Road Test

A road test is mandatory for any customer concern of noise and/or vibration that is not eliminated by the on-hoist check of chassis components. The Road Test Form is arranged to record the conditions that may be heard or felt during this test.

The Road Test Form lists four operating conditions or modes in which some axle noises come and go: Drive, Cruise, Coast and Float.
DRIVE: Accelerating the vehicle; a definite throttle depression applying engine torque.
CRUISE: Maintaining a constant speed with the throttle applied.
COAST: Decelerating with the throttle closed.
FLOAT: Controlled deceleration; backing the throttle continually to prevent either braking or accelerating torque from the engine.

To perform a good diagnostic check it is important to operate the vehicle in all four modes and check off those in which the noise occurs. The modes are defined in the preceding chart. Write down the kilometers-per-hour (miles-per-hour) range at which both noise and vibration occur. On vehicles equipped with overdrive transmissions, evaluate rear axle noise with the transmission in DIRECT DRIVE and not in OVERDRIVE. Transmission noise can be mistaken for rear axle noise when in overdrive.