Author Topic: Erratic Alternator Voltage  (Read 650 times)

Jerry Emert

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Erratic Alternator Voltage
« on: March 11, 2023, 12:21:40 AM »
I noticed the alternator gauge needle was bouncing between 13-16 volts today. While fueling up at sundown I noticed the side marker lights were changing brightness (flickering) in the same manner.
I would normally suspect the regulator but I am not a mechanic.
I just replaced the chassis batteries a few days ago so, of course, I’m my main suspect.
I drove around 60 miles today.
Ideas?
Thank in advance.
Jerry, Chief USN Retired
2003 Patriot Thunder Lexington 40' 3 Slides
C-12 Ser#  2KS89983
4000MH

Eric Maclean

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Re: Erratic Alternator Voltage
« Reply #1 on: March 11, 2023, 01:52:06 AM »
Jerry
If your coach has a duvac type alternator it will have a voltage sense terminal at the back of the alternator this sense terminal is used in a system where there is no connection directly to the battery bank such as when a diode type isolator is used or anytime the battery bank is a long enough distance from the alternator that a volt drop would create a problem.
The sense terminal wire should run directly to the chassis battery bank positive terminal to give the alternator an accurate battery bank voltage for the regulator to act on .

Take a real close look in behind your battery trap to see if maybe you dropped a wire behind while changing out the batteries.
 ( hey it happens)

Your coach likely has a Leese Neville alternator which will have a large ground cable and positive cable.
Make sure the ground cable is good and the connection points are clean.
There should also be two smaller wires one will be powered only will the ignition on. And the other should be powered all the time it's this wire which is the sense wire and it should read the same voltage as the chassis batteries .  Without the sense wire the alternator will over charge or bounce as you described.


Hope this helps
Eric
1997 Patriot Yorktown
3126-B
2009 Chevy HHR
Roadmaster falcon tow bar
Demco Air Force one tow brake.
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Jerry Emert

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Re: Erratic Alternator Voltage
« Reply #2 on: March 11, 2023, 01:34:05 PM »
I do have the duvac  type with sense wire. As I recall there were 3 or 4 single wires attached to the terminals on + and - terminals. One of the ring connectors fell off and had to be replaced. I noticed some corrosion in the wire but it was too short to cut back and I wanted to avoid a splice. Maybe an issue. All terminals and wires were cleaned with a wire brush drill attachment and coated with dielectric grease.
Thanks for your advice I’ll look for any missed wires behind the batteries and check out that wire.
Jerry
Jerry, Chief USN Retired
2003 Patriot Thunder Lexington 40' 3 Slides
C-12 Ser#  2KS89983
4000MH

Eric Maclean

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Re: Erratic Alternator Voltage
« Reply #3 on: March 11, 2023, 11:19:53 PM »
Jerry
If all else fails run a wire from the chassis battery positive to the sense terminal on the alternator to see if your voltage stabilizes
The wire should be fused near the battery bank.
1997 Patriot Yorktown
3126-B
2009 Chevy HHR
Roadmaster falcon tow bar
Demco Air Force one tow brake.
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Jerry Emert

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Re: Erratic Alternator Voltage
« Reply #4 on: March 17, 2023, 07:44:20 PM »
The documentation Ive seen on the sense circuit indicates that there is a fuse in the circuit. Ive checked in the fuse bay above the batteries and cant find any blown fuses there.  Any clue where the fuse might be?
it is incredibly tight behind the alternator. when i installed it i remember having to force it to bend the rubber slightly behind it.  I have no desire to go through that again. so I would have to strip the sense wire or cut it and slice in a new one.  Would rather find a blown fuse. 
so I appreciate any advice on fuse location, or anything else.
Thanks as always.
Jerry
Jerry, Chief USN Retired
2003 Patriot Thunder Lexington 40' 3 Slides
C-12 Ser#  2KS89983
4000MH

Steve Huber Co-Admin

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Re: Erratic Alternator Voltage
« Reply #5 on: March 17, 2023, 11:24:27 PM »
Jerry,
The battery bay is the obvious location but since you didn't find any blown fuses there, it's probably in line between the batteries and the alternator. If you can ID the correct wire you should be able to trace it. Since you had trouble squeezing the alternator into place, I'd suspect that may be your problem area. Sounds like a PIA but maybe the sense wire is broken back there.
Steve
Steve
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Eric Maclean

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Re: Erratic Alternator Voltage
« Reply #6 on: March 18, 2023, 10:22:50 PM »
Jerry
The sence wire is usually fused close to where it picks up it's power source because the wire if shorted is a direct connection to a high amperage capability and would burn.

In your coach that sence terminalmay be connected into the battery side of the boost solenoid on the isolator board under the bed.
As Steve suspects the wire may be broken near the back of the alternator from man handling it into place .
As I suggested before it might be more expedient to run a new wire

Eric
1997 Patriot Yorktown
3126-B
2009 Chevy HHR
Roadmaster falcon tow bar
Demco Air Force one tow brake.
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Jerry Emert

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Re: Erratic Alternator Voltage
« Reply #7 on: March 19, 2023, 03:13:09 AM »
Any ideas what the fuse value should be?  It wasn’t listed on the diagram I found.
Thanks
Jerry
Jerry, Chief USN Retired
2003 Patriot Thunder Lexington 40' 3 Slides
C-12 Ser#  2KS89983
4000MH

Steve Huber Co-Admin

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Re: Erratic Alternator Voltage
« Reply #8 on: March 19, 2023, 12:45:16 PM »
Jerry,
The sense wire may not be fused. See pages 36-37 of the 2003 Marquis wiring diagram in Coach Assist. Pg 36 shows 3 wires to the alternator with ring connections; #450 Alt out, #23 Ign pwr, and 1A which may be the sense wire.
In an earlier post you mentioned a corroded wire that you didn't want to splice. Did you correct it or leave it be?
Steve
Steve
2015-          07 Contessa Bayshore C9,  400 hp
2013-2015: 00 Marquis Tourmaline, C12, 425 hp
2005-2013: 01 Contessa Naples, 3126B, 330 hp

Eric Maclean

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Re: Erratic Alternator Voltage
« Reply #9 on: March 19, 2023, 05:29:14 PM »
Jerry
Steve is likely correct as Beaver commonly makes  the 12 volt power supply wires as either (1 or 1A) andit may not have been fused from the factory .
If you find a wire marked 1 or A near the back of the alternator check to see if it has a constant battery voltage if so that's your sense wire ( remember the alternator is looking at this voltage to determine the Chassis battery state of charge if there is a voltage difference between the terminal at the alternator and the battery and voltage the system work properly.
If for some reason the sense wire is broken the alternator output will either fluctuate seeking a response from the sense wire or there will be a no charge condition .
If there is a volt drop on the sense terminal wire ( ei corrosion ) the alternator will try to raise the alternator end of that sense wire voltage to it's regulated voltage ( 14.2 volts ) which will cause the alternator to output higher by the amount of volt drop in the sense wire causing an over charge condition. Therefore it is important that the sense wire be in good shape with no poor or bad connections .
It is best to have a sense wire that runs directly to the battery bank it is intended to monitor this is the reason for the fuse as the wire is usually a 12 or 14 awg wire connected directly to a high amperage capacity ( a 10 amp fuse here is lots)
Beaver on the other hand has likely picked up it's sense voltage from another source possibly the stater cable at the starter or perhaps directly to the boost solenoid connections .
Remember the alternator must have a good ground connection as well to have a correct sense capability.

Hope this helps
Eric
1997 Patriot Yorktown
3126-B
2009 Chevy HHR
Roadmaster falcon tow bar
Demco Air Force one tow brake.
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Jerry Emert

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Re: Erratic Alternator Voltage
« Reply #10 on: March 19, 2023, 05:56:20 PM »
Jerry,
The sense wire may not be fused. See pages 36-37 of the 2003 Marquis wiring diagram in Coach Assist. Pg 36 shows 3 wires to the alternator with ring connections; #450 Alt out, #23 Ign pwr, and 1A which may be the sense wire.
In an earlier post you mentioned a corroded wire that you didn't want to splice. Did you correct it or leave it be?
Steve
Yes I replaced the ring connector. No help. It was on the battery.
I couldn’t get to the connector on the sense wire where it attaches to the alternator. I shaved some of the insulation on the wire near the alt. and it does not have 12 volts on it either when started or engine or ignition key off.
I hesitate to connect it to 12 volts just because of my lack of experience working on engine related stuff. I don’t want to cause more harm. I’ll make that decision today.
Thanks
Jerry, Chief USN Retired
2003 Patriot Thunder Lexington 40' 3 Slides
C-12 Ser#  2KS89983
4000MH

Steve Huber Co-Admin

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Re: Erratic Alternator Voltage
« Reply #11 on: March 19, 2023, 07:35:10 PM »
Jerry,
If the sense is fused (Duvac diagrams show a fuse) and it's not near the batteries/isolator, it almost has to be near the alternator. I would look for fuse mounted to the side of coach floor/side with the bedroom access panel removed. The fuse shown in the wiring diagram shows a surface mount holder.
Steve
Steve
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2013-2015: 00 Marquis Tourmaline, C12, 425 hp
2005-2013: 01 Contessa Naples, 3126B, 330 hp

Jerry Emert

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Re: Erratic Alternator Voltage
« Reply #12 on: March 19, 2023, 07:55:55 PM »
I was gathering courage to hook 12 volts up to the sense terminal. I took everything apart again. Cleaned up the wires and made sure all connections were good. Checked voltages one last time and found 12 volts on the sense wire. So it looks like I inadvertently fixed it for now. I’m hoping for good. So as usual I seem to create problems while fixing others.  I did discover that none of the wires connected to the battery or back of the alternator are the original beaver wiring. There are even red wires connected to the battery ground posts.
Thanks for all your help.
Jerry
Jerry, Chief USN Retired
2003 Patriot Thunder Lexington 40' 3 Slides
C-12 Ser#  2KS89983
4000MH

Steve Huber Co-Admin

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Re: Erratic Alternator Voltage
« Reply #13 on: March 19, 2023, 09:50:30 PM »
Sounds good. Let us know the outcome.
Steve
Steve
2015-          07 Contessa Bayshore C9,  400 hp
2013-2015: 00 Marquis Tourmaline, C12, 425 hp
2005-2013: 01 Contessa Naples, 3126B, 330 hp
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