You need a flush machine to really change an automatic transmission's fluid. You can't get the fluid out of the torque converter which is half the total volume. You need a flush machine that hooks into the trtans cooler lines and exchanges the fluid quart for quart. A drian and refill is like finding a gallon of sour milk in the frig and emtpying half of it and filling up with fresh milk. It's still sour.