Multi - Plate Clutch Operation

In order to carry out a shift in ratio, fluid needs to be applied or released from the Multi - Plate Clutch (or Brake). As shown in the example , the following sequence occurs:

1. Fluid from a shift valve in the valve body is applied to the clutch assembly. (Figure A, see Fig 1.)
2. Fluid pressure builds behind the apply piston and overcomes the resistance from the diaphragm spring. (Figure A, see Fig 1.)
3. The friction and steel discs are compressed together and become locked, preventing any slippage between them. (Figure A, see Fig 1.)
4. Two planetary members are now locked together.
5. When fluid pressure is released, the steel and friction discs are allowed to unlock. (Figure B, see Fig 1.)
6. The diaphragm spring pushes against the apply piston and returns the piston back to the rest position. (Figure C, see Fig 1.)
7. The check ball in the apply piston is unseated by centrifugal force which allows the clutch to drain completely.
   Fig 1: Identifying Multi - Plate Clutch Operation

   Courtesy of BMW OF NORTH AMERICA, INC.