Wrap the alternator with a trash bag before doing anything. I use this stuff. It's expensive but it works great and doesn't harm any under-hood plastics. I don't trust stuff like Gunk or whatever. Spray on cool engine, brush, rinse with LOW PRESSURE water. It did a great job on my old Volvo and my F30.
https://www.amazon.com/P21S-13001B-A.../dp/B0002LBKCE