Alternator not charging at idle?

[Number21]

In my '73 I can tell how fast I'm going by how bright the headlights are. I checked the voltage at the battery at idle with the headlights on and it's only 12.5 volts. I just added a new wire from the alternator to the battery. Voltage goes up to over 14 if I rev it up any.

Why is the voltage so low at idle? Is it because my engine simply isn't spinning fast enough, should I get a smaller pulley for the alternator? Could this be a bad voltage sense wire on the alternator? The wiring in my truck is pretty hacked up....

[jhngardner367]

Chances are,you have the 37 amp alternator.My '85 did the same thing,and at times the problem was so bad,that with the lights,heater,and wipers on,at the same time,none of them worked properly.Usually,the wipers would stop,or go so slowly,as to be useless!
I ended up getting a 900cca battery,and a 105 amp alt,from a '94 Grand Am.
It solved the problem.
Look in the electrical threads,here,and there should be one to show you how.

[INSIDIOUS '86]

^ that could be the issue but you also may have a dead cell in the battery
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[jhngardner367]

GM alternators,generally, were low-amp,cheapest-one-available,unless the vehicle had A/C,a diesel,or some option(i.e.,the rear-mounted heater in the 'burbs/vans),that required more. The big problem with that,though,is that some engines' idle speed were too low for the alternator to charge. Some cured the problem with a smaller pulley on the alt. Others,like me,opted for a larger-amp alt.& larger battery.While a dead cell might cause it,it would also show other symptoms( slow starter speeds,etc.).