Hello everyone, I have a 47 Chevy with a 261cui, 90amp Powergen 12V alternator and 12V battery. My ammeter gauge is not reading anything. Is this because I'm running a 12V? Or is it because I'm running an alternator? Thanks in advance.

Because of the alternator.
An ammeter will not work with an AC alternator.

Most of the time the ammeter is bypassed when converting to an alternator. That is because the alternator can put out more current than the ammeter can handle. I recently purchase a non Chevrolet car that had been converted to an alternator. I wanted information on the health of the system while operating the car. I purchased a charge monitor from an Ebay vendor in the UK. It is a tiny LED display that changes color for different voltages. So with the car not running, turning on the ignition causes it to display green, if the voltage is 11.8 or above. It will show yellow below that and red if it gets further down. If the charging voltage gets too high, it will flash.

Thanks for the replies. There's an article on the Stovebolt where someone swapped out the ammeter with a voltmeter; he took the original ammeter face and replaced the modern voltage gauge face. Looked sharp!