Automatic Transmission Accumulators
Accumulators are shift modifiers that affect transmission shift timing and quality. They absorb the initial shock of high pressure in the apply circuit. When a clutch or band's piston or servo first strokes, there is little pressure in the circuit. As soon as the piston or servo bottoms and the device starts to apply, pressure increases rapidly. Without a shift modifying device like an accumulator, the device applies suddenly, and harsh shifting occurs.
An increase in throttle pressure increases the pressure on the piston's bottom, assisting the spring and providing a firmer shift. During light acceleration, the piston strokes easily for a smoother, softer shift feel. During hard acceleration or wide-open throttle, throttle pressure increases, preventing the piston from stoking so easily, providing a firmer shift.