BAC RV Club Forum
General Boards => Technical Support => Topic started by: David Stump on March 26, 2026, 02:35:13 AM
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I have a 2006 Monterey with the cat c9 installed. I was having a fluctuating voltage issue (dash volt gage) where the voltage was fluctuating between 12 and 14 volts every half second or so. Dash lights and transmisssion indicator display would dim along with voltage. Worse at low idle. At any rate I was cleaning the connections on the back of the alternator and managed to break off the positive stud. So I get to buy a new alternator anyway.
Here is the actual question:
There are only three connections on the alternator: pos out, neg out and an unidentified small gage wire that disappears into a wiring harness. The small gage wire is connected to the regulator section of the alternator but is not marked. Not connected to a terminal but disappears into the regulator. Leese Neville tells me that this alternator does not require a “sense wire”. Since my system uses a bird and a big boy I don’t think it requires a sense wire either. So does anyone know what the wire may be?
Alternator is a Leese-Neville LBP2180
Thanks for any insight anyone can provide!
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David
It would be my guess that the small wire you are referring to is the excite or light wire which is supplying switched ignition power to the regular to bring the alternator to life often this is done through a dash charge indicator lamp which will light when the ignition is on and the alternator is not charging the light will go out once the alternator starts to charge and produce a charge voltage.
Disconnect the wire and verify that tit has 12 volts only when the ignition switch is turn on.
Hope this helps
Eric
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Thanks for the reply. In this case the small gage wire has a constant battery voltage. Ignition on or off. Chassis battery switch on or off. I would love to know where exactly the other end is attached and why it is wired to the regulator positive wire.
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David
It sounds to me like that wire is a voltage sensing wire and is like connected to or near the chassis battery positive .
On that particular alternator there is a cap which covers the back of the regular and brush slip rings ,out of each side of that cover there should be a wire .
One runs to the ground stud on the back of the alternator known as( B-) or battery negative, that stud should be connected to a good battery ground
Ideally the battery negative terminal but most often to the rear ground stud of the starter.
The other small wire which exits the rear of the regular cover is usually connected to the ( B+) or battery positive terminal stud of the alternator
This small wire is providing the battery sense voltage for the regular allowing the regular to regulate its output accordingly.
The problem with this type of setup is that any volt drop ( caused by poor connections long wire runs or a Diode type battery isolator as where used in earlier coaches )Would cause the regular to effectively see its own output and not true battery voltage resulting in under charged batteries.
Because these coaches often have their battery a distance from the alternator the length of the battery cables and the afore mentioned volt drop associated
Some of these coaches where set up with an external sensing wire to give the alternator a more accurate battery voltage to work towards.
On this setup the small usually red wire running out of the right rear of the regular cover to the B+ terminal stud is supplemented with a small stud on the top of the regular where the alternator receives it's battery voltage sensing usually marke with an "S"
Although this bulletin is not for your particular model it is close enough to give you the layout.
https://studylib.net/doc/18030815/universal-alternator-wiring-diagrams
Hope this helps
Eric