Overdrive Assembly: Description and Operation

Description:

DESCRIPTION
The automatic overdrive unit used on the 1984-88 Corvette, and the 4 speed manual transmission is essentially a combination of two separate transmission units. The first unit is a conventional 4 speed (83 mm) manual system with a 1.1 ratio in 4th gear. On 1984 models, the second unit is a two speed overdrive system electronically controlled by the ECM which operates with a 1.1 or 0.68 to 1 ratio. On 1985 models, the second unit is a two speed overdrive system electronically controlled by the ECM which operates with a 1.1 and a 0.68 to 1 ratio or 1.1 and a 0.59 to 1 ratio. The two speed or overdrive unit performs its functions using a planetary gear set in combination wit two sets of clutch components.

The ECM is programmed to control the shift solenoid and the overdrive unit after receiving information regarding vehicle speed, throttle position and engine RPM from various sensors. The overdrive mode cannot occur when the transmission is in first gear. It can occur when the transmission is in the remaining three gears.

Description:

The hydraulic circuit, consists of a gerotor-type pump on the overdrive unit output shaft, a shift valve and solenoid, an accumulator valve, and a pressure relief valve located in the valve body assembly. In the normal (direct drive) mode, fluid circulates from the pump through a pressure relief valve and through the cooler lines, then back to the pan or sump. When the overdrive switch is closed by the ECM, the solenoid plunger is activated, opening the shift valve passage for fluid flow into the piston. When the solenoid deenergizes, the shift valve returns to its original position.

The overdrive mode can be manually deactivated at any time by means of a switch on the shift lever knob.