If the brakes are applied lightly and the pulsation feltin the steering wheel increases, refer to the Brakes section of the servicemanual that deals with brake-induced pulsation. If you can start to hear the vibrationas a low boom noise (in addition to feeling it), but cannot see it, the vehicle likelyhas a first order (one pulse per propshaft revolution) driveline vibration. Drivelinefirst order vibrations are high enough in frequency that most humans can start tohear them at highway speeds, but are too high to be able to be easily seen. Theseissues can be causedby driveline imbalance or misalignment. If the vehicle exhibits this low boom andthe booming pulses in-and-out on a regular basis (like a throbbing), chances are goodthat the vehicle could have driveline vibration. This type of vibration is normallyfelt more in the “seat of the pants” than the steering wheel.Next, record the Hertz (Hz) reading as displayed by the EVA onto the tire data worksheetfound at the end of this bulletin. This should be done after a tire break-in periodof at least 24-km (15-mi) at 72-km/h (45-mph) or greater, in order to eliminate anypossible tire flat-spotting. This reading confirms what the vehicle vibration frequencyis prior to vehicle service and documents the amount of improvementoccurring as the result of the various steps taken to repair. Completing the SteeringWheel Shake Worksheet below is required. A copy of the completed worksheet must besaved with the R.O. and a copy included with any parts returned to the Warranty PartsCenter for analysis. A reading of 35 to 50 Hz typically indicates a first order propshaftvibration. If this is the situation, refer to the latest version of Corporate BulletinNumber 08-07-30-044.Generally, a reading between 10 and 20 Hz indicates a tire/wheel vibration and ifthis is the reading obtained, continue using this bulletin.
If the tire 1st order vibration goes away and stays away during this evaluation, thecause is likely tire flat-spotting. Tire flat-spotting vibration may come and go atany speed over 72-km/h (45-mph) during the first 10 minutes of operation, if vibrationcontinues after 10 minutesof driving at speeds greater than 72-km/h (45-mph), tire flat-spotting can be ruledout as the cause for vibration.
If flat-spotting is the cause, provide the explanation that this has occurred dueto the vehicle being parked for long periods of time and that the nature of the tireis to take a set. Refer to the latest version of Corporate Bulletin Number 03-03-10-007:Information on Tire/Wheel Characteristics (Vibration, Balance, Shake, Flat Spotting)of GM Original Equipment Tires.If the road test indicates a shake/vibration exists, check the imbalance of each tire/wheelassembly on a known, calibrated, off-car dynamic balancer. Make sure the mounting surface of the wheel and the surface of the balancer are absolutelyclean and free of debris. Be sure to choose the proper cone/collet for the wheel,and always use the pilot bore for centering.