Grounds are the most important part of the electrical system so as stated above check, clean and tighten all of the ground points.
Then check the battery/alt again and see what happens.
When Auto Zone checks an Alternator or battery, they do a load test which puts a strain on the system. This is turn will (sometimes) excite an alternator or battery enough to get a good reading for a little while then things will crap up again !
When checking the grounds, start at the battery cables, then the battery ground point, then the frame/block points, also with a multimeter check to see if the alternator case is grounded back to the battery grounding on the fender.
Also, if/when you replace the alternator consider doing the battery too, unless it has been replace earlier and don’t forget new cabling from the alternator to the battery and as much of the ground wire as possible and go one size larger if you can.
Sounds like a lot but putting in a new alternator/battery is nice and will work ok for a wile but if the current can’t flow then the system will not work well for long.
4x4, 5sp, Std Cab, Stepside, E4 Red
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