FD alternators on underdriven pulleys?
 

I bought one of the FD alts from Silver rotor, and I swear either I have a defective one or the pulleys are underdriven.

The symptom is straight up, lack of charging. At idle (~850 rpm) the battery will read ~11.5 volts. Brand new optima redtop, alternator is about 6 months old. The old battery was a diehard gold, but it had the same issue.

Usually revving at about 3krpm it provide 13.2-13.5v at the battery. I checked for shorts with a test light at the battery terminal and got a faint pulse, it was pulsing in time with the security light in the center console so I assumed that was correct.

Any ideas? My next step is to just throw the old s5 alternator back on the car and deal with it.

Have the alt tested.

I will eventually, but I'd rather find out first if anyone else has had a similar issue, and second, if the pulley is underdriven.