Overdrive Brake

The overdrive (O/D) brake is applied only in overdrive. Pressure is supplied via the oil pump housing. A piston is located in the bottom of the overdrive brake housing, and is held in position by return springs, a retainer and a snap ring. Fluid entering the underside of the overdrive brake housing is retained by neoprene (rubber) O-rings inside and outside of the piston.

When pressurized fluid is applied to the underside of the piston, the piston is forced against the return springs. As the piston moves, it compresses the discs and plates, engaging the brake. The coil (return) springs provide no mechanical advantage, but they do provide a cushion to soften the brake application.

The discs (teeth on the inner edge) are splined to the overdrive brake hub. The plates (teeth on the outer edge) are splined to the overdrive brake housing, which is bolted to the transmission case. The discs and plates are not engaged if the piston is not actuated.

A specific clearance is required between the discs and plates to allow them to move freely when not engaged. This clearance is controlled by the thickness of the discs and plates. Excessive clearance is usually caused by disc and/or plate wear. Insufficient clearance is usually a result of improper assembly.

In operation, the discs engage the plates, locking the overdrive brake hub (which is directly connected to the overdrive sun gear) to the overdrive clutch housing. As a result, the overdrive brake stops the overdrive sun gear from rotating.

Additionally, there is a check ball in the piston. This check ball traps pressure when the piston is applied and allows residual pressure to escape after the piston is released. Two snap rings are used in the overdrive brake assembly. One snap ring holds the spring retainer in place and the other snap ring holds the clutch pack.