Analysis of Vibration
Analysis of VibrationWARNING: A vehicle equipped with a Ford Traction-Lok differential or a Track tech 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 injuryextensive 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 annual inspection.
Driveshaft
1. Road test the vehicle to determine the critical vibration points. Note the road speed, the engine rpm, and the shift lever positions at which the vibration occurs.
2. Stop the vehicle, place the transmission lever in NEUTRAL, and run the engine through the critical speed ranges determined in Step 1.
3. If vibration is felt, balance the driveshaft.
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 other tires inflated to their normal operating pressures.
4 Rotate the transmission output yoke until vertical. This will simplify taking measurements.
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 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 in 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.
An incorrect driveline (pinion) angle can often be detected by the driving condition in which the vibration occurs.
^ A vibration during coast-down from 72 to 56 km/h (45 to 35 mph) is often caused by an excessive U-joint angle at the axle (pinion nose downward).
^ A vibration during acceleration, from 56 to 72 km/h (35 to 45 mph) may indicate an excessive U-joint angle at the axle (pinion nose upward).
If the tires and driveline angle are not the cause, carry out the NVH tests to determine whether the concern is caused by a condition in the axle.
Universal Joint (U-Joint) Wear
Place the vehicle on a frame hoist and rotate the driveshaft by hand. Check for rough operation or seized U-joints. Install a new U-joint if it shows signs of seizure, excessive wear, or incorrect seating.
Drive Pinion Stem and Pinion Flange
Check the pinion flange runout when all other checks have failed to show the cause of vibration.
One cause of excessive pinion flange runout is incorrect installation of the axle drive pinion seal. Check to see if the spring on the seal lip has been dislodged before installing a new differential ring gear and pinion.
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.