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."