Road Test

The road test and customer interview (if available) will provide much of the information needed to find the source of a vibration.
During the road test, drive the vehicle on a road that is smooth and free of undulations. If vibration is apparent, note and record the following:
- the speed at which the vibration occurs
- what type of vibration occurs in each speed range - mechanical or audible
- how the vibration is affected by changes in vehicle speed, engine speed, and engine torque
- type of vibration sensitivity - torque sensitive, vehicle speed sensitive, or engine speed sensitive
Use the following explanation of terms to help isolate the source of the vibration.

Torque Sensitive
The condition can be improved or worsened by accelerating, decelerating, coasting, maintaining a steady vehicle speed, and applying engine torque.

Vehicle Speed Sensitive
The vibration always occurs at the same vehicle speed and is not affected by engine torque, engine speed, or gear selection.

Engine Speed Sensitive
The vibration occurs at varying vehicle speeds when a different gear is selected. It can sometimes be isolated by increasing or decreasing engine speed with the transmission in NEUTRAL, or by stall testing with the transmission in gear. If the condition is engine speed sensitive, the condition is not related to tires.
If the road test indicates that there is tire whine, but no shake or vibration, the noise originates with the contact between the tire and the road surface.
- A thumping noise usually means the tire has flat or soft spots making a noise as they slap the roadway. Tire whine can be distinguished from axle noise because axle noise is diminished or changes according to load or speed. Tire noise remains the same over a range of speeds.

CAUTION: Be sure to deflate tires to their correct pressures after the following check is completed.

- To verify that tire noise is not associated with shake or vibration, inflate the tires one at a time to 345 kPa (50 psi) and check for a change in the sound. The pitch or whine will change as the increased pressure changes the tire frequency.