The stock GX470 alternator is BARELY rated for what the truck requires to run. Most are older and are getting tired. Average alternator failure is around 150,000 miles. OEM replacement will get you close to 120A and the truck takes 90A to run. If you add systems an upgrade is recommended. There are several options to upgrade. Anything more than 150A is a major improvement if you have aux gear and batteries to charge and run. There are manufacturers making up to 270A alternators that work on these vehicles, but new cables and a protective bus bar are required.