VIBRATION/DRONE NOISE EMANATING AT REAR OF VEHICLE. GMC Truck GMC C Pickup

SUBJECT: VIBRATION/DRONE NOISE EMANATING AT THE REAR OF THE VEHICLE (REPLACE REAR SPRING 2ND STAGE LEAF SPRINGS)

MODELS: 1992-93 C/K TRUCKS WITH 6.5L (L65) TURBO DIESEL ENGINES AND AUTOMATIC TRANSMISSIONS (EXCLUDING RPO C5B - 15,000# GVW)

CONDITION:

SOME OWNERS MAY COMMENT ON AN AUDIBLE LOW FREQUENCY NOISE (MOAN) EMANATING IN THE VEHICLE CAB. THIS CONDITION IS ONLY APPARENT ON THE ROAD WITH THE TRANSMISSION TORQUE CONVERTER CLUTCH (TCC) LOCKED AND THE ENGINE SPEED AT AROUND 1700 RPM. THE MOAN MAY BE LOUDER ON THE DRIVER'S SIDE OF THE CAB.

IF THE NOISE EXISTS UNDER ANY OTHER CONDITIONS (I.E. IN NEUTRAL, OTHER ENGINE SPEEDS, WITH TCC UNLOCKED), THAN THE CAUSE OF THE NOISE IS MOST LIKELY THE RESULT OF SOME OTHER CONDITION.

CAUSE:

AT APPROXIMATELY 1700 RPM, THE FREQUENCY OF THE ENGINE PULSATIONS APPROACHES THE NATURAL FREQUENCY OF THE REAR SPRING SECOND STAGE SPRING LEAF (BOTTOM LEAF). THE EXCITATION OF THE 2ND STAGE SPRING LEAF MAY RESULT IN THE VIBRATION/DRONE CONDITION.

CORRECTION:

VERIFY THAT THE SPRING LEAF EXCITATION IS CAUSING THE CUSTOMER CONCERN BY PERFORMING THE FOLLOWING:

1. DUPLICATE THE CUSTOMER CONCERN BY TEST DRIVING THE VEHICLE. NOTE THE ENGINE SPEED AT WHICH THE CONDITION EXISTS.

2. DRIVE THE VEHICLE WITH THE TCC UNLOCKED AT THE SAME ENGINE SPEED AT WHICH THE CONDITION WAS NOTED IN STEP 1. TO TURN OFF THE TCC, APPLY A CONSTANT LIGHT PRESSURE ON THE BRAKE PEDAL UNTIL AN ENGINE SPEED CHANGE IS NOTICED FOR A PARTICULAR VEHICLE SPEED.

NOTE: THE TCC CAN ALSO BE TURNED OFF USING THE MODE 4 FUNCTIONS OF THE TECH 1.

IF THE CONDITION REMAINS, REFER TO SECTION 0C (VIBRATION DIAGNOSIS) IN THE APPROPRIATE SERVICE MANUAL.

IF THE CONDITION IS ELIMINATED, BOTH REAR SPRING 2ND STAGE LEAF SPRINGS WILL NEED TO BE REPLACED AS DESCRIBED IN THE PROCEDURE BELOW. THE NEW SPRINGS ARE SHORTER AND, THEREFORE, HAVE DIFFERENT NATURAL FREQUENCY CHARACTERISTICS.

SERVICE PROCEDURE:

1. REMOVE THE LEAF SPRING ASSEMBLY FOLLOWING THE PROCEDURE IN SECTION 3D OF THE APPROPRIATE SERVICE MANUAL.

2. REPLACE THE 2ND STAGE REAR SPRING LEAF (BOTTOM LEAF) WITH P/N 15976383.

3. REPEAT THE PROCEDURE FOR THE OPPOSITE REAR SPRING.

SERVICE PARTS INFORMATION

P/N DESCRIPTION QTY

15976383 2ND STAGE REAR SPRING LEAF 2

PARTS ARE EXPECTED TO BE AVAILABLE FROM GMSPO ON 10/11/93.

WARRANTY INFORMATION

FOR VEHICLES REPAIRED UNDER WARRANTY, USE:

LABOR OP DESCRIPTION LABOR TIME

E5147 REPLACE BOTH REAR USE PUBLISHED 2ND STAGE SPRING LEAFS LABOR OPERATION TIME

FIGURES: 0

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.