I. Road Test Procedure

The initial road test is the most important step in the diagnosis and subsequent correction of vehicle vibration. Select a road test area with as smooth a road surface as available and where a speed of 55 mph or greater is possible. Check and adjust all four tires for proper air pressure as recommended in the owner's manual. Inspect tires for misalignment, cupping, or scalping conditions. Road test the vehicle with the customer, if possible, to determine the exact vibration complaint, recording the speed at which the condition occurs.

A neutral coast test is a good method to determine if the engine/accessories, driveline, or wheel/tires are causing the vibration problem. To do this, accelerate to 55-65 mph and place transmission selector in "neutral," allowing engine to return to idle speed. If the problem exists in this condition, the investigation can be confined to driveline and tire/wheel assemblies. If the problem does not exist during "neutral" coast, the investigation can be confined to engine/accessories.

Another good test is to carefully overinflate all four tires to approximately 10 psi above the maximum stated on the tire sidewall. Then, repeat the road test over the same road and at the same speeds. If the disturbance is eliminated or changed noticeably, deflate one front tire to normal operating pressure and repeat the test. If the problem does not return, continue adjusting tire pressure and road testing, one tire at a time, until the original problem returns. The last wheel/tire adjusted to normal operating pressure will usually be the one requiring service.

Remember, vibrations that occur at less than 40 mph are usually related to runout, while those that occur above 40 mph may be caused by either runout or imbalance.