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Alternator only putting out 12.5V

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Capt.America! View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Capt.America! Quote  Post ReplyReply Direct Link To This Post Topic: Alternator only putting out 12.5V
    Posted: Aug/29/2018 at 3:43pm
Ok so I have seen a couple similar posts about this, but not exactly the same I’m at a bit of a loss. So I have a 1969 Javelin, straight 6 originally, I swapped for a 401. Have everything put together and mostly working, but my alternator is only putting out about 12.5V. I measured with my multi meter at the battery and alternator, I just replaced my voltage regulator, no change. So new battery, regulator and alternator, only putting out 12.5V, not enough to charge the battery good while it’s running. I just re wired it, because it was done incorrectly the first time, but I’m confident now the wiring is good and still made no difference. Not sure what else to check at this point. Any help anyone has to offer would be much appreciated! Thanks
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Post Options Post Options   Thanks (1) Thanks(1)   Quote matty 401 Quote  Post ReplyReply Direct Link To This Post Posted: Aug/29/2018 at 4:05pm
did you check it with a load light lights on and blower moter on high ?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Capt.America! Quote  Post ReplyReply Direct Link To This Post Posted: Aug/29/2018 at 4:20pm
No, just the engine running with nothing else on. I can check that in a bit
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Aug/30/2018 at 7:13am
Another thing -- rev the engine up for a couple seconds then check. Some of the early alternators have to go over 1000 rpm for a second or so to start working. One "excited"  they should charge. Turning lights on and such to increase the load may make it start putting out even at idle. Doesn't always work like that, especially for things other than an alternator... lights on may have opposite effect...
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Capt.America! Quote  Post ReplyReply Direct Link To This Post Posted: Aug/30/2018 at 8:09am
Thanks, I’ll check that out as well. I checked it with everything turned on last night, and that did not seem to make a difference in the output I was reading
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Aug/30/2018 at 9:44am
Try taking the battery terminal off the alternator and checking the voltage at the alternator.
If the voltage is up significantly then the new voltage regulator is bad.
From reading your post so far it seems that your alternator output is excitation voltage.
If the voltage stayed the same with lights on then the alternator is providing output and your not just draining the battery.
If you have the old voltage regulator, try plugging it in to see what happens.
Another thought, and the experts can chime in please, that voltage regulator case needs a good ground for reference voltage as memory serves.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Capt.America! Quote  Post ReplyReply Direct Link To This Post Posted: Aug/30/2018 at 10:02am
Alright, thanks for the input. The regulator I have is a prestolite I believe, picked it up at advance auto parts. I can get the older version online somewhere if it looks like that’s what it is, I was having the same problem with the old regulator that’s why I replaced it, but didn’t change anything
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Aug/30/2018 at 11:54pm
i would not run an alternator without a battery attached. alts rely on the battery to regulate. the voltage could spike and ruin many things electrical in the car. harm could be done in milliseconds.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Aug/31/2018 at 7:42am
 agree not to disconnect the battery completely from the circuit but disconnecting the charge terminal from the alternator should not let the alternator run away. The excitation voltage is still supplied by the battery and maxed out at that voltage.
This will only check to see if the alternator can put out more then excitation voltage at no load.
It should be in the 16-18 VDC range. This voltage is not connected to anything.
The point is to determine if the alternator is bad or the voltage regulator.
The alternator could conceivably be wired wrong internally and only put out excitation voltage.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Aug/31/2018 at 9:53am
I would check the ground wire from the engine to the chassis. I did that and went from mid twelves to the proper 13.5 or so volts instantly.

Chris
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