Preliminary Diagnosis

Question the Customer

The preliminary diagnosis begins with the service technician asking the customer enough diagnostic questions; especially for conditions of noisy operation and/or vibration.

Always ask the customer about:

1. The exact nature of the symptom. If it is a vibration, how is it felt? If a noise, what kind? High-pitched, rumble, loud, etc.?

2. The driving conditions in which the symptom appears; road speed, drive or coast, on turns, or on a grade only.

3. Whether the condition is constant or variable; does a noise increase in pitch with speed, etc.?

   All of this information should be written on the service order so that the diagnostician (foreman or technician) will know where to start looking. Whoever is doing the diagnosis must be able to identify the axle concern from what is written on the order following the road test.

   It is, of course, preferable to have the customer demonstrate the symptom on a road test whenever possible.

Axle Diagnosis Check Sheet

If there is enough reason to suspect an axle concern, the service technician should also initiate an Axle Diagnosis Check Sheet.

It is most important to record all the information related to the concern to diagnose it accurately and quickly.

Road Test

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

The Diagnosis Check Sheet lists four operating conditions or modes in which some axle noises come and go: Drive, Cruise, Coast and Float.

Mode	Conditions
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.

It is important to a good diagnostic check to operate 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, it is also important that rear axle noise be evaluated with the transmission in DIRECT DRIVE and not in OVERDRIVE. Transmission noise can be mistaken for rear axle noise when in overdrive.