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Hello, my name is***** year, model, make, and engine size of vehicle do you have? Thanks.
Has the alternator been replaced?
It sounds like your computer may need to be replaced. At alternator verify large black/gray wire has voltage with key off. Verify small dark green/orange wire has power with engine running. The dark green wire should have ground with engine running. If missing ground verify dark green wire is good between alternator and pin 4 of connector 1 at PCM. If everything is good up to this point but no ground on green wire replace the PCM. Here is a wiring diagram to help with testing. Let me know if you have more questions, thanks.
If above tests were done correctly then computer is what controls alternator output. This will be around a $400 job. Let me know if you have more questions. Thanks.