He's referring to the contact pattern. The teeth wear a pattern that matches between them. When anything happens to move a gear a little, or if the contact pattern never was adjusted perfectly, the teeth will make a slight noise when they mesh. As the gears turn, the teeth slide across each other's points of contact and that leads to the noise you can hear.
The two really large "carrier" bearings can become noisy too. Most noise occurs on acceleration. That's when the load on the gears makes it harder for the teeth to slide across each other, and it's when there's the most pressure on the carrier bearings. In some cases the noise gets worse during coasting. That's when the tires are pulling the engine instead of the other way around, and the other sides of each gear tooth makes contact.
In either case, a special yellow ink is used to observe the tooth contact area. It is applied to the sides of some of the teeth on one gear, then the drive shaft is turned while trying to hold back on the wheels. The ink will rub off where the teeth from the other gear made contact. To adjust the contact pattern the large ring gear can be adjusted left or right, or the pinion gear can be adjusted forward and rearward with shims. Changing the pinion gear is a pretty involved job, but typically if that adjustment is not right, either something is coming apart or it wasn't set up correctly before. It doesn't just change on its own without there being some other underlying problem.
If the contact pattern looks good there are other possible causes of gear noise. The most logical one is the gear lube was contaminated with water from driving through deep water. Under normal conditions the gear lube doesn't need to be changed periodically because there are no detergents or other additives that wear out like in engine oil.
Probably the most common cause of a differential failure is getting stuck, then abusing it by trying to get unstuck. If only one wheel is turning the small axle and spider gears will be turning real fast. Normally they only turn a very little when you go around a corner and one wheel turns faster than the other one. Those four little gears don't ride on bearings, just gear lube, because they don't turn much. With one wheel spinning when you're stuck, those gears can overheat and destroy the lubricating properties of the lubricant. That is something you do not want to smell! I had to work next to a fellow one time who had to rebuild an overheated differential. That smell is 100 times worse than a skunk and will clear the entire shop!
Wednesday, June 6th, 2012 AT 9:46 PM