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Old 12-02-2014, 11:16 AM   #1
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Starting up of the alternator

Hello my USfriends. When I start the motor of my excella, the alternator doesn't produce more than 11.5V. I have to accelerate once to 3000tr/mn for get a voltage of 12.5V (on the batteries, and 13.5V on the diode bridge but my main battery was low ). After 3000 tr, when the engine get down to 1000tr, the voltage rest ok 12.5V.
So when I drive, if I never pass through 3000, my batteries will not reloaded !
Is it normal the alternator doesn't begin to produce down 3000tr ? And no more than 13.5V ?
Thank you ( ouf ! sorry for my fran-glish )
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Old 12-02-2014, 11:20 AM   #2
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It's not normal and sounds like you have a bad alternator. Most likely a bad regulator in the alternator.

At about 2000 engine RPM you should see approximately 14 volts.
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Old 12-02-2014, 01:44 PM   #3
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I would agree, bad regulator in the Alternator. Common part here in the states, but not so sure how easy it is to get in France.

The voltage is temperature compensated. At lower temps, you should see in the 14.5 volt range with a normal regulator. But if yours is bad, it may only go to 13.5 volts all the time.
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Old 12-02-2014, 01:50 PM   #4
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A second thought for you. Try removing the wire from the alternator to what you call the diode bridge, and attaching it directly to the lug which goes to the engine battery. See how it behaves. If it changes to operate more normally, the "diode bridge" (we call them battery isolater here) may be internally defective rather than the regulator in the alternator.
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Old 12-03-2014, 01:51 AM   #5
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Thank you so much. Do you know if there are several kinds of alternator for an excella 81 or only one ???
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Old 12-03-2014, 06:20 AM   #6
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Quote:
Originally Posted by bigbidule View Post
Thank you so much. Do you know if there are several kinds of alternator for an excella 81 or only one ???
Several. You will need the number off the casting on the housing to get a proper replacement.
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Old 12-05-2014, 10:29 AM   #7
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If you are seeing 13.5 off the alternator is would suspect corroded connections. I'd clean them all up before chasing other more pricy possibilities.
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Old 12-06-2014, 01:46 AM   #8
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With news batteries the alternator voltage is now 14.1V on the batteries separator and 13.5 on each battery park. It seems ok. BUT the alternator still begins to produce this 14.1 at 2500 rpm, not before. There are 2 wires on the back / 12 hours, of the alternator. There is permanent 12V on the 2. What about the 1 ?
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Old 12-06-2014, 09:28 AM   #9
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GM alternators have a "sense" wire which determines the voltage after the battery isolator and resistance through a lot of the wiring up to the ignition switch. I am not sure just where it is attached under the dash, but the purpose is to compensate for the voltage drop in the chassis wiring.

In general, there is a plug in the back of the Alternator with two small wires coming from it. One is this sense wire, and I believe the other is for what we call the Idiot light... the light which comes on when the alternator is not working at all.

Then there is the large wire which is the Alternator output, and should go directly to the battery isolator center terminal.

There will be + 12 volts on the large output wire but only when the alternator is operating. I think there will be +12 volts on the small "sense" wire. There would probably not be voltage on the "idiot light" wire.

These are my recollections, I am not truly confident of my comments though. Perhaps others will chime in and offer better information.
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