A pressure control or adjusting valve used in the hydraulic system of the automatic transmission . When the vehicle is under heavy load or full throttle, the modulator increases the pressure to hold the clutches in place. In a light load situation, the modulator reduces pressure to give smoother shifts. If the vehicle tends to stay in low gear , shifts with difficulty, or produces a white smoke, and has low transmission fluid or has leaked transmission fluid , you have a transmission problem. It may be cured by an expensive repair or by simply replacing the modulator. Also called "vacuum modulator ."