my voltage is good until I've been driving for a few miles and then it slowly starts to drop off... it'll read well above 12 for miles and everything will work fine and then suddenly it will drop to 12 and slowly go down as I drive and use accessories and lights... I can stop and idle for a while and voltage climbs slowly back up but then when I start going again it drops back down... it'll slowly drop so far that the car will barely run...

 

I've replaced the alternator and all the connections at it (they were old, crusty, and corroded)... also put a new battery in it...

 

what else can it be? where are the other common trouble spots that could cause this problem? are there some usual bad connections I should be checking, cleaning and maybe rewiring?

 

--Spiffy

Make sure the battery connections, fusible links and alternator connections are in good clean shape. Check the voltage of each wire going to alternator. They all should be close to the battery voltage. Also make sure the battery warning light turns on when the key is turned from OFF to ON.

if your getting around 13v or more to your battery terminals whilst the engine is running.rev the enginme a little just to make sure the alternater provides a steady 13v or more,and does not drop away...to a lower voltage..and all your battery /alternater and ground wires are clean and tight...then the only thing it can be is a dying battery..change it before it leaves you stranded if all else is good..cheers

Have you verified the reading with a voltmeter, or are you going by the notoriously inaccurate dash gauge?

 

This could also be caused by a defective ground that increases its resistance as it heats up.