General Maintenance & Repairs - Low Voltage

I just put on my Bo White ported throttle body and was taking my car for a test drive and coming out of 2nd gear I noticed that my check gages light came on. Only thing I could see was that their is a low voltage output. It reads at about 9-10 on the gauge and doesn't change much or go up at all.

When I get up about 2500-3000 RPMs the check gages light goes away but it still reads 9-10 causing me to believe that a pulley is to blame for this. I've checked all the pulleys and the only thing I think may be to blame is the tensioner assembly.

Anyone had a similar problem and can the tensioner be to blame for this?

Just me, but first thing I'd check is the battery/alt output with a digital volt meter.

Thanks. I had one but my brother took it. I went to autozone to get a free check and my battery was a little low but that didn't seem to be the problem. Their was a problem with my charging system, probably the alternator. I had ruled it out mostly because I couldn't see anything wrong with the pulley. I had also taken the drive belt off and could easily turn it without any problem. The guy that did the testing told me that my check gages light would come off when I got to about 3000 RPMs because it was spinning fast enough to get it to charge. So I'm going to drop the alternator and go get a replacement.