Analysis of Gear Noise

Gear Howl and Whine

Before disassembling the rear axle housing (4010) to diagnose and correct gear noise, it is important that the tires, exhaust, trim items, roof racks and axle shafts/wheel bearings be checked as possible causes.

The noises described under Road Test 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.

Chuckle

Chuckle that occurs on the coast driving phase is usually caused by excessive clearance between the differential gear hub and the differential case bore, 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 and eliminate the noise simply by cleaning up the gear tooth nick or ridge with a small grinding wheel. If the cleaned up or damaged area is larger than 3.2mm (1/8 inch), it is advisable to replace the gearset.

To check the differential ring gear and pinion (4209) , 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. Also look for cracks or other damage.

If either gear is scored or damaged badly, the differential ring gear and pinion 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.

Damaged Gear Teeth






Scored Gear Teeth

Knock

Knock, which can occur on all driving phases, has several causes. In most cases, you will discover one of the following conditions:

1. A gear tooth damaged on the drive side is a common cause of the knock. This can usually be corrected by grinding the damaged area.

2. Occasionally, the rear axle differential gear case bolt (4216) will knock against the inside of the rear axle housing . The cause may be too little clearance due to casting flash or bumps. In this case, the differential case (4204) can be removed and the interference points ground out. Another possibility is simply that one or more rear axle differential gear case bolts are slightly backed-out. Remove, replace and properly tighten the rear axle differential gear case bolts using Stud and Bearing Mount E0AZ-19554-BA or equivalent, meeting Ford specification WSK-M2G349-A1 on the threads.

3. Knock is also characteristic of excessive end play in the axle shaft (4234) . Up to 0.762mm (0.030 inch) is allowed in most axles. The frequency of knock will be less because the axle shaft speed is slower than the driveshaft (4602) .

   NOTE: Be sure to measure the end play with a dial indicator, not by feel. A guess usually feels like far more end play than there actually is.

Clunk

Clunk is due to backlash in the driveline or loose rear suspension components. To determine whether driveline clunk is caused by the axle, make a check of the total axle backlash as follows:

1. Raise the vehicle on a frame or twin post hoist so that the rear wheels are free. Refer to Section 00-02.

2. To maintain driveline balance, mark the rear driveshaft yoke and rear axle universal joint flange (4851) so they may be installed in their original positions.

3. Remove the flange bolts and disconnect the driveshaft from the axle rear axle universal joint flange . Support the driveshaft .

4. Install Companion Flange Holding Tool T78P-4851-A between the rear axle universal joint flange and a part of the frame or body so that the rear axle universal joint flange cannot move.

5. Lock the LH rear wheel to keep it from turning.

6. Using a torque wrench, and locating on one of the wheel hub bolt nuts (1012) , rotate RH wheel slowly until a torque of 6.8-8.1 Nm (5-6 lb-ft) is obtained. Hold a chalk marker on side of tire 304.8mm (12 inch) from center of wheel.
   
   
   
   

7. Rotate wheel slowly in opposite direction until a torque of 6.8-8.1 Nm (5-6 lb-ft) is obtained.

8. Measure the length of the chalk mark to obtain the total axle backlash. Total axle backlash should be 25.4mm (1 inch) or less.

   If the backlash is within this limit, the clunk will not be eliminated by going into the axle.

   Check for these conditions if the backlash is excessive:

   • Elongation of the differential pinion shaft holes in the differential case .
   • Missing differential pinion thrust washer (4230) or differential side gear thrust washer (4228) .
   • Galling of the differential pinion shaft (4211) and bore.

   If none of the above conditions show up, there may be a loose fit of the axle shaft to the differential side gear splines. You should continue as follows until the correction is made:

9. Install new differential side gears (4236) and check the backlash.

10. Install two new axle shafts .

11. Replace the parking brake lever boot retainer (2405) .

12. Install the driveshaft so the index mark on the rear yoke is in-line with the index mark on the rear axle universal joint flange . This ensures original driveline balance. If vibration exists after installation, refer to Section 00-04.

13. Tighten all flange bolts to 95-130 Nm (70-95 lb-ft).

Bearing Whine

Bearing whine is a high-pitched sound like a whistle. It is usually caused by malfunctioning pinion bearings, which are operating at driveshaft speed. Bearing noise occurs at all driving speeds: this distinguishes it from gear whine, which usually comes and goes as speed changes. However, if there is only one differential pinion bearing (4621) that is malfunctioning, the noise may vary in different driving phases.

Differential pinion bearings are frequently replaced on axles with low mileage under 24,139 km (15,000 miles) 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 bearing's original blend radius has worn to a sharp edge, the differential pinion bearing should be replaced.

Bearing Rumble

Bearing rumble sounds like marbles being tumbled. This condition is usually caused by a malfunctioning wheel bearing. The low pitch is because the wheel bearing turns at only about one-third of driveshaft speed. In addition wheel bearing noise may be high-pitched, similar to gear noise but will be evident in all four driving modes.

Remember that an exterior luggage rack or tires can make a low-pitched rumble like that of a damaged wheel bearing.

Analysis of Inoperative Conditions

In the event the axle fails to operate, there will be obviously broken or jammed-up parts. It is important to try to analyze why the parts broke to prevent the condition from recurring.

Broken Axle Shaft

Inspect a broken axle shaft for a twist in the spline next to the break.

Breaks that occur away from the spline, or that are not accompanied by a twist in the spline, may be caused by forging flaws and/or fatigue of the shaft.

Conditions that can cause a broken axle shaft include forging flaws or a collision.

Broken Pinion Stem

The drive pinion stem is subject to conditions that can break it anytime the pinion bearing preload is not set properly. If there is no preload, the differential bearing (4221) will turn relative to the shaft. This galls the roller shaft and generates heat, which can draw the temper away from the shaft.

Too much preload also generates bearing heat with the same result. The break normally occurs between the two differential bearings .

Broken Welds

Axle housing welds, if completely broken, require replacement of the rear axle housing .

Wheel Bearing Malfunctions

Because of the severe loads they must handle, rear wheel bearings (1225) may require replacement at high mileage. If a rear wheel bearing fails at low mileage, it is often caused by overloading.

Rear wheel bearing noise might be mistaken for differential pinion bearing noise, so be sure to look at the rear wheel bearing carefully before tearing down the axle.

Rear wheel bearings are pressed into the axle housing tubes, making them more difficult to check. However, the axle shaft is the inner race for the bearing. If the bearing is damaged, the roller surface on the axle shaft may be damaged as well. The rollers run on approximately the center of the polished surface.

Axle Lock-up

Lock-up or seizure of the rear axle housing is usually associated with a low lubricant condition. The lubricant, of course, prevents friction and helps cool the parts as they heat up. Without enough lubricant, the heated parts get soft and may eventually weld together. Check for lubricant leaks after servicing.