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  #1  
Old December 5th, 2011, 07:51 PM
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Alternator Question

My random Argentine alternator only charges above a certain RPM, around 1000 or so, so I assume it must be from some other vehicle. Wouldn't a normal alt charge at idle?
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  #2  
Old December 5th, 2011, 08:22 PM
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We had a similar issue with a scout alt. that would kick in and charge when revved up. I'm completely guessing here but it may be the hot wire that feeds the regulator or the regulator itself. Does it continue to charge once it kicks in or do you have to keep it revved up for it to charge?
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Old December 5th, 2011, 08:26 PM
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It's so damn sporadic. The truck actually died today at the end of a test drive. So the alt doesn't know when to charge.
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  #4  
Old December 5th, 2011, 08:39 PM
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is it a generator or alternator?
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  #5  
Old December 5th, 2011, 08:57 PM
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Quote:
Originally Posted by D90user
is it a generator or alternator?
Alternator
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Old December 5th, 2011, 09:18 PM
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gotcha... should work then
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  #7  
Old December 6th, 2011, 03:14 AM
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Im having the same problem and mines an alt. It charges above 850-900 but at an idle at a stop sign it will start to disscharge. Its a GM type alt im thinking its just turning to slow at idle. It has a hand throttle so i bump it up in the winter or at night so it will charge in stop and go traffic. I have thought about a smaller pully to get it to spin faster?. Its a real pain and would like to fix it if possible.


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  #8  
Old December 6th, 2011, 06:41 AM
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Shoot, I think you are on to something I didn't catch. The alternator has the larger pulley swapped over from the generator, so it's likely spinning slower anyway.
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  #9  
Old December 6th, 2011, 08:32 AM
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Is it worth the hassle? Just get a proper alternator and pulley... It should charge at idle.
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Old December 6th, 2011, 08:46 AM
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Originally Posted by Red90
Is it worth the hassle? Just get a proper alternator and pulley... It should charge at idle.
No, hence the question. I'll be getting a new alt.
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  #11  
Old December 6th, 2011, 09:19 AM
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An alternator at idle will be putting out such a paltry few amps that it is virtually not charging at all. There are some diodes that are what they call "Low turn on" in that the alternator will begin charging at a much lower RPM. It isn't unusual that you aren't seeing any amps below 1000.
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  #12  
Old December 6th, 2011, 09:35 AM
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Quote:
Originally Posted by o2batsea
An alternator at idle will be putting out such a paltry few amps that it is virtually not charging at all. There are some diodes that are what they call "Low turn on" in that the alternator will begin charging at a much lower RPM. It isn't unusual that you aren't seeing any amps below 1000.
Understood, but by that logic the motor is running off the battery, which doesn't make sense in a low RPM truck that could spend much of it's life off road below 1000 rpm.

Either way, the alt is intermittently charging.
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  #13  
Old December 6th, 2011, 10:39 AM
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What flavor and model number is it? The alt. that is.
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  #14  
Old December 6th, 2011, 10:47 AM
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Quote:
Originally Posted by James Pierce
What flavor and model number is it? The alt. that is.
No clue. Argentine special!
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  #15  
Old December 6th, 2011, 11:10 AM
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Quote:
Originally Posted by Wolf Fabrication View Post
Understood, but by that logic the motor is running off the battery, which doesn't make sense in a low RPM truck that could spend much of it's life off road below 1000 rpm.

Either way, the alt is intermittently charging.
It's not "logic" it's the way it is, no matter what alternator you have. Yes, alternators do charge intermittently. The alternator voltage sense wire is connected to the battery to determine the battery's voltage (or state of charge if you will). The alternator voltage regulator turns on the alternator output based on a factory low set point, usually somewhere around 13 volts.
Once the VR turns on the output the alternator will charge as much as it possibly can given a particular RPM. When the battery voltage once again reaches the high set point the alternator will turn itself off. The engine could be revving at 4000 rpm and the alternator will not be charging. This kind of VR is "dumb" in that it's power output curve is linear. There are external VRs that are "smart" in that they provide stepped increments of battery charge voltage based on the battery state of charge. That is, bulk charge, absorption charge, and float charge (there's a fourth charge rate called equalization that an alternator can't really handle).

If your battery is at, say, 14.4 volts at idle, then the alternator is not going to put out any charge current because the battery is full.
Hope that makes sense to you.

If anything, your alternator's VR is bad, but since you say it does put out charge, it is probably not that.
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  #16  
Old December 6th, 2011, 11:17 AM
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It puts out a charge, but is intermittent for some reason. At idle, it won't charge, but when I rev the motor it does. Then when I drive the truck it starts to run off the battery.

I still disagree that a motor idling is running off the battery (my logic point), as a motor is supposed to run by itself if you disconnect the battery. Am I off base on that? I've always diagnosed engine electrics that way.
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  #17  
Old December 6th, 2011, 11:40 AM
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Quote:
Originally Posted by o2batsea View Post
An alternator at idle will be putting out such a paltry few amps that it is virtually not charging at all. There are some diodes that are what they call "Low turn on" in that the alternator will begin charging at a much lower RPM. It isn't unusual that you aren't seeing any amps below 1000.
BS. My stock 200TDI alternator gives a solid 13.5 volts at idle with the lights on and heater on full. I have a dash mounted digital gauge and know how the alternator is performing all of the time.
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Old December 6th, 2011, 11:45 AM
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Just went out to the garage and checked the system again. It won't charge at idle, and when I rev the motor I get about 13v if I'm lucky. When I disconnect the battery the truck dies. In old skool auto mechanics this points to the alt as the culprit. The VR is an Echlin VR-32 and is new.
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  #19  
Old December 6th, 2011, 12:57 PM
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On a vehicle with an alternator, don't disconnect the battery while the engine is running. You can fry the diodes in the alternator as there will be no sense voltage and the alternator can run up the voltage pretty quickly.

Sounds like you need a new one. And, a proper sized pulley. I have some extra pulleys here if you need one but they are available elsewhere.


Quote:
Originally Posted by Wolf Fabrication View Post
Just went out to the garage and checked the system again. It won't charge at idle, and when I rev the motor I get about 13v if I'm lucky. When I disconnect the battery the truck dies. In old skool auto mechanics this points to the alt as the culprit. The VR is an Echlin VR-32 and is new.
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  #20  
Old December 6th, 2011, 12:59 PM
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Quote:
Originally Posted by Ren Ching View Post
On a vehicle with an alternator, don't disconnect the battery while the engine is running. You can fry the diodes in the alternator as there will be no sense voltage and the alternator can run up the voltage pretty quickly.
like in less than half a second. There is such a thing as a "zap stop" for knuckle heads who do that or have a 1,2,BOTH,OFF switch and turn it to off when the engine is running.

------ Follow up post added December 6th, 2011 01:04 PM ------

Quote:
Originally Posted by Red90 View Post
BS. My stock 200TDI alternator gives a solid 13.5 volts at idle with the lights on and heater on full. I have a dash mounted digital gauge and know how the alternator is performing all of the time.
Your battery should be reading 14.2-14.4 when the alternator is actually producing current. At 13.5 it's doing zippo. That's resting voltage for a fully charged battery. In addition a dash mounted gauge will be suffering some amount of voltage drop due to the wire resistance. Only when you put a multimeter across the battery terminals will you get a proper reading.
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