Analysis of Vibration
Analysis of VibrationWARNING: A vehicle equipped with a Ford Traction-Lok differential or a Tractech-Truetrac differential will always have both wheels driving. If only one wheel is raised off the floor and the rear axle is driven by the engine, the wheel on the floor could drive the vehicle off the stand or jack. Verify that both rear wheels are off the floor.
Few vibration conditions are caused by the front or rear axle. On a vibration concern, follow the diagnosis procedure unless there is a good reason to suspect the axle.
Tires
WARNING: Do not balance the wheels and tires while they are mounted on the vehicle. Possible tire disintegration/differential failure could result, causing personal injury/Extensive component damage. Use an off-vehicle wheel and tire balancer only.
Most vibration in the rear end is caused by tires or driveline angle.
Vibration is a concern with modern, high-mileage tires if they are not "true" both radially and laterally. They are more susceptible to vibration around the limits of radial and lateral runout of the tire and wheel assembly. They also require more accurate balancing.
Wheel and tire runout checks, truing and balancing are normally done before an axle inspection.
Driveline Angle
Driveline angularity is the angular relationship between the engine crankshaft, the driveshaft, and the rear axle pinion. Factors determining driveline angularity, include ride height, rear spring, and engine mounts.
Driveline Angle
Calculate the driveline operating angles as follows.
1. Preliminary setup procedures.
1 Inspect the U-joints for correct operation.
2 Park the vehicle on a level surface such as a drive-on hoist, or back onto a front end alignment rack.
3 Verify the curb position ride height is within specifications with the vehicle unloaded, and all of the tires inflated to their normal operating pressures.
4 Rotate the transmission output yoke until vertical. This will simplify taking measurements.
5 Calibrate the special tool by placing tool on clean, flat level section of the frame rail and press the ALT-ZERO button.
2. Using the special tool, measure the slope of the components. Record the measurements and the direction of the component's slope.
Example for Calculating U-Joint Operating Angles
3. Calculate the difference in the slope of the components to determine the U-joint operating angle.
- When two connected components slope in the same direction, subtract the smaller number from the largest to find the U-joint operating angle. When two connected components slope us the opposite direction, add the measurements to find the U-joint operating angle.
- The U-joint operating angle is the angle formed by two yokes connected by a cross and bearing kit. Ideally, the operating angles on each end of the driveshaft must:
- be equal or within one degree of each other.
- have a three degree maximum operating angle.
- have at least one-half of one degree continuous operating angle.
If the tires and driveling angle are not the cause, carry out the NYH tests to determine whether the concern is caused by a condition in the axle.
Drive Pinion Stem and Pinion Flange
Check the pinion flange runout when all other checks have failed to show the cause of vibration.
Coupling Shaft/Center Bearing Alignment
Vehicle noise and vibration can be caused by a dislodged or failed driveshaft center bearing support rubber insulator, a contaminated driveshaft center bearing support or excessive compression of the rubber insulator.
Bearing Shimming
Drive-away shudder is the predominant symptom associated with driveline angles condition on vehicles with two-piece driveshafts. Drive-away shudder can usually be corrected by shimming down the driveshaft center bearing bracket.
If the drive-away shudder cannot be corrected by shimming down the driveshaft center bearing bracket, check the driveline angles.