SHAKE/VIBRATION FROM FRONT OF VEH. (INSPECT FRONT END

SUBJECT: SHAKE/VIBRATION FROM FRONT OF VEHICLE (INSPECT FRONT END COMPONENTS)

MODELS: 1990-98 CHEVROLET AND GMC 7H MEDIUM DUTY CONVENTIONAL MODELS, 6-7T SCHOOL BUSES EQUIPPED WITH 8,100#, 9,000#, 10,000#, 11,000#, 12,000#, AND 14,600# FRONT AXLES (RPO'S FL1, F43, FM6, FL2, FS7, FL3)

CONDITION:

SOME OWNERS MAY COMMENT ABOUT A SHAKE OR VIBRATION IN THE FRONT END OF THE VEHICLE. THIS CONDITION HAS MAINLY AFFECTED, BUT IS NOT LIMITED TO, VEHICLES EQUIPPED WITH 8,100# FRONT AXLES.

CAUSE:

THESE CONDITIONS MAY BE CAUSED PRIMARILY FROM A WHEEL/TIRE IMBALANCE. HOWEVER, MANY FACTORS MAY CONTRIBUTE TO FRONT END SHIMMY OR VIBRATION. THIS WHEEL/TIRE IMBALANCE MAY MANIFEST ITSELF AS A STEERING WHEEL SHAKE. THIS MAY BE INITIATED BY EITHER CONTACTING A BUMP IN THE ROAD OR BY MOMENTARILY DRIVING THE RIGHT FRONT TIRE ONTO THE SHOULDER OF THE ROAD.

WHAT IS SHIMMY?

A SHIMMY CONDITION IS A BOUNCING OR WOBBLING OF THE FRONT TIRES, ACCOMPANIED BY MOVEMENT IN THE STEERING WHEEL. THIS IS ACTUALLY A LOW-TO-MEDIUM FREQUENCY/SIDE-TO-SIDE VIBRATION THAT SETS UP IN THE FRONT TIRE/WHEEL ASSEMBLIES. AS THE WHEELS ROTATE, EVEN VERY SLIGHT IMBALANCES IN THE FRONT WHEELS MAY GENERATE A VIBRATION THAT TRANSMITS, OR AMPLIFIES, THROUGH THE CHASSIS. THE TRANSMITTING PATH FOR SHIMMY IS THE FRONT SUSPENSION, AND THE RESPONDING COMPONENT IS THE STEERING WHEEL THAT YOU FEEL SHAKING.

SHIMMY MAY BE CAUSED FROM THE FOLLOWING CONDITIONS:

- EXCESSIVE TIRE/WHEEL ASSEMBLY, BRAKE ROTOR OR DRUM RUNOUT - FRONT TIRE/WHEEL ASSEMBLY, BRAKE ROTOR OR DRUM IMBALANCE - LOOSE, WORN, OR DAMAGED FRONT AXLE AND/OR SUSPENSION COMPONENTS (I.E. SHOCK ABSORBERS, TIE ROD ENDS, PITMAN ARM, WHEEL BOLT HOLES AND STUDS) - IMPROPER STEERING GEAR LASH ADJUSTMENT - INSUFFICIENT KING PIN THRUST BEARING PRELOAD - FRONT WHEEL ALIGNMENT OUT OF SPECIFICATIONS

CORRECTION:

MAKE A COMPLETE INSPECTION OF ALL OF THE POSSIBLE CAUSES LISTED ABOVE. THIS MAY INVOLVE THE REDUCTION OF THE IMBALANCE OR DAMPENING OUT THE FORCES THAT RESULT FROM IMBALANCE. INSPECT THE FOLLOWING ITEMS:

TIRE/WHEEL ASSEMBLY, BRAKE ROTOR OR DRUM INSPECT FOR EXCESSIVE RUNOUT OF THE WHEEL/TIRE. VEHICLES EQUIPPED WITH CAST SPOKE WHEELS MAY BE MORE PRONE TO EXHIBIT EXCESSIVE RUNOUT. CORRECT AS REQUIRED.

FRONT TIRE AND WHEEL ASSEMBLIES INSPECT FOR ANY IMBALANCE IN THE TIRES, THE WHEEL ASSEMBLIES, AND THE BRAKE ROTORS AND DRUMS. CORRECT AS REQUIRED.

FRONT AXLE AND/OR SUSPENSION COMPONENTS INSPECT FOR LOOSE, WORN, OR DAMAGED SHOCK ABSORBERS, TIE ROD ENDS, PITMAN ARM, AND WHEEL BOLT HOLES AND STUDS. REPLACE AS REQUIRED.

STEERING GEAR INSPECT FOR PROPER CENTERING OF THE STEERING GEAR AND SECTOR LASH ADJUSTMENTS. REFER TO SECTION 3B OF THE APPROPRIATE SERVICE MANUAL.

KING PIN THRUST BEARING INSPECT FOR WORN OR DAMAGED SEAL, KING PIN, AND BEARINGS. DO THE FOLLOWING KING PIN ARTICULATION TEST

KING PIN ARTICULATION TEST INSPECT PRELOAD ON THE KING PIN THRUST BEARING BY MEASURING HOW MUCH FORCE IT TAKES TO ROTATE THE STEERING KNUCKLE. USE THIS TEST TO DIAGNOSE A SHIMMY CONDITION AFTER YOU HAVE COMPLETED ALL OF THE ABOVE INSPECTIONS. REFER TO SECTION 3 OF THE MEDIUM DUTY SERVICE MANUAL.

IMPORTANT: BEFORE VERIFYING PRELOAD, THOROUGHLY GREASE THE UPPER KING PIN BEARING AND THE LOWER BUSHING.

IN ORDER TO VERIFY PRELOAD, YOU WILL NEED A STRONG PIECE OF STRING AND A TENSION SCALE.

1. POSITION THE STRING BETWEEN THE SCALE AND THE SPINDLE, AT THE INBOARD EDGE OF THE THREADS. IF THE SCALE IS EQUIPPED WITH A METAL HOOK, USE THE COTTER KEY HOLE IN THE SPINDLE FOR THIS MEASUREMENT.

2. PULL THE STEERING KNUCKLE THROUGH ITS FULL RANGE OF TRAVEL WITH THE TENSION SCALE.

3. NOTE HOW MUCH FORCE IT TAKES TO PULL THE STEERING KNUCKLE. DISREGARD THE ADDITIONAL FORCE NEEDED TO SET THE STEERING KNUCKLE IN MOTION (I.E. FIRST TUG). THE ARTICULATION TIGHTENING SPECIFICATION IS 11-50 N.M (8-37 LB FT). HOWEVER, ANYTHING LESS THAN A 133 N (30 LB) PULL WILL NOT PREVENT SHIMMY. IN ORDER TO HELP REDUCE SHIMMY, SET THE ARTICULATION TIGHTENING SPECIFICATION TO A 133-165 N (30-37 LB) PULL.

4. TAKE TWO READINGS, IN OPPOSITE DIRECTIONS, AND AVERAGE THE TWO READINGS.

ADJUST THRUST BEARING PRELOAD COMPRESSING THE BEARING BETWEEN THE SPACER SHIMS AND THE KING PIN NUT CREATES PRELOAD. PROPER KING PIN THRUST BEARING PRELOAD IS ESSENTIAL, AND SHIM SELECTION IS CRITICAL. PRODUCTION ONE-PIECE SPACER SHIMS ARE STAMPED ACCORDING TO SIZE IN MILLIMETERS.

PRODUCTION SHIMS ARE ALSO SIDE-SPECIFIC. BECAUSE THESE SHIMS HAVE DIFFERENT THICKNESSES, YOU CANNOT SWAP SPACER SHIMS FROM SIDE-TO-SIDE. IF A PRODUCTION SHIM CAME OFF THE LEFT SIDE OF THE TRUCK, THE SHIM MUST GO BACK ON THE LEFT SIDE, NOT THE RIGHT, AND VISA-VERSA.

IN ORDER TO INCREASE BEARING PRELOAD AND KING PIN ARTICULATION TORQUE, REPLACE THE ONE-PIECE PRODUCTION SHIMS WITH A STACK OF FIVE THINNER SERVICE SHIMS IN THE STEERING KNUCKLE ASSEMBLY. IN ORDER TO DECREASE THRUST BEARING PRELOAD AND KING PIN ARTICULATION TORQUE, USE THICKER SPACER SHIMS.

IN ORDER TO SIMPLIFY THE SHIM SELECTION, A NEW SHIM STACK CODE CHART IS SUPPLIED IN THIS BULLETIN. VARIOUS SHIM PACKS ARE AVAILABLE FROM GMSPO. REFER TO THE SHIM SELECTION CHART AND THE PART NUMBERS LISTED BELOW.

USE THE CHART IN THE FOLLOWING WAY:

1. REMOVE THE PRODUCTION SHIMS FROM THE VEHICLE.

2. READ THE STAMPED NUMBER ON THE SHIM.

3. FIND THAT NUMBER ON THE SHIM SELECTION CHART.

4. DETERMINE THE PROPER COMBINATION OF 5 SERVICE SHIMS THAT WILL GIVE YOU THE NEXT LOWER NUMBER ON THE CHART. THERE IS A DIFFERNCE OF 0.015 MM BETWEEN EACH SHIM COMBINATION. REDUCING THE SHIM HEIGHT BY 0.015 MM IS USUALLY ENOUGH TO PROVIDE FOR AN ARTICULATION TIGHTENING SPECIFICATION OF 133-165 N (30-37 LB). FOR EXAMPLE, IF THE THICKNESS IS 13.01 MM, THEN THE NEXT THINNER STACK IS 12.95 MM. THE SHIM CODE FOR 12.95 MM IS 5,5,5,2,2. THIS MEANS YOU SHOULD USE THREE #5 SHIMS AND TWO #2 SHIMS.

IMPORTANT: DO NOT OVERTIGHTEN THE KING PIN NUT TO INCREASE THE BEARING PRELOAD. THE KING PIN NUT TIGHTENING SPECIFICATION IS 550 N.M (406 LB FT). VERIFY PRELOAD AFTER INSTALLATION OF THE SERVICE SHIMS.

IMPORTANT: IF THE SHIM PACK IS TOO THIN, TOO MUCH THRUST BEARING PRELOAD AND HIGH ARTICULATION TORQUE MAY EXIST AND IF THE SHIM PACK IS TOO THICK, A LOW THRUST BEARING PRELOAD AND LOW ARTICULATION MAY EXIST.

PARTS INFORMATION:

P/N DESCRIPTION/RPO QTY --- ---------------- --- 15569813 SHIM, FRONT AXLE (FL1, F43) REFER TO 15569812 SHIM, FRONT AXLE (FL1, F43) SHIM SELECTION 15569811 SHIM, FRONT AXLE (FL1, F43) CHART 15569810 SHIM, FRONT AXLE (FL1, F43) 15569809 SHIM, FRONT AXLE (FL1, F43) 15569818 SHIM, FRONT AXLE (FM6, FL2, FS7, FL3) 15569817 SHIM, FRONT AXLE (FM6, FL2, FS7, FL3) 15569816 SHIM, FRONT AXLE (FM6, FL2, FS7, FL3) 15569815 SHIM, FRONT AXLE (FM6, FL2, FS7, FL3) 15569814 SHIM, FRONT AXLE (FM6, FL2, FS7, FL3)

PARTS ARE CURRENTLY AVAILABLE FROM GMSPO.

FRONT WHEEL ALIGNMENT INSPECT FOR PROPER ALIGNMENT OF THE FRONT WHEELS. REFER TO THE SERVICE PROCEDURE IN THE APPROPRIATE SERVICE MANUAL. ADJUST AS REQUIRED.

SCHOOL BUS AND CONVENTIONAL MODEL MEDIUM DUTY ALIGNMENT SPECIFICATIONS ARE SIMILAR. REFER TO SECTION 3A IN THE APPROPRIATE SERVICE MANUAL FOR FRAME ANGLE CASTER ADJUSTMENT PROCEDURE.

IN ORDER TO MAINTAIN ACCEPTABLE STEERING RETURNABILITY, THE TECHNICIAN SHOULD BE ADVISED TO SET CASTER TO THE HIGH END OF THE CURRENT SPECIFICATION ON THOSE VEHICLES WITH HIGH KING PIN BEARING PRELOADS 133-165 N (30-37 LB). VEHICLES WITH LOWER PRELOADS CAN TOLERATE CASTER SETTINGS AT THE LOW END OF THE SPECIFICATION.

THE VEHICLE SHOULD BE TEST DRIVEN AFTER INSPECTION OF ALL OF THE ITEMS LISTED ABOVE TO ENSURE THAT THE SHIMMY AND/OR VIBRATION CONDITIONS ARE RESOLVED.

WARRANTY INFORMATION:

FOR VEHICLES REPAIRED UNDER WARRANTY, USE:

LABOR OPERATION DESCRIPTION LABOR TIME --------- ----------- ---------- E2950 KING PIN BEARING PRELOAD 2.3 HRS - ADJUST

FIGURES: 00 ATTACHMENTS: 02

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.

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