M/T - Neutral Gear Rattle, Replace Prom, Clutch & Pilot

BULLETIN NUMBER: 92-7B-149A

SECTION: 7B Manual Transmission

NUMBER: 1

CORPORATE REFERENCE NUMBER: 2672O1R

DATE: August 1993

SUBJECT:
NEUTRAL GEAR RATTLE (REPLACE CLUTCH DISK, PROM AND PILOT BUSHING)

MODELS:
1990-92 C/K AND 1990-92 S/T WITH 4.3L AND 5 SPEED MANUAL TRANSMISSION

This bulletin cancels and replaces Truck Bulletin 92-7B-149 (corp. # 267201R) dated December 1992, and is being revised to add the 1992 model year S/T Trucks with 4.3L engines (RPO LB4) and to add the "Detonation and Neutral Gear Rattle Proms" section.

CONDITION
Some owners of 1990-92 C/K and 1990-92 S/T Trucks with 4.3L engines (RPO LB4) and 5-speed manual transmission (RPO's MG5, MY2) may comment that the transmission rattles when in neutral with their foot off the clutch pedal and the engine at idle. The customer may describe the rattle as loose bearings.

CAUSE
This rattle noise is created when the turning transmission gears contact against each other. The contact is created by the firing impulses of the engine which causes rapid acceleration and deceleration of the individual gears.

CORRECTION
This condition may be corrected by the installation of the following parts:

- A revised clutch driven disc which features a live degree neutral stage. This neutral stage helps to smooth the rapid acceleration and deceleration of the individual transmission gears.

- A revised PROM which raises engine idle speed to 650 RPM on C/K and 700 RPM on S/T. This increase in idle speed helps to smooth the engine firing pulses.

- A clutch pilot bearing rather than a clutch pilot bushing. This bearing helps to isolate the transmission from the engine idle acceleration/deceleration.

SERVICE PROCEDURE

Important
Incorrect engine idle can contribute to neutral gear rattle. Before installing the parts listed in this bulletin, refer to the appropriate year Light Duty Truck Fuel and Emissions Service Manual, Section 2, Driveability Symptoms "Rough, Unstable or Incorrect Idle Stalling" to verify there are no existing conditions contributing to a rough idle which may aggravate neutral gear rattle. Clutch Disc and Pilot Bearing Replacement: Refer to the appropriate year and model Service manual, Section 7C "Clutch Assembly and Pilot Bearing Replacement."

Important
C/K trucks require pressure plate P/N 15974649 which was first used in production mid-1991. 1990 and 1991 vehicles built prior to the following VIN breakpoints require installation of P/N 15974649 unless it has already been installed in a service repair:

Ft. Wayne (Z) MZ516555

Oshawa (1) M1526826

Pontiac (E) ME51G465

PROM Replacement: Refer to the appropriate year Light Duty Truck Fuel and Emissions Service Manual, Section 3 "On-Vehicle Service, PROM".














SERVICE PARTS INFORMATION

WARRANTY INFORMATION





For vehicles repaired under warranty use: