Understanding and Diagnosing Rear Differential Noise
“The rear differential plays a very large role in the working assembly of a car, truck or semi. We all know that a vehicle works via its engine’s piston motion which is powered by fuel combustion. But how exactly does the motion of the piston or crankshaft convert to the rotational motion of the wheels? Let’s take a look at the magic of the rear differential and the problems that arise from rear differential noise.“
The differential is the final end mechanism that enables the creation of torque in the crankshaft before it is distributed through the transmission, to the drive shaft, and then to the wheels. This part serves to transmit engine power to the wheels while also enabling them to have different rotation speeds. This very purpose is the reason behind the name ‘differential’—because it can distribute different kinds of speeds to wheels especially during a turn.
The pair of driving wheels in a vehicle does not rotate at the same speed during a turn. In situations like this, the wheel on the turning side (inside wheel) rotates slower than its counterpart (outside wheel). This is made possible with the help of the rear differential’s side gears and spiders. To put it simply, this mechanism is the one that decides the speed and the frequency of the turning of the axle and wheels in relation to the turning of the drive shaft. This is often referred to ring and pinion ratio. If for example you have a rear gear ratio of 3.73:1, this simply means that the drive shaft (which is connected directly to the pinion gear) is going to rotate 3.73 times to the wheels turning one, full rotation.
Rear Differential Noise
The rear differential is made up of many gears with teeth entwined with each other. The effectiveness of its function is dependent on how precise these gears are arranged and oriented relative to each other. If, for example, lubrication runs low on the mechanism or the arrangement of the gears goes out of place, rear differential noise occurs. Gear whine, bearing noise and clunking are common rear differential noises that should be a concern to vehicle owners.
There are various situations which can create rear differential noise. For example, howling of gears is a sure sign of wearing. If you notice that the howling noise only occurs during deceleration, then it is a good indicator that the pinion-bearing preload has loosened.
Howling under acceleration at various speeds, however, indicates that the gears are already worn out or are out of alignment or depth with each other. If rear differential noise occurs while accelerating the car only at a certain speed, it is likely because the gears have become worn due to overloading and lubrication failure. If your gears are newly installed and still create a howling noise, double-check its preload and make sure that the teeth are properly aligned.
Rumbling and whirring noises at speed over 20 mph, moreover, can be the result of worn carrier bearings. For vehicles with C-clip axles, the rear differential noise may change at different turns. Generally, worn out pinion bearings can create whirring noises at various speeds, be it may during deceleration and/or acceleration. If the pinion bearings are the problem, they create more of a whirring noise than a rumble because it turns several times faster than the carrier assembly. Regular clunking every few feet can also be an indicator of a broken pinion gear and/or chipped and damaged ring gear.
Overly worn out bearings tend to make a howling noise when they do not properly support the gears. Rumbling while turning, on the other hand, is a sign of bad wheel bearings. Clunking and banging noises on the corners can be due to lack of sufficient posi-traction lubrication, broken spider gears, or worn posi-traction or limited-slip clutches. Broken spider gears, moreover, can also immobilize the differential and create a loud, crunching sound during final departure. If the rear differential noise is characterized by clunking every two or three feet, then there is a great chance that a broken ring gear is the problem with the section with the broken teeth banging or grinding as it tries to engage the pinion.
Having a chipped or missing high spot on a gear tooth sounds closely similar to a broken gear except that the rear differential noise only happens while accelerating or decelerating. This is because the problem is just present on one side of the offending tooth. This rear differential noise is described as a heavy clicking type of sound which occurs every eight feet or so. If the pinion is the one that has a high spot, the noise occurs every two or three feet and is much more pronounced because of its higher frequency.
Rear differential noise, when ignored, can lead to major problems and permanent damage on the ring and pinion. Repair of the differential is usually recommended to be left in the hands of professionals because of the complexity of the system.
Instead, what you could do is to boost up the lubrication on the differential instead. Providing proper lubrication levels will also lessen heat and friction and make the movements of gears against each other much smoother and quieter. This is merely a preventive step, however, and a replacement of the entire mechanism is still recommended for those who have been delayed in fixing the problem.
In terms of lubricants, one very good example is Boost Performance Product’s CleanBoost® EMT™ Engine and Metal Treatment. This product is formulated with stabilizing anti-oxidants and special metal deactivators that ensures the smooth performance of the engine and gear applications. When applied, the gear oil in the differential carries this Engine and Metal Treatment to the asperities of the metals to form a covalent, galvanic bond on it. It provides 2 to 4 micron penetration into the surface, therefore lessening heat and friction and quieting rear differential noise where applicable.
For more information about this product, simply head here.
About the Author
Passionate about Jeeps, mechanics and all the new ways to improve engine performance.