Section 9 (8HP70 Automatic Transmission (Service Information)): Operation: Clutches

The 8HP70 transmission uses two multi-plate holding clutches (A and B) and three multi-plate driving (rotating) clutches (C, D, and E). Gear shifts from one - eight and eight - one are synchronous shifts, meaning one of the clutches must continue to transmit torque at lower pressure until the other clutch is able to accept the input torque.

Clutches A and B are holding clutches. Clutch A is applied by hydraulic pressure and released by a return spring, similar to the driving clutches. Clutch B operation is unique because it has fluid chambers on both sides of the apply piston, and therefore it is applied and released hydraulically. Regulation of fluid pressure on each side of the piston allows precise positioning of clutch B for apply, release, and controlled slip conditions.

Clutches C, D, and E operate in a similar manner. Regulated pressure is available at the clutch C shift solenoid and the clutch valve. When the shift solenoid is de-energized, fluid flows through the shift solenoid and applies pressure to the clutch valve. During the shift, pressure moves the clutch valve, which begins to apply the clutch. Pressure from the holding valve regulates the opening of the clutch valve, softening the shift. Above a set threshold, the holding valve seats and allows the clutch valve to open completely.