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Old 09-20-2010, 01:04 PM   #1
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Alternator frying batts need help!!!

I just replaced the alt in my rv about a month ago and it wasn't putting out enough volts. I found that the wire from the back of the alt had a bad connection on it so I replaced it. That seem to fix the problem.

Untill yesterday. We got stopped at a red light and the engine died. Looked at the batts and one was boiling and spewing batt acid. finally they got cooled down enough to get to a parking lot. I took the alt out and took it to auto zone and had it tested it was bad. Picked up another new alt hooked it up and it was giving me 18 volts off the back of the alt. and 17.5 at the isolator and 17.5 to the batts. No wonder I fryed the batts. I thought I might have got a bad alt again so I took that one back and had it tested and it was fine.

So i thought it might be the batts that were bad. I took them to auto zone and had them tested, 3 out of the 4 were bad. :( ( they were only 6 months old) I replaced those. And am still having the same problem. I cannot figure this out. all the cables to the batts are nice and tight.

I think im going to start at the wires going to the voltage regulator on the alt and see if they are fryed. and then check the ground going from the batts to the engine. I might also check the ground from the engine to the chassis too. I would think It has to be a grounding issue. Due to not having any place for the extra volts to go.

I am at a loss and can't figure it out any help would be great.
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Old 09-20-2010, 02:17 PM   #2
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That sounds like the voltage regulator is kaput.
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Old 09-20-2010, 05:30 PM   #3
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l would suspect the voltage regulator also. Some regulators are adjustable, but sounds like yours is bad.

If you have a DVOM you can check for voltage drop between the connections. If you have more than 1/2 volt drop you need to figure out why, bad connection, corrosion, etc.
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Old 09-20-2010, 07:56 PM   #4
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My vote is for the voltage regulator as well.
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Old 09-21-2010, 07:34 AM   #5
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I would wager that you have a dodge components from the sound of it. And if so, the volt reg sounds kaput.
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Old 09-21-2010, 12:09 PM   #6
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Regulator is toast

The alternator will output voltage a lot higher than the system needs, it does what it is told to do.

The regulator controls the output of the alternator, and given that you have changed it and the replacment was the same type and assumed working then I suspect the regulatorhas a shorted output control.

It has the field wire shorted to the "on" position thus the alternator is running in a "floored" condition.

Locate the regulator and unplug it, then re-test, if the output of the alternator now reads the same as before the engine was running then the alternator is not self-exciting, if the voltage is still too high the alternator may be self exciting or there may be a shorted wire in the harness that needs to be confirmed.

If the alternator tests ok at the parts house and it was reading high with the regulator unplugged then locate the wire shorted to battery.

If the alternator is not running a high voltage then replace the regulator.

Be sure to get a regulator that is adjustable so the voltage can be set to correctly maintain your batteries with the voltage drop across the isolator.
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Old 09-21-2010, 04:16 PM   #7
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Mic I have a 82 chevy 6.2 diesel.

I took the alt back to auto zone and they tested it and it was fine. but I insisted the give me a new one. They did and I also had them test that one and it was fine. Got it installed and the problem persists.

The alt that I have has the regulator built into it. It has what looks to be like a pos wire and a neg wire going to it in a harness that you plug into the alt. I jammed my volt meter into the pos wire coming out of the regulator harness and it gives me 12.63 volts with the engine running. I would assume its regulating just fine. Now for the pos on the back of the alt I get a reading of 18. and a reading of 17.5 at the isolator and 16 to 16.5 at the batt terminals. I have checked the ground from the batts to the frame and the frame to the engine. and they seem fine.

I have no clue whats going on and hope someone can help me figure it out.
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Old 09-21-2010, 07:45 PM   #8
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The plug you speak of does two things: viewing from the rear forward the wire to the right (from ignition) provides the excitement to start the alternator working(an input) The wire to the left is a feedback (an output)for the idiot light you probably don't have.
You can try unplugging that plug from the alt. and restarting the engine.....you should have no output on the rear terminal (it will read batt. voltage).
I'm curious what output voltage Auto Zone read on the bench...or did their test station only say "good"?
To try and narrow things down you may run only a dedicated wire from the alternator output direct to the engine battery and check voltage.
Whats the output between the single post at the rear of the alt. and chassis ground? And battery ground?
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Old 09-22-2010, 11:47 AM   #9
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I would hook the battery wire from the alternator directly to the chassis batteries at the isolator. Isolators can give the alternator a false reading and it will continue to charge. This is very common.

You do not need the chassis batteries hooked to the isolator, just the coach batteries.
There should be 3 terminals on the isolator. Center is usually alt. output, move the chassis battery wire to this terminal, leave the coach batteries on one of the other terminals. If all else is good this might fix your problem.

The diodes in the isolator are a one way device, will let current go thru to the batteries, but not from the batteries to the alternator. There is a hefty voltage drop going thru the isolator also. You can hook the chassis batteries direct to the alternator and use a constant duty solenoid to charge the coach batteries. solenoids have almost no voltage drop. You can either wire this so it is on when the ignition is in the run position, or I recomment turning it on with a toggle swithc. My Itasca has the switch and it works great this way.
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Old 09-22-2010, 12:12 PM   #10
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Quote:
Originally Posted by TheDubs View Post
Mic I have a 82 chevy 6.2 diesel.

I took the alt back to auto zone and they tested it and it was fine. but I insisted the give me a new one. They did and I also had them test that one and it was fine. Got it installed and the problem persists.

The alt that I have has the regulator built into it. It has what looks to be like a pos wire and a neg wire going to it in a harness that you plug into the alt. I jammed my volt meter into the pos wire coming out of the regulator harness and it gives me 12.63 volts with the engine running. I would assume its regulating just fine. Now for the pos on the back of the alt I get a reading of 18. and a reading of 17.5 at the isolator and 16 to 16.5 at the batt terminals. I have checked the ground from the batts to the frame and the frame to the engine. and they seem fine.

I have no clue whats going on and hope someone can help me figure it out.
Yup, you still have that damned Diode Isolator that I told you to get rid of when you brought up your charging dilemma the first time. Diode = 1 way current, Alternator can't sense the voltage of the batteries, and thus is running at full tilt.

Get
Rid
Of
The
Diode
Isolator

Get
A
Constant-Duty
Solenoid.
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Old 09-23-2010, 09:57 PM   #11
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Given you have a 82 Winnebago, you should have a SI series alternator (most likely rated at 63 Amps). The B+ lead is red. There should be 2 more wires connected via plug to the back of the alternator. 1 of those wires is brown (field exitation from from ignition switch via either a idiot light or resistive wire). The other one is red (sensing wire). I am going to make a guess that the 12.63VDC reading (engine running) was on the red wire in the plug. The red wire is the sense signal that this version of alternator uses to determine system output. Automotive regulators are designed to maintain approx 14.1 VDC on the sense line. The 12.63 VDC reading indicates you have a bad connection (high resistance) in that wire and resulting in the alternator putting out the high voltage. It's trying to raise the 12.63 VDC reading up to 14.1VDC. I recently fixed the same type problem in a friends Ford based system. BTW, the 17.5VDC indication at the isolator indicates that the circuit (wiring) from alternator to isolator is ok. It's the red sense wire that is causing the problem

Good Luck,
Dave

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Old 10-09-2010, 08:15 AM   #12
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Dubs, how about an update on the problem ? Did you get it figured out ?
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