NEUTRAL GEAR RATTLE (RPL. PROM CLUTCH DISK, PILOT BUSHING)
SUBJECT: NEUTRAL GEAR RATTLE (REPLACE CLUTCH DISK, PROM AND PILOT BUSHING)
VEHICLES AFFECTED: 1990-92 "C/K" AND 1990-91 "S/T" TRUCKS WITH 4.3L ENGINE (VIN Z, RPO LB4) AND 5SP MANUAL TRANSMISSION (RPO's MGS, MY2)
THIS BULLETIN SUPERSEDES AND REPLACES CORPORATE BULLETIN REFERENCE NUMBER 267201." THIS BULLETIN IS BEING REVISED TO ADD REPAIR INFORMATION AND UPDATE THE MODEL YEARS INVOLVED.
Some owners of 1990-92 C/K and 1990-91 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.
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.
This condition may be corrected by the installation of the following parts:
o A revised clutch driven disc which features a five degree neutral stage. This neutral stage helps to smooth the rapid acceleration and deceleration of the individual transmission gears.
o 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.
o 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 15668537 which was first used in production mid - 1991. 1990 and 1991 vehicles built prior to the following VIN breakpoints require installation of P/N 15668537 unless it has already been installed in a service repair:
GMC Chevrolet
Ft. Wayne (Z) MZ516555 MZ159936 Oshawa (1) M1526826 M1178933 Pontiac (E) ME516465 ME152577
PROM Replacement: Refer to the appropriate year Light Duty Truck Fuel and Emissions Service Manual, Section 3 "On-Vehicle Service, PROM".
SERVICE PARTS INFORMATION
Model Description Part Number ----- ----------- ----------- 1990-92 C/K, 4.3L, 5sp Plate, Clutch Driven 15961141*
1990-91 S/T, 4.3L, 5sp Plate, Clutch Driven 15960226
1990-92 C/K, S/T, 4.3L Bearing, Clutch Pilot 14061685
*This clutch disc can ONLY be used with pressure plate P/N 15668537. See Service Procedure information above for more details.
Service B/cast Scanner Year Model Eng Trans Emission Type PROM Code I.D. ---- ----- --- ----- ------------- ------- ------ ------- 1992 C1,2 LB4 MG5 Fed (NA5)/Alt (NA6) 16185334 BDCW 5565 1992 K1,2 LB4 MG5 Fed (NA5)/Alt (NA6) 16184269 BCZP 4445 1992 C1,2 LB4 MG5 Cal (NB2) 16185336 BDCX 5575 1992 K1,2 LB4 MG5 Cal (NB2) 16184270 BCZR 4455
1991 CK1,2 LB4 MG5 Fed (NA5)/Alt (NA6) 16184264 BCZK 4405 1991 CK1,2 LB4 MG5 Cal (NB2) 16184267 BCZM 4425
1990 CK1,2 LB4 MG5 Fed(NA5)/Alt (NA6) 16184264* BCZK 4405 1990 CK1,2 LB4 MG5 Cal(NB2) 16184267* BCZM 4425
*These PROMs can ONLY be used with Service ECM P/N 16144288.
Service B/cast Scanner Year Model Eng Trans Axle PROM Code I.D. ---- ----- --- ----- ---- ------- ------ ------ 1990-91 S LB4 MY2 2.73 (GU2) 16184103* BCWS 4201 1990-91 S/T LB4 MY2 3.08 (GU4) 16184104* BCWT 4211 1990-91 S/T LB4 MY2 3.42 (GU6) 16184106* BCWU 4221
*These PROMs can ONLY be used with Service ECM P/N 16144288.
WARRANTY INFORMATION:
For vehicles repaired under warranty use: Labour Time ----------- Labour Operation C K S T ---------------- ------------------ - PROM Replace 0.6 0.6 0.6 0.6 K0800 -Clutch Disc and Pilot Bearing Replace 2.3 3.1 3.0 4.1
Add: W/Skid Plate 0.2 0.2
**BULLETIN REFERENCES: --------------------- Corporate Ref. 267201
General Motors of Canada 92-5-113
Chevrolet Division 92-187-7B
GMC Truck Division 92-7B-86
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.
