I've had it recently rebuilt and also had a different one entirely and they both do the same thing. They are not putting out enough at idle or low RPM. With A/C, fog lights, headlights on, the volts drop to just above 10. With the A/C off it really doesn't make much difference. The turn signal struggles between flashes. Once I get up to highway speed or just hold the RPM up around 2500 then it's fine. I'd like to think the system can manage the load when everything is on.

 

Put a smaller pulley on the alternator.

RBob.

 

Is this a dash gauge voltage reading or a voltmeter reading at the atl or battery?
Stock crank, alt and not under drive pulleys? (I use ford fox mustang alt pulleys because some are much smaller). All Grounds and output cable connections clean and tight?

Sounds like a true ALT output problem but if running and you have 14 volts at the battery and lower voltage showing on the factory gauge, pull the cluster out, remove and clean all the contact points on the printed circuit.
Cleaning those really changed the dash gauge reading on one of my cars that was having issues.

 

I haven't measured at the battery but I know it's low because the turn signal is very slow as I raise RPM the volts come up and the signal speeds up

 

Your alt should be able to be just fine at idle, so I'd start by cleaning every ground connection you can find.

 

When did you last change the Bat. Sounds like you got a bad cell.

 

Easy enough to test this if one has a battery charger, hydrometer and the battery caps are removable.

 

try doing a voltage drop test. theres many videos on youtube, etc.

i added a 2nd, 10 gauge wire from my alternator to battery and it helped my alternator issues. not saying its your problem.

but a voltage drop test will help isolate your problem. could be a simple bad ground, etc.