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Ok so I have a 1977 280z I have done the GM alternator up grade cs10 if I'm not mistaken I had a cs12 which is a 83amp alternator installed when I original did the swap. but I ended up replacing it with the lower amp cs10 due to lack of funds at the time.

 

So to my point I'm having charging issues. It charges at 12.5v no load at idle. I have checked the 2 terminals on the alternator plug I have 12.5v switched going into terminal 1 and 12.5/0 (loaded power) going into terminal 2. I have checked resistance on terminal one key on (battery disconnected) and I'm showing 40ohms. I have consulted with the guys on DPAN and Houston Datsuns on face book and from what I can tell it seems like I just have a bad alternator. To make things more confusing I have a lifetime warranty on this alternator so I am now on my 4th on and every time I seem to replace it the symptoms seem to just get worse. Now on an alternator that's just a day old I am no longer charging at idle but at 1500rpm. Is there something I'm missing? Any help would be really insightful. I am going back to the cs12 tomorrow so if I have any change I will list the results if what gremlins I have to better help the next person. I will try to post pictures later when I get a chance. I also had the battery tested in which it passed. But just to be sure I had it replaced as well. The fuse under the dash is good no problems there.

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You haven't shown that it's charging at all.  The only number you've shown is battery voltage.  Are you saying that the problem is that it doesn't charge until 1500 RPM?  How big is the pulley?  You want a small one to get low RPM charging.  Are they different size between your old one and the one you have now?

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12.5v is not charging. 13.9-14.9v is what you want to see. What voltage are you getting at the alternator terminal. I suspect maybe the voltage is dropping too much to the battery and not charging at all. 

 

Did you have the alternator tested at a separate shop?

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