Charging system only sometimes works

My car died on the interstate last night due to running the battery empty, though it never turned on the dash warning light. So after towing it home we pulled the alternator and took alternator and battery to Autozone for test, both passed. Charged up the battery and the car runs fine, but checking the voltage when it is running shows that sometimes it is getting a boost in voltage from the alternator (~14.2 volts) and sometimes it is battery alone (~12.6 volts). Battery was new in February and alternator new in June. Connections seem to be decent, we can shake them all we want and the voltage doesn’t change. Could this be anything other than a bad connection under the hood? What is in the cable that connects to the alternator, could something be wrong somewhere on the other end of that?

I would suspect an intermittent problem with the voltage regulator.

The other possibility is a connection problem between the alt. and the batt., when checking the charging system are you doing it only at the batt., if so, try checking the output at the alt., [black charge lead], check it at the connection in the fuse box and at the connection on the alt.

Is the charge indicator light working, does it come on when you turn on the ign. switch to run?:stare:94

charging system light does work, though it has typically not come on during this, even when the car died due to lack of power (CEL came on, not battery light). Poking a multimeter at the lug on the alternator seems to give the same results as checking at the battery, though we have not tried that as often since it is harder to reach. Which of the two big leads into the fuse box is the alternator? I’m guessing the one closer to the firewall, but since they are wrapped and common to each other anyway it’s hard to tell.

Sorry, not sure which one it is on a G2, disconnect one of them, the one that still has 12V+ on it, [engine not running] will be the batt. lead.:whisper:94

Finally dragged out the full Helm’s manual, which showed that it was the wire I thought.

Anyway, yeah, turns out the alternator was indeed going in and out. So I waited till it was off, hooked up my boat battery through the jumper cables, and drove to a different autozone. Their in-car tester said 1 amp output, so I pulled the alternator again and swapped for a new one.

Thanks for the help.