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**Dockhead** · 29-01-2019, 08:27

I prophylactically replaced my 100 amp, 24v school bus alternator last spring before my Arctic cruise, so that I would have a spare.

The new one worked fine all summer, and oddly stayed much cooler than the old one while producing max power.

However there is one weird problem -- when the engine is switched off, even when the batteries are isolated, the alternator gets warm and uses power!

WTF? I disconnected the exciter and warning lamp wires last time I left the boat, and that stopped it.

What could this be about? The new alternator is a Leece Neville 8SC3133U, which replaced a 8SC3014U. The new one is supposed to be an exact replacement for the obsolete old one, and they look identical, and have exactly the same terminals on the back.

Next time I'm on the boat, I'll do some experiments:

1. See if I can measure any current running through any of the wires, and which.

2. See if there is voltage on the exciter wire; and see if the phenomenon persists when I cut off the alternator exciter switch.

3. See if the phenomenon persists with the warning lamp wire disconnected.

There should not be any current going through the exciter circuit with the engine shut down, because it's on a relay. I guess there could be a failure, but that would be an odd coincidence.

Anybody have any clues?

hlev00 · 29-01-2019, 08:31

Your 3 steps seem like the next logical troubleshooting.

**GordMay** · 29-01-2019, 10:33

Prestolite (Leece-Neville) has a great Tech Support site.
➥ http://www.prestolite.com/Prestolite - Leece Neville

➥ http://www.prestolite.com/literature...nce_Manual.pdf

four winds · 29-01-2019, 11:12

With the exciter and lamp wires disconnected, the only logical source of current to warm the alternator would be the output wire. Easy enough to check by removing to see if the issue persists.

Isolating the batteries is done with the battery switches I assume. But if the issue goes away after removing the output wire then logically that wire is still connected somehow. So, where does the output wire connect to the system, and what else might be connected at that point as well is a question to consider. With the batteries isolated the bilge pumps are still connection I suppose. Where do they draw power from.

You have a complex electrical system as I recall so there is some complexity to the troubleshooting.

Also consider all your grounding points as they also proving a current path at the alternator, possibly to another system onboard.

For example, let's say the main ground on the engine is weak. And you have a faulty bilge pump. And the alternator has its own ground point and has become the primary source for grounding. And the faulty bilge pump is drawing current continuously through the only ground available, the alternator.

Far fetched wild ass guess no doubt. But if the issue persists after removing the alt output, then the alternator ground working with another issue to provide a current path is a possibility.

Likely all total bollocks, but might be true that you have compounding issues. Probably not easy to track down. Think holistically to all systems onboard.

tanglewood · 29-01-2019, 12:01

Internal regulator? It sounds like the regulator is staying on, probably because the "exciter" wire is remaining powered when the engine is turned off.

hellosailor · 29-01-2019, 13:03

I'm betting you sailed too close to Iceland and dissed some faeries. Or perhaps, because you went elsewhere before visiting them, you got them upset.

But if you insist in rational explanations, yes, testing that exciter relay would be my first shot. In theory NOTHING should be feeding the exciter when the engine power is off.

The other main culprit when an alternator is warm and leaking power, is the diodes. One or more can be "leaky" not just blown or shorted, but weak enough to pass current all the time. In which case you can disconnect the output terminal (which is hot even when everything is switched off, so you're handling a live high capacity hot wire) and see if there's power leaking through all the time. Or access the diode frame, which usually means opening up the alternator, and testing the diodes individually. (Or looking at the waveform from the alternator if you've got a o'scope.)

But yes, first check out the exciter relay.

As to why this alternator would outperform the older one...newer parts, less internal resistance? Better diodes or other electronics in the new one? I don't think it relates to the problem, but is co-incidental.

**noelex 77** · 29-01-2019, 13:18

Quote:

Originally Posted by Dockhead

Next time I'm on the boat, I'll do some experiments:

See if the alternator is magnetic. When the engine is off, put a screwdriver near the shaft. There should be no, or very little magnetic attraction.

**GordMay** · 29-01-2019, 14:08

Quote:

Originally Posted by hellosailor

I'm betting you sailed too close to Iceland and dissed some faeries. Or perhaps, because you went elsewhere before visiting them, you got them upset.
But if you insist in rational explanations ...

I was going to mention "evil spirits"; but didn't have a solution.

evm1024 · 29-01-2019, 14:16

Quote:

Originally Posted by GordMay

I was going to mention "evil spirits"; but didn't have a solution.

The "solution" to pissed off Domovoy is to pour a little Vodka in the house (boat) for them to sip.

smac999 · 29-01-2019, 20:41

the lamp or exciter must still have power with the key off. measure with a meter. they shouldn't.

then switch to external reg

ronstory · 29-01-2019, 21:53

Along the line with what smac said:

if it's draining power while the boat is "off" and/or goes away with removing the exciter (Field in US-speak) and the warning lamp... measure everything with VOM (DVM today

)

At a minimum (most is in your original post) ;^)

Check the alternator case to battery ground and see if there is a voltage potential.

Check the exciter wire to the alternator case

Check the warning lamp voltage with the ignition off to the alternator case

That said, I'm not personally familair with your model paricular of Leece Neville. If it has an isolated ground and it's not grounded ... that could be an issue... but then it would not work better than original.

**Dockhead** · 03-02-2019, 11:39

OK, so back on the boat, and tested this.

There is 26 volts on the exciter wire. Whether or not the "alternator power" breaker is switched. Pulls about 1 amp when both wires are connected to the alternator.

And there is 26 volts on the warning lamp wire. Whether or not. Pulls about 0.5 amps when both wires are connected to the alternator.

This with the engine off and key off.

Same potential to the alternator case, as to the ground post, although I thought this was an isolated ground device.

I guess this is a boat wiring problem, and not an alternator problem -- but ultra weird that it would occur just as I changed alternators.

But if anyone has any insight, I will still be very grateful.

hellosailor · 03-02-2019, 16:34

If they are getting full power when the key is off, one possibility is that the "ignition" switch itself is faulty. I've had that happen, sometimes the rear of the switch is just screwed or crimped onto the body, and if that comes loose, the contacts/wiper can get out of position.

I'd try to physically unplug any power from the ignition switch, and if that cuts the exciter power? Time for a new cylinder, probably.

ronstory · 03-02-2019, 16:46

Agree, it could be an ignition switch issue, and it's easy to check.

With the ignition off, see is you have voltage on the output of the switch (Note: one terminal will be powered from the master). If you see voltage on more than one terminal... disconnect it and see if the Field wire goes off as well.

Ignition switches have tough since they are moving parts, in the weather.

Good luck.