So...this is timely...I'm tracking a low-voltage gremlin in my 87FJ60. I've done the test as Jeff talks about and I'm in the lower range just about 13 at idle, if not a little lower. As an example, when at idle with just my headlights and radio on, my dash-meter reads just under 12 if I put everthing on, it reads closer to 10. Now...if I do the test with voltmeter...I pick up about a .5 to .75v. So, I'm sure the dash meter isn't totally accurate. I also have a toyota Re-man'd/re-wound alt that's pushing about 70A (had it tested). I'm thinking my lights shouldn't dim at stoplights.
How do you test the diodes? and if they check out, where do recommend starting the trouble shooting. I"m stumped after replacing all the battery/chassis grounds. I have the power cable to replace still...
Sorry for the hi-jack, and FWIW I'd really like to go through a electrical 'primer' like the upcoming GPS one.