Pre-Road Test Check

Road Conditions
The road is an important factor in the road test. A smooth asphalt road that allows driving over a range of speeds is best. The brushed concrete road surface found on many expressways and the coarse aggregate sometimes found in concrete can mask many vehicle noises and make diagnosis difficult.

If the customer complains of a noise or vibration on a particular road and only on a particular road, the source of the concern may be the road surface. If possible, try to test the vehicle on the same type of surface. This is the only case where a smooth asphalt road may not be best.

Vehicle Preparation
1. Check and verify if vehicle is equipped with original equipment, type, size and brand of wheels and tires. If non-original equipment wheels or tires are installed, it may be necessary to substitute original equipment level tire/wheel assemblies for diagnosis.
2. Check and set tire pressure.

Hoist Test

NOTE: A test on the hoist may produce different vibrations and noises than a road test because of the effect of the hoist on the rear axle. It is not unusual to find vibrations on the hoist that were not found in the road test and many times these may be ignored. If the condition that was found on the road can be duplicated on the hoist, performing experiments on the hoist may save a great deal of time.

After a road test, it is sometimes useful to do a similar test on a hoist.

1. Elevate the rear wheels slightly, check to make sure that both are turning and run the engine up with the vehicle in gear.

NOTE: Use a frame hoist and use axle stands.

2. Explore the speed range of interest using the drive/cruise/coast/float tests.
3. Perform a coast down in NEUTRAL. If the vehicle is free of vibration when operating at a steady speed and behaves very differently in drive and coast, an axle concern is likely.