NOISE/VIBRATION W/TCC APPLIED (REPLACE TCC)

Model and Year: 1989-90 C, S, T, G, M TRUCK 1990 L TRUCK 1991 S/T TRUCK

THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 91-13-7A, DATED APRIL 1990. THE MODEL AND YEAR HAVE BEEN CORRECTED. ALL COPIES OF 91-13-7A SHOULD BE DISCARDED.

This bulletin covers Service and parts information for HYDRA-MATIC 4L60 (THM 700-R4) transmissions which may produce a noise or vibration with the torque converter clutch (TCC) applied. This may happen if the damper spring stops, in the torque converter damper, bottom out against the damper plate. This condition may occur in the following HYDRA-MATIC 4L60 models that are used with 4.3L LB4 engines:

9FMM, 9MAM, 9MFM, 9MXM, 9THM, 9TLM OCAM, OCBM, OMJM, OMNM, OSHM, OTLM, OWAM

To eliminate this condition, a torque converter containing damper springs with a higher spring rate and improved torque capacity has been released for production and service.

The following dates show which transmissions contain torque converters with damper springs that may bottom out against the spring stops.

MODEL DATE OF PRODUCTION

9FMM, 9MAM, 9MFM, 9MXM, 9THM, 9TLM Some After December 9, 1988 (Julian Date 343) OCAM, OCBM, OWAM All Before February 20, 1990 (Julian Date 051) OSHM All Before February 20, 1990 (Julian Date 051) OTLM All Before February 14, 1990 (Julian Date 045) OMJM, OMNM All Before February 2, 1990 (Julian Date 033)

The following diagnosis information will help you determine if the vehicle has a vibration condition caused by the damper spring stops bottoming out in the torque converter.

- Gear Range: Drive 4 (overdrive) with TCC engaged

- Vehicle Speed: Approximately 40-50 mph with TCC engaged - then step into the throttle slightly

- Conditions: 1. Noise, vibration or rattle

2. Low frequency noise that builds up until the floorboards rattle

3. Low growl or driveline grumble

Road grades or trailer loads may cause the conditions to be more prominent or extended. The conditions should disappear when the TCC is disengaged.

Part Number Description

8678979 Converter Assembly Package 298 mm (DE5F)

Parts are currently available from GMSPO.

Information on converter assembly removal and installation is covered in the Automatic Transmission Unit Repair section of the appropriate Service Manual.

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.