If you are willing to chase down the issue yourself page 2 of this document has a step by step, check (under voltage tests). Bet you a buck it turns out to be a loose connection and another buck it is on the negative or ground cable side.
http://www.cb4x4.com/PDFs/Electrical...%20testing.pdf
After checking connections you can (full field) alt. for a quick test to prove alt. does work or not. look here....
https://reconit.wordpress.com/2013/0...n-the-vehicle/