4 dr, 4.3 L, 2wd

Not sure. Alternator checks out good and so does battery. Both were installed in December (less than 5 months old).

Voltmeter sits about 9 volts while running. Sometimes it will go up to 12.5 volts, but not often. It's discharging.

What can it be???

 

Start with the easy stuff first.
Are your battery cable ends bright & tight? Even a clean external connector can be corroded inside the insulation. Closely check the connection at the negative battery to engine/chassis area. Remove, thoroughly clean and replace. The diode pack inside the new alternator could be bad, giving intermittent readings even though it tested OK. If you can, try a different alternator. Is it possible to have your belt routed backwards? Just a wild thought to throw out there. Double check the connector at the alternator to make sure the pins/plugs are all making contact and are not bent. Use a little di-electric grease at connectors. And double check the battery. Is it sealed or can you check the electrolite level inside? Confirm charge status with a hand-held voltage meter. Hope this helps.