Alternator exciter wire issue

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Alternator exciter wire issue

July 16, 2025, 08:15:35 am

First of all, let me state that I do not like working on electrical problems and electricity doesn't like me. (my DW's phone even turns off when she hands it to me)
OK, here goes. The rebuilt last fall Leese/Neville alternator on our coach, according to the dash gauge, was only putting out 11.5 volts. So, two days ago, with temps up in low 90s and high humidity I crawled under the raised bed and proceeded to remove the alternator. Red wire (+) off first. Then black wire (-) off. Then small red "exciter" wire off. This wire is SUPPOSED to be "dead" until the ignition key is turned on. I think I am OK. The key is out of the ignition and laying on the lower dash. YEAH!!! SURE!!! The exciter wire touched the black ground wire and suddenly I have sparks, smoke, and can't see squat!!! So why is the exciter wire live? I can only see about a foot of the now melted wire so where does this wire go?
Oh, by the way, the rebuilder checked the alternator and 14.4V. He said if the "exciter wire was "hot" all the time that it would draw down the battery. YUP, that was what was happening.

Re: Alternator exciter wire issue

Reply #1 – July 16, 2025, 08:39:45 am

Norm,

The typical Leece/Neville DUVAC alternator requires TWO small wires. One is the "excite" wire, which is hot ONLY when ignition key is ON. The other is the "sense" wire, which is typically hot all the time.

Without knowing exactly what model alternator you are working with, hard to say what the single small wire does or where it goes. However, if it was hot with the ignition key off, it sure sounds more like a "sense" wire than a "excite" wire.

Do you know the model number of your alternator, or can you post a photo of the back side of your alternator?

Re: Alternator exciter wire issue

Reply #2 – July 16, 2025, 09:03:45 am

Only number I could find was #102200 on the Leese/Neville nameplate, along with the 14V number.
I took pics of the back of the alternator, but haven't yet figured how to send them, so here is a description of what it looks like:
Far left (slightly large post) the negative post
Far right (large post) the positive post
About 1" to right of the negative post is the small post referred to as the "exciter" post.
Below this post is a recess with three small posts, with the right one slightly separated from the other two posts.
In all the years we have had the coach, with the alternator working correctly, we have never had more than the positive, negative, and the small wire to the alternator.
On my electrical schematic there is no mention of either the exciter wire or a sense wire. Not even lines or words.
My problem is that I was only a mechanical engineer and left the electronics to the electrical engineering dept.

Re: Alternator exciter wire issue

Reply #3 – July 16, 2025, 09:10:27 am

Photos below (from Forum archives) of the typical L/N DUVAC alternator. Do these look like your alternator?

Re: Alternator exciter wire issue

Reply #4 – July 16, 2025, 09:16:22 am

Quote from: nitehawk – July 16, 2025, 09:03:45 am

Only number I could find was #102200 on the Leese/Neville nameplate, along with the 14V number.

102200 is the voltage regulator number. Should be another plate on the alternator body somewhere with different model number.

Re: Alternator exciter wire issue

Reply #5 – July 16, 2025, 09:29:37 am

Nope. Looked all over while the alternator was on the counter. Where else might it be hidden?

Re: Alternator exciter wire issue

Reply #6 – July 16, 2025, 09:33:20 am

Quote from: nitehawk – July 16, 2025, 09:29:37 am

Where else might it be hidden?

Any paperwork (that might show the model number) from when you bought it or when you last had it worked on?

Re: Alternator exciter wire issue

Reply #7 – July 16, 2025, 09:49:22 am

Quote from: nitehawk – July 16, 2025, 09:03:45 am

In all the years we have had the coach, with the alternator working correctly, we have never had more than the positive, negative, and the small wire to the alternator.
On my electrical schematic there is no mention of either the exciter wire or a sense wire. Not even lines or words.

If your schematic doesn't mention or show either wire, why do you assume the small wire is a "excite" wire? It could just as easily be a "sense" wire. In fact, that would make more sense (sorry - pun not intended).

There are some alternators that are "self excited". That is, they do not require external excitation to start outputting voltage. If this is how your mystery alternator works, then perhaps the small wire is indeed a "sense" wire.

Does your coach have a diode battery isolator?

Re: Alternator exciter wire issue

Reply #8 – July 16, 2025, 09:55:06 am

The rebuilder yesterday showed us how the alternator did not charge without an exciter wire. Zero volts without and 14.4 volts with. He was quite knowledgeable about Leese/Neville and had rebuilt the same alternator about eight years ago.
Guess my getting the start battery cable crossed two years ago didn't help much either. Spark display and heat were nice to watch, tho.
No names here please---just dumbass will do.

Re: Alternator exciter wire issue

Reply #9 – July 16, 2025, 09:58:03 am

Does your coach have a diode battery isolator?

Re: Alternator exciter wire issue

Reply #10 – July 16, 2025, 10:07:47 am

Where would it be and what does it look like?

Re: Alternator exciter wire issue

Reply #11 – July 16, 2025, 10:18:10 am

Quote from: nitehawk – July 16, 2025, 10:07:47 am

Where would it be and what does it look like?

I don't know where it would be located on your coach.
Finned aluminum case with 3 or 4 large battery cable posts...something like the one below.

Re: Alternator exciter wire issue

Reply #12 – July 16, 2025, 10:26:46 am

Mine is on the driver's side frame rail in the engine compartment. Someone built a "platform" above the frame rail to hold the 50A cord. Also, right under that is a small fan cooled tranny radiator. (nice, huh?)

Re: Alternator exciter wire issue

Reply #13 – July 16, 2025, 10:28:13 am

Got to leave for awhile--Drs appointments for my cancer followup.

Re: Alternator exciter wire issue

Reply #14 – July 16, 2025, 10:30:18 am

Re: Alternator exciter wire issue

Reply #15 – July 16, 2025, 03:23:26 pm

I am back now, but it is raining "a little bit". We are in a Flood Watch until 7:00 PM tonight so no sense in going out in the downpour.
But I am still curious as to where the red wire (that smoked and melted) goes to from the alternator.

Re: Alternator exciter wire issue

Reply #16 – July 16, 2025, 07:22:33 pm

Quote from: nitehawk – July 16, 2025, 03:23:26 pm

But I am still curious as to where the red wire (that smoked and melted) goes to from the alternator.

If you can't find the wire on a wiring schematic, then you will have to either physically follow the wire by hand to see where it goes, or use a circuit tracing device to locate the other end of the wire.

If you have a diode based battery isolator on your coach, and if your alternator does not have a "sense" wire connected, then I don't understand how your charging system works.

Re: Alternator exciter wire issue

Reply #17 – July 16, 2025, 07:35:25 pm

On some models of the earlier rear engine diesel engines the alternator had to be revved up to a little over a 1000 rpm to start charging. That has been too long ago for me to remember the wiring, they didn't have duvac alternators then. I know this isn't much help. I wonder what would happen if you hooked up an ignition hot to that terminal and tape off the small wire that is hot all the time.

Re: Alternator exciter wire issue

Reply #18 – July 16, 2025, 10:49:48 pm

My excite wire is about 9 in long. And runs from the fuel solenoid to the little post on the alternator.
I don't think that any other small wires are used.

Re: Alternator exciter wire issue

Reply #19 – July 16, 2025, 11:29:22 pm

Mike? Say what?? The wire goes from the fuel solenoid to which "small post"?

Re: Alternator exciter wire issue

Reply #20 – July 17, 2025, 10:00:40 am

Quote from: nitehawk – July 16, 2025, 11:29:22 pm

Mike? Say what?? The wire goes from the fuel solenoid to which "small post"?

It would go to the SENSE terminal.

There is a voltage loss across the diode-based battery isolator. By having the sense wire on the chassis battery side of the isolator, the voltage regulator compensates for the loss. So for example alternator puts out 14.7 VDC. There is a .7 VDC loss in the isolator. The batteries then get 14.0 VDC.

Re: Alternator exciter wire issue

Reply #21 – July 17, 2025, 01:17:04 pm

Sob, whimper, cry, tear what remaining hair I have out!! My alternator DOES NOT have a post labeled "sensor.

Re: Alternator exciter wire issue

Reply #22 – July 17, 2025, 01:27:14 pm

Quote from: nitehawk – July 17, 2025, 01:17:04 pm

Sob, whimper, cry, tear what remaining hair I have out!! My alternator DOES NOT have a post labeled "sensor.

Then it is not the correct alternator to be used with a diode-based battery isolator, as the isolator reduces voltage by about .7 VDC and the remote sense wire (to the chassis battery side of the isolator) compensates for the voltage loss in the isolator by telling the regulator what voltage is actually getting to the chassis battery. Yes, the sense wire can "come from" any location that shows chassis battery voltage-- does not need to be at any particular connection. On many, it is on the chassis battery lug of the battery isolator.

With engine at 1,000+ RPM, check voltage at alternator B+ to ground and then at the center lug of the diode-based battery isolator to ground and then at the two outer lugs to ground. Let us know what you find.

There ARE workarounds that are a lot less $$ than a new alternator.

Re: Alternator exciter wire issue

Reply #23 – July 17, 2025, 03:35:41 pm

Hey Brett. How do I know if I have a diode-based battery isolator alternator thingee??

Re: Alternator exciter wire issue

Reply #24 – July 17, 2025, 03:57:44 pm

Follow the wire from the B+ terminal of the alternator. Should go right to the center lug of the rectangular, finned diode-based battery isolator.

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