I had under drive pulleys on one of my past mustangs. I did have charging issues and the lights would flicker at idle, driving me nuts. I fixed it with smaller pulley off of a mid 90s Ford Escort. Pullet fit with no issues. After the pulley swap I felt more power pulling off. Before it felt like the car stumbled for a spit second from a light. To my understanding....the crank pulley has to be changed with the alt and wp. If using just the crank one issues can result.