NEUTRAL GEAR RATTLE INFO 4.3L (DIAG. PROCEDURE). GMC Truck S15 Pickup

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 five 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) ME516465

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

SERVICE PARTS INFORMATION

Part Model Description Number ----- ----------- ------ 1990-92 C/K, Plate, Clutch Driven 15961141 4.3L, 5sp

1990-92 S/T, Plate, Clutch Driven 15960266 4.3L, 5sp

1990-92 C/K, Bearing, Clutch Pilot 14061686 S/T, LB1, 5sp

This clutch disc can only be used with pressure plate P/N 15974649. See Service Procedure information above for more details.

C/K PROMS FOR NEUTRAL GEAR RATTLE ONLY -------------------------------------- Emission Service B/cast Scanner Year Model Engine Trans. 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

*Combination detonation and neutral gear rattle PROM available. See DETONATION AND NEUTRAL GEAR RATTLE below.

**To use these PROMs you need Service ECM P/N 16144288. If the vehicle is not already equipped with the service ECM, it must be ordered and installed with the new PROM.

S/T PROMS FOR NEUTRAL GEAR RATTLE ONLY -------------------------------------- S/T Manual Transmission Applications Use One (1) PROM for Nationwide Emissions (No Federal, California, or Altitude).

Service B/cast Scanner Year Model Engine Trans. Axle PROM Code I.D. --------------------------------------------------------------------- 1992 S/T LB4 MY2 2.73(GU2) 16196114* BHSF 6145 1992 S/T LB4 MY2 3.08(GU4) 16196118* BHSH 6155 1992 S/T LB4 MY2 3.42(GU6) 16196121 BNSJ 6165 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

*Combination detonation and neutral gear rattle PROM available. See DETONATION AND NEUTRAL GEAR RATTLE below.

**To use these PROMs you need Service ECM P/N 16144288. If the vehicle is not already equipped with the service ECM, it must be ordered and installed with the new PROM.

DETONATION AND NEUTRAL GEAR RATTLE PROMS

If, in addition to a neutral gear rattle condition, a 1992 vehicle also has a detonation condition or had a detonation fix PROM installed, use the following part numbers.

Service B/cast Scanner Year Model Engine Trans. Axle PROM Code I.D. ----------------------------------------------------------------------------- 1992 C1,2 Fed. LB4 MG5 All 16196152 BHSM 6195 1992 K1,2 Fed. LB4 MG5 All 16196134 BHSN 6205 1992 C1,2 CA LB4 MG5 All 16196131 BHUT 6215 1992 K1,2 CA LB4 MG5 All 16196153 BHUU 6225 1992 S/T LB4 MY2 2.73(GU2) 16196188 BHWN 6235 1992 S/T LB4 MY2 3.08(GU4) 16196192 BHWP 6245 1992 S/T LB4 MY2 3.42(GU6) 16196199 BHWR 6255

Parts are currently available from GMSPO.

WARRANTY INFORMATION

For vehicles repaired under warranty use:

Labor Labor Time Operation C K S T --------- ------------------------------- T0500-PROM 0.6 Hr. 0.6 Hr. 0.6 Hr. 0.6 Hr. Replace K0800-Clutch Disc and Pilot Bearing See Applicable Labor Time Replace Guide Add: W/Skid Plate

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.