How to Wire an Alternator

[Dockhead]

As some of you may know from an earlier thread, I burned up my school bus alternator a couple of months ago.

I had it completely rebuilt. But when I put it back, it didn't work.

The "no charge" light doesn't come on, and the engine room blowers don't come on, so there's something fishy.

I have four terminals on my alternator (a Leece-Neville school bus job).

The back of it looks like this:



I have an "R" terminal, which I understand is for relays or a tach. It is connected to my windlass and blower relays.

I also have an Exciter terminal, which is apparently connected through the "no charge" light, although the wiring diagram doesn't show this. In any case, when I disconnect this wire from the alternator, it has 25 volts on it with the ignition on, and 0 with the ignition off. I am guessing that if I were to ground it with the ignition on, the "no charge" light would come on, but I haven't done that experiment.

Then there is a small negative terminal with a black wire on it which goes God knows where -- it's not shown in the wiring diagram. (What is that for, by the way?)

I started pulling wires off and testing voltages.

I have 25 volts (normal system voltage) ON ALL TERMINALS, including even the negative one. The small negative terminal has 25 volts potential with the chassis of the alternator -- weird.

And all that is whether or not I have the ignition switched on. So the source of the voltage is the main large output cable which is hard-wired to the batts.

I assume that means there is some internal short in the alternator -- so something was not installed right when the alternator was rebuilt.


Anyone have any insights? My understanding of electrickery is very weak and I will be grateful for any tips.

[Hydra]

Dockhead
I enclose the wiring diagram for my engine (Nanni) and alternator (Valeo). The letters on the terminals aren't the same as on yours but it might help anyway.

Alain

[Dockhead]

Originally Posted by Hydra

Dockhead
I enclose the wiring diagram for my engine (Nanni) and alternator (Valeo). The letters on the terminals aren't the same as on yours but it might help anyway.

Alain

Thanks!

So B+ is output.

D+ is "Charge Alt" -- I guess is connected to the "no charge" lamp? Like my "Exc" terminal.

W is tach -- like my "R" terminal.

Where in the world does the black wire go? If it's simply grounded, why do you need the wire?

[OceanSeaSpray]

Your alternator appears to have an insulated ground, which is great, so you need two heavy cables to the battery for a start: on the "+" and "-" posts. The reason why you are measuring 25V everywhere is because you haven't "grounded" your alternator.

"EXC" should most likely be connected to the charge light, but put it straight to the battery positive for testing if you want. It should be the equivalent of "D+". "D+" is an isolated B+ used to feed the regulator/field internally. You feed into it from the battery until the alternator kicks in only.

If you have a "W" tach output, it won't show any voltage at rest. "W" is taken from one of the stator phases, before the rectifiers. There is an AC voltage on it when the alternator is running.
Don't connect it to anything until you are sure about what it is. The alternator doesn't need it to operate. Your "R" might be "W", but it is not certain and what you are saying about "relays" is very suspicious.

Anyway, this is all I can say from the photo you posted.

[svlamorocha]

Originally Posted by Dockhead

As some of you may know from an earlier thread, I burned up my school bus alternator a couple of months ago.

I had it completely rebuilt. But when I put it back, it didn't work.

The "no charge" light doesn't come on, and the engine room blowers don't come on, so there's something fishy.

I have four terminals on my alternator (a Leece-Neville school bus job).

The back of it looks like this:

Attachment 111677

I have an "R" terminal, which I understand is for relays or a tach. It is connected to my windlass and blower relays.

I also have an Exciter terminal, which is apparently connected through the "no charge" light, although the wiring diagram doesn't show this. In any case, when I disconnect this wire from the alternator, it has 25 volts on it with the ignition on, and 0 with the ignition off. I am guessing that if I were to ground it with the ignition on, the "no charge" light would come on, but I haven't done that experiment.

Then there is a small negative terminal with a black wire on it which goes God knows where -- it's not shown in the wiring diagram. (What is that for, by the way?)

I started pulling wires off and testing voltages.

I have 25 volts (normal system voltage) ON ALL TERMINALS, including even the negative one. The small negative terminal has 25 volts potential with the chassis of the alternator -- weird.

And all that is whether or not I have the ignition switched on. So the source of the voltage is the main large output cable which is hard-wired to the batts.

I assume that means there is some internal short in the alternator -- so something was not installed right when the alternator was rebuilt.


Anyone have any insights? My understanding of electrickery is very weak and I will be grateful for any tips.

The easiest way to get 100% reliable help on this starts with knowing exactly what model of alternator you have. I am not sure I understand what model of Leece Neville alternator you have. Is it a 8SC3068VC?

[jimbunyard]

I've no experience with this alt at all, and don't know if this helps, but this is from Leece Neville.

[svlamorocha]

Originally Posted by Dockhead

Anyone have any insights? My understanding of electrickery is very weak and I will be grateful for any tips.

If I am guessing correctly the model you have (8SC), then this is how I would connect it:

* Ignition terminal in top right does not need to be connected to anything because there is an ignition lamp.
* “R” (aka D+) goes to lamp, then to ignition switch
* B+ goes to battery positive
* B- goes to battery negative via ground bus etc (I suspect this one is not connected)

I know it is worrying to see a wire that goes nowhere. In these alts you use either the "ignition" wire or the "R" wire depending on whether you have a lamp.

[OceanSeaSpray]

Thanks Jim, I couldn't find that diagram when I tried. It looks close enough to be very relevant.

"EXC" should go to the ignition key or an oil pressure switch that comes on after the engine starts.
"R" is the charging light, which is in itself normally connected to the same place as "EXC".
You need battery cables on both "+" and "-" as stated earlier. Many alternators won't operate at all if both EXC and D+/R are not connected.

There seems to be no tach output.

[smac999]

Originally Posted by Dockhead

I also have an Exciter terminal, which is apparently connected through the "no charge" light, although the wiring diagram doesn't show this. In any case, when I disconnect this wire from the alternator, it has 25 volts on it with the ignition on, and 0 with the ignition off. I am guessing that if I were to ground it with the ignition on, the "no charge" light would come on, but I haven't done that experiment.

stick a 1a fuse on end of the wire and ground it. you'll know if it's the lamp circuit (light turns on) or an ignition circuit (fuse blows). it's the fastest way I've found to test a lamp / ignition wire to figure out which was which. and yes I blow the odd fuse... beats spending hours tracing a wire. (and tests the bulb)


sometimes a ground wire at the alt is ground for something on the boat. the last alt I worked on it was actually the ground for the ignition system. so when the alt was disconnected the key wouldn't power up the boat... stupid. but I don't know why the ground wire would have 25v on it. but is this connected to the alt or not connected?

[Stu Jackson]

Years ago there was a download for The 12 Volt Doctor's Alternator Handbook, by Edgar Beyn, published by Spa Creek. May still be around, try Google. Good luck. It's a very good text. If you can't find it, send me a pm, it's pdf.

[Mikado]

The very first thing I would do is take it back to the rebuilder or the.nearest auto electric shop and have them bench test your alternator to eliminate the question of whether your alternator is good or not.

[exMaggieDrum]

Is the alternator internally regulated, or external? The small black wire "might" be for an internal regulator. Your pic does not show it and the drawings don't either.

You need to be careful reading some of the helpful replies here as some of them direct contradict the others.

I'll throw in my two cents just to stir the pot. "+" is the output and "-" is the output negative. Size both wires for the full output of the alternator and run length. "Ignition" turns on the alternator. If you are using an external regulator (smart or not) the lead from it goes here. "Indicator lamp" goes to a lamp on the panel (if it has one) that says the alternator is putting out (or at least turned on).

If it has internal regulation and that is what you want (you shouldn't with this size alternator though) then you are good. If you have external regulation (should have) then the internal regulator has to be disconnected, and how you do that is dependent on the alternator.

Recommend you get a good alternator shop and/or experienced tech to explain this to you in person. There are literally hundreds of wiring types of alternators and they don't all follow the same terminology and case markings. So if someone has a different alternator, even another Leece-Neville large frame, don't go with what they have - necessarily. Alternators are the most confusing things I have every worked with in all my years as a marine electrician. I found that even alternator shops can be wrong and give the wrong advice. Almost none of them have any experience with external regulators but they don't mind guessing so they don't sound ignorant.

If it were me I would take the windlass out of the equation but that has been gone over in other threads ad nauseum. You are a capable operator and can decide on your own when you want the engine running to pull up the anchor. Normally you would but you would always start the engine before you unhook anyway.

[Dockhead]

Thanks for all the helpful replies.

The alternator is a Leece-Neville 8SC3014U.

It is one of two alternators on my main engine -- the first one is the standard Hitachi, which does nothing but charge the main engine start batt.

It is externally regulated using an Adverc, which is unique in that the standard internal regulator is NOT disconnected or removed and remains fully functional -- you can just unplug the Adverc if you suspect a malfunction and want to revert to the standard regulator.

Here is the wiring diagram:

http://www.prestolite.com/literature...ng_Diagram.pdf


I have a sickening feeling I know what the problem is. I thought the thing was grounded through the case. But I see from the wiring diagram that B- is supposed to have a large ground wire. Damn. I didn't connect one up; didn't remember it, and it's not in my boat's wiring diagram.


Will I have blown the diodes, the way you do if you start up an alternator with no battery connected to it?

If so, are they hard to replace? Can I do it myself?

[svlamorocha]

Originally Posted by Dockhead

I have a sickening feeling I know what the problem is. I thought the thing was grounded through the case. But I see from the wiring diagram that B- is supposed to have a large ground wire. Damn. I didn't connect one up; didn't remember it, and it's not in my boat's wiring diagram.


Will I have blown the diodes, the way you do if you start up an alternator with no battery connected to it?

If so, are they hard to replace? Can I do it myself?

I believe you are spot on the root cause.

I am hopeful that because this is an internally regulated alt with sensing at the alternator itself and the ground was not disconnected while running, the diodes will be fine. Please let us know how it goes.

I guess you learned not to trust the boat´s wiring diagram. In fact form similar experience I have convinced myself that it is worthwhile having a proper "as built" diagram.

Please let us know how it goes.

[exMaggieDrum]

Originally Posted by Dockhead

Thanks for all the helpful replies.

The alternator is a Leece-Neville 8SC3014U.

It is one of two alternators on my main engine -- the first one is the standard Hitachi, which does nothing but charge the main engine start batt.

It is externally regulated using an Adverc, which is unique in that the standard internal regulator is NOT disconnected or removed and remains fully functional -- you can just unplug the Adverc if you suspect a malfunction and want to revert to the standard regulator.

Here is the wiring diagram:

http://www.prestolite.com/literature...ng_Diagram.pdf


I have a sickening feeling I know what the problem is. I thought the thing was grounded through the case. But I see from the wiring diagram that B- is supposed to have a large ground wire. Damn. I didn't connect one up; didn't remember it, and it's not in my boat's wiring diagram.


Will I have blown the diodes, the way you do if you start up an alternator with no battery connected to it?

If so, are they hard to replace? Can I do it myself?

Glad you are sorting it out. I don't think you will have blown anything up in the alternator by not having the negative on. It just wouldn't work, and neither would the other functions - lamps, etc. I think you will use the 2nd diagram of the two on the reference.

Alternator diodes typically blow up when the alternator is running and putting out juice and then the battery is disconnected before the excitation circuit is disconnected. Yours wasn't running at all. It was turning for sure but not running.