Transmission Architecture and Power Flow
A conventional hydraulic automatic transmission uses a combination of a torque converter, planetary gear sets, and hydraulically actuated clutch packs and bands to provide multiple forward gear ratios and smooth, driver-free gear changes. The torque converter couples the engine to the transmission — it is a fluid coupling that allows engine power to be transmitted while permitting the engine to idle with the vehicle stationary. At higher speeds, the torque converter's lockup clutch mechanically locks the converter turbine to the engine crankshaft for direct power transfer and improved fuel economy. Inside the transmission, multiple planetary gear sets (each consisting of a sun gear, planet gears, a planet carrier, and a ring gear) are interconnected. Different gear ratios are achieved by selectively holding or releasing specific members of the planetary set through hydraulically applied clutch packs (multiple friction discs clamped together by hydraulic pressure) and bands (external steel bands wrapped around a drum, applied by a band servo). In first gear, a specific clutch pack is applied while others release — the planetary set produces a high-ratio multiplication of engine torque. As the vehicle accelerates, the transmission control module (TCM) or hydraulic valve body commands sequential clutch apply/release events, shifting through each gear ratio. Shift quality depends critically on clutch pack clearance (the amount of free travel before the clutch begins to engage) and fluid condition. Worn friction discs (burned, glazed, or missing material) cause slipping on apply, flare-ups between shifts, and delayed engagement.