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Old 01-19-2018, 03:23 PM   #1
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Alternator charge light

Alternator charge light came on and a few minutes later it was off. Checked voltage on Scangauge D and volts were 12.5 and fluctuating to 12.3
Pulled over at first opportunity to check belt and any obvious issues.
Turned off all electrical loads and drove 4 more hours to South Padre Island.
Checked again for loose pulley, belt, wires etc.
Itís a 19 year old coach so I just replaced the Leece Neville 200 amp alternator.
I should be getting around 13.7 volts to the battery but Iím not.
Now Iím wondering if the battery isolator is bad.

Has anyone had a similar experience?
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Old 01-19-2018, 03:50 PM   #2
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Did you monitor your voltage while running with your new alternator ? If Its not the correct DuVac alternator, it will overcharge and burn out, boil your batteries or blow the isolator.

You can check the isolator with a multi meter.
An isolator is a pair of diodes potted in a heat sink. The center post of the isolator is the Anode of both diodes and the outer posts are the 2 cathodes.

All that means is alternator output goes in the center and out each side. It can not come back thru.

Set the multi meter on diode test and check from center to each side, reversing the meter leads to test. An open ( bad ) diode will not make the meter beep either way.
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Old 01-19-2018, 04:37 PM   #3
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Quote:
Originally Posted by twinboat View Post
Did you monitor your voltage while running with your new alternator ? If Its not the correct DuVac alternator, it will overcharge and burn out, boil your batteries or blow the isolator.

You can check the isolator with a multi meter.
An isolator is a pair of diodes potted in a heat sink. The center post of the isolator is the Anode of both diodes and the outer posts are the 2 cathodes.

All that means is alternator output goes in the center and out each side. It can not come back thru.

Set the multi meter on diode test and check from center to each side, reversing the meter leads to test. An open ( bad ) diode will not make the meter beep either way.


A couple of things are bothering me.
When I took the cover off of the low voltage/ high current plate I noticed that the isolator was loose from the heat sink. Testing with the multi meter the diodes seemed to be fine ( open in one direction but showing resistance in the opposite direction) I was expecting continuity with little or no resistance.
I then took the alternator wire and bolted it directly to the chassis battery wire and ran the engine for a minute. This seemed to be the answer but the voltage was to high. Almost 17 volts! Iím wondering if I bought a faulty alternator or do the voltage regulators typically need adjusted right of the shelf?
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Old 01-19-2018, 05:09 PM   #4
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That says the alt is still working but it is not getting battery voltage to the voltage regulator.

You should have a battery voltage wire from the chassis battery to the Duvac terminal on the alt. It may be there but fused.

If that's not connected the alt can't sense battery voltage and charges at max output.
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Old 01-19-2018, 06:34 PM   #5
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Quote:
Originally Posted by Scotialaddie View Post
... Testing with the multi meter the diodes seemed to be fine ( open in one direction but showing resistance in the opposite direction) I was expecting continuity with little or no resistance.
Do you have a meter that can measure the forward voltage drop? It will be between (typically) 0.2 and 0.7 volts. There are two types of diodes, the lower voltage drop ones (Shockley have lower voltage drops).

Resistance readings are a bit misleading.

Quote:
Originally Posted by Scotialaddie View Post

I then took the alternator wire and bolted it directly to the chassis battery wire and ran the engine for a minute. This seemed to be the answer but the voltage was to high. Almost 17 volts! Iím wondering if I bought a faulty alternator or do the voltage regulators typically need adjusted right of the shelf?
Sounds very much like the regulator was trying to compensate for the isolation diodes. With the diodes out of the circuit using a direct connection the voltage would in fact be high.
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Old 01-19-2018, 06:40 PM   #6
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As quick test jump all three of the posts of the isolator together. This will put the regulator online as long as it does not have a blown fuse etc.

If the voltage is behaving properly you can then progress backwards by unstacking the wires.
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Old 01-19-2018, 07:32 PM   #7
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Quote:
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That says the alt is still working but it is not getting battery voltage to the voltage regulator.

You should have a battery voltage wire from the chassis battery to the Duvac terminal on the alt. It may be there but fused.

If that's not connected the alt can't sense battery voltage and charges at max output.


I think you nailed it. When I bypassed the isolator I neglected to add the battery sense wire. This would explain the excessive voltage.
Will have to wait till tomorrow morning to know for sure. Iíll post my findings.
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Old 01-20-2018, 03:24 PM   #8
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Thanks everyone for the info!
It was indeed the battery isolator.
I put the chassis cables on the alternator stud and am getting 13.8 to the battery. This got me to San Antonio where my new isolator will be delivered on Tuesday morning.
The added bonus is that the house battery bank should also charge while weíre driving!
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Old 01-20-2018, 03:34 PM   #9
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The house battery should have been charging thru the isolator all along.
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Old 01-21-2018, 08:03 PM   #10
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Quote:
Originally Posted by twinboat View Post
The house battery should have been charging thru the isolator all along.


That should have been my first clue.
It seems that diode failed a month or so ago but I didnít bother to investigate since Iíve been only traveling short distances every few days. However when the chassis diode pooped I saw that right away on my scangauge as I watched the voltage slowly drop.
The big takeaway is how much Iíve been learning about this coach.
Much thanks to you Twinboat and everyone else for your responses.
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