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I remember reading this before. I think a diode is probably shot. My alternator charges enough to keep the battery charged but it is weak. I'm gettin 13.0V at idle with no electrical load. With the lights on it drops to 12.7 and if I turn on the blower fan it drops to 12.4. Any thing else on will cause the motor to start to stumble at idle. I checked the wiring and the grounds and all seem to be in order. I get less then 1 ohm resistance between the negative battery terminal and the case of the alternator. When I go down the road the ammeter jumps from the center position to the charge side. The rythem of the pulse varies with engine RPM and electrical load. I'm assuming that I just need to pick up a new alternator.
Before I spend the money... Any ideas?
 

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You don't say what you are working on but I had a similiar problem. Spent $11.00 for a new regulator and the problem was solved.
 

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Sounds like the regulator kicking in and out when the needle moves to the charge side. The regulator opens the field winding in the alternator thus essentially turning it off. Your regulator may be fine and your alternator may be weak.
 

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Had a similar problem in my 73 XR7 and I replaced the regulator and now ammeter gives a good constant reading and only cost 12 dollars.
 
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