electrical system diagnostics

[lnirenberg]

Yesterday was the 1st time I had the Chevelle out in weeks because of the sh***y weather. Drove 30 miles to a friend's house for a gathering and when I got home the car wouldn't start and the volt meter was only showing 10 volts although the car ran fine until I tried to restart it and got it started by rolling down our little hill and popping the clutch in reverse. I take this a giving the alternator a clean bill of health. Hooked up the trickle charger and I just checked the battery, an almost new red top optima, and it had a full 12 volts. Started the car and revved up to about 4500 the volt meter slowly drifted from 12 to 11 usually it would have gone up to 14. Shut down and checked battery and it was down to 11.8 volts. What is the logical process of elimination to troubleshoot?

[MonzaRacer]

OK are you useing the stock external regulator alternator, if so hook a battery powered wire to the F terminal on the regulator the alternator should full field, make 15-16 volts, if so the regulator is bad and make sure its grounded good.
If you have newer internal regulator alternator there will be a a D shaped hole and if you put a small screwdriver in there about 1/4 to 1/2 in, no more and you move it to ground out against the case it will full field the regulator.
A car with an alternator can still run only making 5 or less amps and run down till below 10.5 volts, especially with an AGM battery like an optima.
Below 10.5 volts an HEI or M SD will get sloppy in switching, points may keep it running or not.
I would bet you may need to replace both, big issue with alternator is that are the diodes still good.
If you need give me a call at 812-709-1006, Ill try to talk you through a few tests.

[lnirenberg]

I'm running a cammed up LS6 small block so I assume its an internal regulator but its a detail I didn't pay attention to. See pic. Its got a stock GM LS starter so I wasn't surprised that it didn't take a lot of voltage drop to make it not work.

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