results in the caution not to turn off the A/Both/B switch and allow the alternator to go no load. The higher voltage may blow the diodes.

I'd try loading the alternator with a moderate load - say an old battery - for a few minute test - I suspect you will find it is working normally and your diodes may not have been damaged by the short run. If it does, replace the breaker and see what happens.
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Old 26-04-2022, 19:22   #8
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Re: Alternator showing 28-29 volts open circuit

It could be the voltage regulator. If it's a N type alternator, and the field wire shorts to ground, it will put out as much voltage as it physically can. It's what happened to me, but with an external regulator that went bad. The alternator was a regular internally regulated one, where the regulator was gutted to pass the field wire out of them.
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Old 26-04-2022, 20:09   #9
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Re: Alternator showing 28-29 volts open circuit

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IMHO.. Alternator's circuit should never be open when engine is running.For that reason there should not be any breakers or fuses in the circuit.

I agree. But that was the situation with which I was faced. Maybe if I HAD bypassed the breaker, everything would have been fine - but if it was a 24 volt alternator I didn't want to take the risk and do more serious damage. Hence this post.

On Scorpius I have master switches between the alternators and their respective battery banks. However, those master switches, when switched "off" also disconnect the field circuits of each alternator.
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Old 26-04-2022, 20:10   #10
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Re: Alternator showing 28-29 volts open circuit

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Alternators usually show a higher than nameplate voltage when not under load. That results in the caution not to turn off the A/Both/B switch and allow the alternator to go no load. The higher voltage may blow the diodes.

I'd try loading the alternator with a moderate load - say an old battery - for a few minute test - I suspect you will find it is working normally and your diodes may not have been damaged by the short run. If it does, replace the breaker and see what happens.

Good idea. I'll see about giving it a shot.
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Old 26-04-2022, 21:37   #11
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Re: Alternator showing 28-29 volts open circuit

If you are pressed for time Try bypassing or replacing the breaker and give it a run
If it still goes to way above 13.6 it is a 24 v alternator
But if you have time I would remove the alternator and get it checked
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Old 26-04-2022, 23:42   #12
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Re: Alternator showing 28-29 volts open circuit

I thought you could run the alternator without load as long as you don't cut the load when it's running (voltage spike -> diod failure)???
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Old 27-04-2022, 10:09   #13
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Re: Alternator showing 28-29 volts open circuit

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IMHO.. Alternator's circuit should never be open when engine is running.For that reason there should not be any breakers or fuses in the circuit.


Eek. Automakers indeed fuse the B+ fat wire. Reason being, if diodes do short, you will be looking at wire melting current. Engine off or on.
I dont fuse mine,but i run into diode based isolators.
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Old 27-04-2022, 10:28   #14
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Re: Alternator showing 28-29 volts open circuit

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I thought you could run the alternator without load as long as you don't cut the load when it's running (voltage spike -> diod failure)???

Not really. The AO )alternator output) needs a place to go. One can run the alternator if the regulator is disconnected because it is the regulator that tells the alternator to produce.


Cutting the load when running almost always kills the diodes although there is anecdotal evidence that a very low load won't kill the diodes if suddenly disconnected. I say anecdotal because a friend told me it happened to him. Once. I wouldn't try it though.
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Old 27-04-2022, 10:30   #15
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Re: Alternator showing 28-29 volts open circuit

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Eek. Automakers indeed fuse the B+ fat wire. Reason being, if diodes do short, you will be looking at wire melting current. Engine off or on.
I dont fuse mine,but i run into diode based isolators.



I think you have this backwards. You are not fusing the alternator output, you are fusing the wire that leaves the battery bank from a battery short. You should add a fuse at the battery end. Consider the battery as a big bomb.
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