LOSS OF OVERDRIVE 4-SPD M/T WITH AUTOMATIC OVERDRIVE

VEHICLES AFFECTED: 1984-85 Chevrolet Corvette

Some 1984-85 Chevrolet Corvettes equipped with the 4-speed manual transmission and automatic overdrive may experience a loss of overdrive operation.

This condition may be caused by a hang up of the direct clutch thrust washer if the washer's tanged fingers (6) have worn a groove in the overdrive unit's aluminum case, the washer may then hang up in this groove preventing the overdrive clutch applying.

On transmissions beginning with serial number D4L 10837CD, a new direct clutch thrust washer has been released, this washer (P/N 14091496) incorporates notches in all the tangs as well as anti-rotational tabs (3) on alternate fingers (see Figure 1) which prevent the washer from contacting the case wall.

Vehicles that may experience this condition will have transmissions built prior to serial number D4L 10837CD. These transmissions are to be repaired by disassembling the overdrive unit according to the appropriate section in the vehicle service manual to enable the following to be carried out:

1. Inspect the pressure plate outside circumference for signs of contact with the case wall. If an such indication is present, the case and pressure plate should be replaced. If no sign of contact is present then the case and pressure plate should be reused.

NOTICE: The pressure plate is the aluminum cast part which is located behind the stamped thrust washer and is identified as Item 51 in the exploded view of the overdrive unit in the service manual.

2. Replace all burnt direct and indirect clutch plates.

3. Replace the direct clutch thrust washer with the new, P/N 14091496, washer. Install the new washer with its tangs facing the pressure plate and ensuring that the anti-rotational tabs are located in the pockets in the face of the pressure plate, and that the finger on the washer that has the drilled hole is located to the bottom of the case (6 o'clock position) as shown in Figure 2.

NOTICE: Whenever an overdrive unit from a transmission built prior to serial number D4L 10837CD is disassembled for any reason, it is recommended that a new, P/N 14091496, direct clutch thrust washer be installed to prevent potential future washer hang-up occurring. The overdrive unit's serial number is on a metal tag attached to the oil pan by one of the pan retaining bolts on the front rail.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.