Driveline NVH Complaints

1. Find the speed where the concern is most severe.
2. Turn the air conditioning system off and on to isolate air conditioning system impact on the noise. Accelerate gently through this speed to a few mph above it and then coast back down a few mph below it and note if the concern changes character when driving through or coasting down.
3. Repeat this procedure if necessary to get a feeling for the behavior. Then drive about five mph above the speed, put the transmission in neutral and coast down. Note any change in behavior.
4. Try "floating" the driveline by backing off slightly on the throttle at the problem speed. The idea is to unload the axle gears and the universal joints as much as possible. This test is difficult to master and sometimes it helps to test on a slight downgrade so that gravity is counteracting the aerodynamic drag. The throttle should be operated in much the same way it is operated when shifting out of gear without using the clutch. In that case, the throttle is used to unload the transmission gears; here it is used to unload the driveline in general.

If the concern does not change in all these modes of operation, the cause may well be driveline imbalance since the imbalance is not changed by the throttle position.

If there is a distinct change in the character or intensity between the drive and coast modes of operation, the concern is most likely not imbalance but is most likely due to the axle or the universal joints since they are sensitive to the load on the driveline. (The axle pinion gear teeth are machined on two separate machines. One machine does the drive side of the teeth, another does the coast side. Hence there can be a great difference in performance between driving and coasting in gear.) If the noise and/or vibration goes away when the driveline is "floated" or when coasting through the speed in neutral, the diagnosis is strengthened. Driveline imbalance does not go away in a neutral coast, but axle and joint concerns may.

To become familiar with the symptoms of imbalance and to determine if driveline imbalance exists on the vehicle, intentionally imbalance the driveline. This can be done by installing three hose clamps with screw heads aligned on the rear of the driveshaft. Test drive the vehicle again. If the same vibration is still present but now intensified, the concern is driveline imbalance. If a new vibration is now present as a result of the intentional imbalance, the original condition would not be caused by driveline imbalance.