Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Understanding Rear Axles


The rear axle is a special type of transmission that reduces the speed of the drive from the engine and transmission and drives the power to the rear wheels. Power enters the rear axle from the driveshaft via the companion flange. The flange is mounted on the drive pinion shaft. The drive pinion shaft and gear which carry the power into the differential turn at engine speed. The gear on the end of the pinion shaft drives a large ring gear the axis of rotation of which is 90- away from that of the pinion. The pinion and ring gear reduce the speed and multiply the power by the gear ratio of the axle, and change the direction of rotation to turn the axle shafts which drive both wheels. The rear axle gear ratio is found by dividing the number of pinion gear teeth into the number of ring gear teeth.

The ring gear drives the differential case. The case provides the two mounting points for the ends of a pinion shaft on which are mounted two pinion gears. The pinion gears drive the two side gears, one of which is located on the inner end of each axle shaft.

By driving the axle shafts through this arrangement, the differential allows the outer drive wheel to turn faster than the inner drive wheel in a turn.

The main drive pinion and the side bearings, which bear the weight of the differential case, are shimmed to provide proper bearing preload, and to position and the pinion and ring gears properly.

The proper adjustment of the relationship of the ring and pinion gears is critical. It should be attempted only by those with extensive equipment and/or experience.

Limited-slip differentials include clutches which tend to link each axle shaft to the differential case. Clutches may be engaged either by spring action or by pressure produced by the torque on the axles during a turn. During turning on a dry pavement, the effects of the clutch are overcome, and each wheel turns at the required speed. When slippage occurs at either wheel, however, the clutches will transmit some of the power to the wheel which has the greater amount of traction. Because of the presence of clutches, limited-slip units require a special lubricant.


First, determine when the noise is most noticeable.

Drive Noise: Produced under vehicle acceleration.
Coast Noise: Produced while the car coasts with a closed throttle.
Float Noise: Occurs while maintaining constant car speed (just enough to keep speed constant) on a level road.

Road Noise See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Noise Diagnosis

Brick or rough surfaced concrete roads produce noises that seem to come from the rear axle. Road noise is usually identical in Drive or Coast and driving on a different type of road will tell whether the road is the problem.

Tire Noise

Tire noises are often mistaken for rear axle problems. Snow treads or unevenly worn tires produce vibrations seeming to originate elsewhere. Temporarily inflating the tires to 40 lbs will significantly alter tire noise, but will have no effect on rear axle noises (which normally cease below about 30 mph).

Engine/Transmission Noise

Determine at what speed the noise is most pronounced, then stop the car in a quiet place. With the transmission in Neutral, run the engine through speeds corresponding to road speeds where the noise was noticed. Noises produced with the car standing still are coming from the engine or transmission.

Front Wheel Bearing Noise

While holding the car speed steady; lightly apply the footbrake; this will often decrease bearing noise, as some of the load is taken from the bearing.

Rear Axle Noises

Eliminating other possible sources can narrow the cause to the rear axle, which normally produces noise from worn gears or bearings. Gear noises tend to peak in a narrow speed range, while bearing noises will usually vary in pitch with engine speeds.