DRIVELINE CLUNK. GMC Truck GMC P RV/Van/Commercial Chassis 1996

REVISION: 10/04/96

THIS BULLETIN IS BEING REVISED TO INCLUDE 1996 AND 1997 MODELS. PLEASE DISCARD CORPORATE BULLETIN NUMBER 56-44-01 (SECTION 4 - DRIVE AXLE). GMC RECOMMENDS THAT A COPY OF THIS BULLETIN ALSO BE PLACED IN SECTION 7-TRANSMISSION. *********************************************************************

SUBJECT: INFORMATION ON DRIVELINE CLUNK

MODELS: 1997 AND PRIOR LIGHT DUTY TRUCKS

SOME OWNERS OF LIGHT DUTY TRUCKS EQUIPPED WITH AUTOMATIC TRANSMISSIONS MAY COMMENT THAT THEIR TRUCK EXHIBITS A CLUNK NOISE WHEN SHIFTING BETWEEN PARK AND DRIVE, PARK AND REVERSE OR DRIVE AND REVERSE. SIMILARLY, OWNERS OF TRUCKS EQUIPPED WITH AUTOMATIC OR MANUAL TRANSMISSIONS MAY COMMENT THAT THEIR TRUCK EXHIBITS A CLUNK NOISE WHILE DRIVING WHEN THE ACCELERATOR IS QUICKLY DEPRESSED AND THEN RELEASED.

WHENEVER THERE ARE TWO OR MORE GEARS INTERACTING WITH ONE ANOTHER, THERE MUST BE A CERTAIN AMOUNT OF CLEARANCE BETWEEN THOSE GEARS IN ORDER FOR THEM TO OPERATE PROPERLY. THAT CLEARANCE OR FREEPLAY (ALSO KNOWN AS LASH) CAN TRANSLATE INTO A CLUNK NOISE WHENEVER THE GEAR IS LOADED AND UNLOADED QUICKLY, OR WHENEVER THE DIRECTION OF ROTATION IS REVERSED. THE MORE GEARS YOU HAVE IN A SYSTEM, THE MORE FREEPLAY THE TOTAL SYSTEM WILL HAVE.

THE CLUNK NOISE THAT CUSTOMERS SOMETIMES HEAR IS BELIEVED TO BE THE RESULT OF A BUILDUP OF FREEPLAY (LASH) BETWEEN COMPONENTS IN THE DRIVELINE.

FOR EXAMPLE, THE POTENTIAL FOR A DRIVELINE CLUNK WOULD BE GREATER IN A 4-WHEEL DRIVE OR ALL-WHEEL DRIVE VEHICLE THAN A 2-WHEEL DRIVE VEHICLE. THIS IS BECAUSE IN ADDITION TO THE FREEPLAY FROM THE REAR AXLE GEARS, THE UNIVERSAL JOINTS, AND THE TRANSMISSION (COMMON TO BOTH VEHICLES), THE 4-WHEEL DRIVE TRANSFER CASE GEARS (AND THEIR ASSOCIATED CLEARANCES) ADD ADDITIONAL FREEPLAY TO THE DRIVELINE.

IN SERVICE, DEALERS ARE DISCOURAGED FROM ATTEMPTING TO REPAIR DRIVELINE CLUNK CONDITIONS FOR TWO REASONS:

1. COMMENTS OF DRIVELINE CLUNK ARE ALMOST NEVER THE RESULT OF ONE INDIVIDUAL COMPONENT WITH EXCESSIVE LASH, BUT RATHER A RESULT OF THE ADDITIVE AFFECT OF THE FREEPLAY (OR LASH) PRESENT IN ALL THE DRIVELINE COMPONENTS.

SINCE ALL COMPONENTS IN THE DRIVELINE HAVE A CERTAIN AMOUNT OF LASH BY DESIGN, CHANGING DRIVELINE COMPONENTS MAY NOT RESULT IN A SATISFACTORY LASH REDUCTION.

2. WHILE SOME CUSTOMERS MAY FIND THE CLUNK NOISE OBJECTIONABLE, IT WILL NOT ADVERSELY AFFECT DURABILITY OR PERFORMANCE.

IMPORTANT: THE CONDITION DESCRIBED IN THIS BULLETIN SHOULD NOT BE CONFUSED WITH DRIVELINE STOP CLUNK, DESCRIBED IN CORPORATE BULLETIN NUMBER 964101R (CHEVROLET 92-265-7A, GMC TRUCK 91-4A-77, OLDSMOBILE 47-71-20A, GM OF CANADA 93-4A-100).

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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.