I've been trying to diagnose an erratic voltage gauge for a few weeks that would fluctuate between 9 and ~14v. When I had the battery and alternator checked they passed with flying colors. So I bought a new gauge, and experienced the same issues. Figured it had to be electrical, and probably a bad ground so I checked and found they were good. I also checked the 12v source to the gauge as far as I could.
At 65 with a bad neck, back, eyes, and certainly attitude I was ready to give up. Then I membered I had Randy's number and maybe ought to call him. After a couple of questions he nailed it. Since I looove to crawl under the dash, I had reached under the dash to snap a pic of the fuse box, but didn't look closely at the pic. While troubleshooting with Randy he mentioned the accessory post on the fuse box. I looked at the pic, oops...a loose terminal on the stud. Erectricity 101.
Thanks MidLife.