EVALUATING DRIVELINE VIBRATION (DIAGNOSIS FAULT TREE)

MODEL: 1980-89 LIGHT DUTY TRUCKS

The purpose of this bulletin is to provide a logical approach to evaluating vehicles tor driveline generated vibrations and imbalance occurring between 5O-62mph. The following diagnosis tree is designed to be used as a guide, and when utilized properly can help isolate and identify causes of driveline disturbances.

PROCEDURE

Prior to road test, determine if disturbance is engine related by following the procedure below:

1. With engine at operating temperature and vehicle parked, set parking brake and place transmission shift lever in neutral.

2. Slowly accelerate engine from idle to 4000 RPM (diesels 3200 RPM), noting RPM at which vibration is felt (if any).

3. If vibration is felt, disregard use of this bulletin and isolate and repair responsible engine component (transmission, clutch/flywheel, torque converter, engine accessory, etc.).

Road test vehicle on a smooth level road to verify customer comment. It is usually beneficial to road test the vehicle with the customer present. Note the speeds and conditions at which the disturbance is most noticeable. Refer to Remark A in this bulletin for diagnosis information.

TO USE DIAGNOSIS TREE:

Locate symptom on the diagnosis tree which best describes the disturbance. Follow arrows and perform the required check or service action. Never bypass or skip an inspection or assume that a component is OK. All steps must be performed as called out.

DEFINITION OF TERMS

"Boom" - Continuous bass drum roll sound caused by body cavity resonance excited by drivetrain. Sometimes felt as pressure in ears.

"Buffeting" -High pressure pulsation.

"Droning" - A low heavy reverberating often muffled sound. Sounds like a bowling ball rolling down alley.

"Drumming" - A drum roll similar to "Boom."

"Howling" - A mid-range continuous sound usually associated with driveline gear noise.

PROCEDURE

1. Measure run out of companion flange, as described in Figure 2. Indicate high point with a mark.

2. Remove companion flange as described in Section 4B of the Service Manual.

3. Measure run out of pinion stem at radius between nut thread and spline. Mark low point.

4. Reinstall pinion companion flange with marks aligned. Tighten pinion nut to specification torque. Refer to Section 4B of the Service Manual.

5. Remeasure companion flange runout. If excessive runout still exists, do Step 6.

6. Additional reindexing of the companion flange to the pinion stem splines by one or two teeth may further reduce runout.

REMARKS

A. Verify concern. Drive on smooth road up to 65 mph as speed limits permit. Note speeds and severity of disturbance. Assure that disturbance is not engine or transmission related. Refer to Section OC "Vibration Diagnosis Chart" in the 1989 Service Manual. If disturbance is determined to be engine/transmission related, disregard use of this bulletin.

B. Disturbance may peak within this speed range.

C. Visually check components for damage, looseness, missing balance weights, etc.

D. This may compensate for companion flange run out.

E. Excessive companion flange runout may occur as high prop runout. Replace companion flange as required, see Figure 2.

F. IMPORTANT: If runout is reduced to below 0.005-inch and the flange possesses a compensation weight, this part is out of spec. See Figure 2.

G. Ensure the companion flange run out is still well within specifications.

H. If this point has been crossed several times, it is recommended that the Technical Assistance Center be contacted.

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.