Section 00-04: Noise, Vibration and Harshness
1996 F-150, F-250, F-350, Bronco and F-Super Duty Workshop Manual
Noise or Vibration, 4-Wheel Drive Vehicles
Verify the condition by road testing.
For 4-wheel drive vehicles, verify concern with known quality conventional tires. If symptom still exists, perform the following:
Shift transfer case into two-wheel drive.
Unlock front hub locks (or remove front driveshaft (4602)).
If condition disappears, concern is in front axle or driveshaft.
If condition persists, remove rear driveshaft and road test using front drive.
If condition disappears, concern is in rear driveline or axle.
If condition persists, refer to Symptom Charts in the Diagnosis and Testing portion of this section or other sources.
For front wheel tire concerns, refer to Match Mounting Tires in the Adjustments portion of this section.
Axle Pitch Line Runout and/or Boom
This condition can be misdiagnosed as driveline imbalance. The symptoms are very similar: vibration and/or boom in the range 72-97 km/h (45-60 mph). It is important to test carefully. Refer to Section 05-00.
Driveline imbalance yields a vibration and/or boom that is present in all modes of operation: drive, coast and neutral coast. Axle pitch line runout symptoms may change between drive and coast because the pitch line runout can be different on the drive side and coast side of the gear teeth and will definitely improve in neutral coast where the gears are unloaded.
If the source of the problem is high pitch line runout, it is necessary to change the ring and pinion gears as a set.
Hub Bearing, Rear
Bearing malfunctions normally will be obvious at disassembly. Pinion bearings make a high-pitched, whistling noise, usually at all speeds. However, if there is only one pinion bearing that is malfunctioning, the noise may vary in different driving phases.
Pinion bearings are frequently replaced unnecessarily when correcting gear noise. They should not be replaced unless they are actually scored or damaged, or there is a specific pinion bearing noise. Examine the large end of the rollers for wear. If the original blend radius has worn to a sharp edge, the bearing assembly should be replaced.
Remember that the low-pitched rumble of a malfunctioning wheel bearing can be duplicated by a wind noise from externally mounted accessories or by tires.
On semi-floating type axles, wheel bearing noise might be mistaken for pinion bearing noise, so be sure to look at the wheel bearings carefully before disassembling the axle.
Wheel bearings on semi-floating type axles are pressed into the housing at the axle tube ends. The axle shaft is the inner race for the bearing. The rollers can run on about the center of the polished surface. So if the bearing is damaged, the roller surface on the shaft will be damaged as well.
Analysis of Gear Noise
The noises described in this section usually have specific causes that can be diagnosed by observation as the unit is disassembled. The initial clues are, of course, the type of noise heard on the road test and the driving conditions.
Gear Howl and Whine
When disassembling the axle to diagnose and correct gear noise, it is assumed that the tires, exhaust, and trim items have first been checked as possible causes. Examination of the gear tooth contact pattern, as shown in the following illustration, will aid diagnosis of gear whine/howl. Refer to procedure in Section 05-00.
Ring and Pinion Gear Wear Pattern Interpretation
|A||—||Normal or Desirable Pattern (Part of 4209)|
|B||—||Backlash Correct; Thinner Pinion Position Shim Required (Part of 4209)|
|C||—||Backlash Correct; Thicker Pinion Position Shim Required (Part of 4209)|
|D||—||Pinion Position Shim Correct; Decrease Backlash|
(Part of 4209)
|E||—||Pinion Position Shim Correct; Increase Backlash|
(Part of 4209)
Chuckle that occurs on the coast driving phase is usually caused by excessive clearance due to differential gear wear or by a damaged tooth on the coast side of the pinion or ring gear.
Any damage to a gear tooth on the coast side can cause a noise identical to chuckle. Even a very small tooth nick or ridge on the edge of a tooth is enough to cause the noise.
You can often correct this condition simply by cleaning up the gear tooth nick or ridge with a small grinding wheel. However, if the cleaned up or damaged area is larger than 3.175mm (1/8 inch), it is advisable to replace the gearset.
To check the gearset, remove as much lubricant as possible from the gears with clean solvent. Wipe the gears dry or blow them dry with compressed air. Look for scored or damaged teeth. Refer to the following illustration. Also look for cracks or other damage.
If either gear is damaged or scored badly, the gearset must be replaced. Also, if there is metal broken loose, the carrier and housing must be cleaned to remove particles that could cause damage later. Any other damaged parts in the axle must be replaced.
Clunk is due to backlash in the driveline, but not necessarily in the axle. To determine whether driveline clunk is caused by the axle, make a check of the total axle backlash. Refer to Section 05-00 for procedure.
A low-frequency booming noise which may be heard or sensed as a pressure in the ear drums at or above idle, possibly indicates a bound-up exhaust system or engine mounts, or can be caused by tire or driveline imbalance above 64 km/h (40 mph).
Check Bearing Preload
On F-150 models only. The absence of differential bearing preload causes noise as driving loads tend to move the gear pattern to the outside (heel) of the ring gear. In fact, if the preload is not set correctly, it can move out to the edge and score the gears.
Both the pinion bearing and differential bearing preload must be checked to ensure that the pattern will stay in place under load.
Check the pinion bearing preload by putting a Nm (lb-in) torque wrench on the pinion nut and measuring the torque effort it takes to turn the pinion.
Compare the reading with the preload specification for used bearings. Refer to the appropriate axle section in Group 05.
The pinion seal will offer a resistance equivalent to 0.56-0.68 Nm (5-6 lb-in). If the bearings are preloaded, a reading higher than 0.56-0.68 Nm (5-6 lb-in) on the torque wrench scale must be obtained. If not, there is no preload.
Another indication of pinion bearing preload is some threads protruding from in front of the pinion nut. Normally about 2.29-2.54mm (0.090-0.100 inch) of threads protrude if the preload is set properly. If the nut is flush with the end of the stem, chances are the preload was not set. Refer to the appropriate axle section in Group 05 to reset the proper preload.
NOTE: On F-150 models only, the absence of preload may indicate that the spacer between the pinion bearings is crushed and should be replaced.