I have a 06 gmc envoy xl 4.2...battery meter fluctuates back and forth from 14 Down to 9. When it does this and u have to shut it off it will not start back up without a jump. Alternator and battery were both tested and are both good. Any ideas?

And don't forget to check that drive belt tension

Ignition switch could be faulty......sends a bad signal to everything......common issue with that year Envoy

Check the connections between battery and alternator. The regulator in the alternator could be going even though it test good

Bad plug or broken wire at the plug, at the alternator. Pull the plug off the alternator. Turn the key on and check for power and ground at the leads. No reading, probe the wires a couple inches back. If a read there fix or replace the plug. If you have a 3 wire plug, best to use an ohm meter. Test each wire separately. At the pin and a couple inches back on the wire. This test may include testing the wire back to the PCM and fuse panel. You'll need wiring schematics for this. The PCM is what fields the alternator to charge and supply power when the engine is running.