I will try lay this out as best I can.
99 Ford F-150
Noticed a flickering in my lighting, (gauges, dome lights head lights etc) figured alternator
was getting tired.
Few weeks go by
Noticed the alternator gauge moving a lot one day on my dash. Soon followed the battery light.
Checked voltage on my battery with a good multi meter. 12.32 volts
Figured my alternator finally gave up. Went to Napa and got a new alternator and also bought a new battery (it was 9 years old).
Installed both. Started vehicle. Battery light is still on. Checked voltage. Steady at 12.41 volts, drops slowly over time (since no output on alt).
Took new alternator back to Napa. Had them test it. It was good. For giggles I had them test my old one I gave them earlier in the day. Tested good.
Went back and reinstalled new alternator and checked everything. This time its charging, but wildly. Range is 12.21-15.33 volts. Everything pulses.
Battery light comes on and off. If you take the battery positive off (I know its not supposed to be done) charging is normal. 14 volts. Hook the battery up and everything goes weird again.
Eventually the alternator stops charging altogether. everything stops pulsing but it dim.
If you let the truck sit for a while (overnight) Alternator will charge again but only for a while, and sporadic charging voltage and pulsing lights.
This happens with 3 alternators, old one, new one, new new one.
Is this a ground that has gone bad? So confused.