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Old 03-07-2014, 09:42 AM   #1
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Voltage gauge fluctuates high/Low

2000 HR with Cummins Engine, new/rebuilt alternator. When the batteries are low and the alternator appears to be putting out. The battery indicator on the dash lights up and the gauge above the dash indicates 14.2 but drops down to around 12.3 then jumps back to 14.2. this happens every 10/15 seconds. Voltage going into the isolator from the alternator reads around 15.3. on the other side of the isolator going into both battery banks voltage is 14.2. I have not checked the voltage from the alternator to the isolator to see if it is also fluctuating but I will next time Im at the MH. Any ideas what could cause this fluctuation?
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Old 03-07-2014, 09:56 AM   #2
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Are you sure the shop that did the rebuild was a reputable one and the alternator was rebuilt correctly?

That's the first place I would check since it wasn't doing it prior to the rebuild.

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Old 03-07-2014, 10:29 AM   #3
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If the voltage at the batteries are a steady 14.2 with the engine running but the dash gauge is fluctuating then the connection to the dash gauge would be suspect.

At least, you are then getting the correct charging effect from the alternator to the batteries.
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Old 03-07-2014, 10:44 AM   #4
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I think before blaming the alternator or rebuilder it would be prudent to clip another meter to the circuit and VERIFY that it is a REAL voltage fluctuation and not a loose connection to the dashboard gauges or a bad ground. You might also remove, inspect, and clean all battery connections.
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Old 03-07-2014, 10:56 AM   #5
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Another vote on Alternator, or it's connection to the battery bank. A bad connection to the gauges wouldn't coincidentally read 12.3V (the battery voltage), it would likely drop to zero. Have fun chasing!

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Old 03-07-2014, 11:32 AM   #6
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Thanks for your input, I will do as you suggest and check the connections. Is there a chance there is a bad solenoid somewhere is the circuit? Would a bad diode in the alternator act like this?
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Old 03-07-2014, 11:47 AM   #7
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Keep in mind that the voltage drop thru the isolator is about a 1/2 volt.
Does your system use a out board regulator if so that might be your problem.
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Old 03-07-2014, 12:13 PM   #8
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Not sure what a out board regulator is. I have a Big Boy ( or at least I think I do) attched to the isolator and that controles the boost between the two banks
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Old 03-07-2014, 12:29 PM   #9
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You either have a internal regulator a part of the alternator or you have a alternator
that uses a out board regulator.
You say you have a isolator and that allows two or more batteries to be charged with
neither battery or banks of batteries being effected by the other ie the reason it is called a isolator.
You need to find out if the alternator has either a internal or external regulator.
A flaky regulator will cause the problems you indicate as will several of the items
others have posted.
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Old 03-07-2014, 01:23 PM   #10
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Again, thanks eveyone, I will run the test you suggest and see what I get. I do have an internal regulator so am am leaning toward a bad alternator.
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Old 03-07-2014, 01:48 PM   #11
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Quote:
Originally Posted by Rehbergl View Post
Thanks for your input, I will do as you suggest and check the connections. Is there a chance there is a bad solenoid somewhere is the circuit? Would a bad diode in the alternator act like this?
Could be a solinoid but I hope one is not installed. I too think a regulator problem may be there. Do you have a fl chassis? If so my voltmeter went to 0 and turned out to be the vdc. Look up vdc on rv2. A bad diode would cause your alternator to scream while charging as the load is no longer the same throughout the revs.
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Old 03-07-2014, 05:12 PM   #12
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if I understand correctly you have a isolator that allows the regulator to charge both battery banks. The very easy check to make is to disconnect the isolator from the charging system. Start the engine with only the chassis batteries in circuit. See if the problem still exists. If not your alternator is ok and the problem is with the isolator or the coach batteries.
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Old 03-07-2014, 06:47 PM   #13
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what is the temperature where you are at

I recall going through this same problem, possibly on another forum, but the same engine and year. It was determined that, If it is below 50 degrees the voltage will change as the preheater comes on and off.

after a few minutes it should stabilize. my meter does the same thing. 14 plus then it drops then it goes up then it drop - eventually it stays at 14
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Old 03-07-2014, 09:28 PM   #14
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Same for me. During cold weather, this happens for about 2 minutes, engine warms up then stabilizes. I had it checked out this past December and it was the preheater doing it.

Quote:
Originally Posted by ladagobago View Post
I recall going through this same problem, possibly on another forum, but the same engine and year. It was determined that, If it is below 50 degrees the voltage will change as the preheater comes on and off.

after a few minutes it should stabilize. my meter does the same thing. 14 plus then it drops then it goes up then it drop - eventually it stays at 14
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