What Does the Differential on a Car Act Like When it Goes Bad?

Broken or badly chipped gear teeth will typically cause a sort of "skip" in the power transfer, which typically manifests as a vibration that increases with speed. A chipped tooth on your pinion and outer ring gears -- which always carry power -- will cause a skip or vibration at all times. A broken tooth on the pinion -- "spider" -- gears will result in vibration when turning in either direction; chipped teeth on the axle gears will cause vibration when turning in one direction, but not necessarily the other.

While you may hear a bit of rattle or thunk associated with a single missing tooth, you'll almost certainly hear it when you've got more than one. Teeth tend to break in sets, two or three in sequence. Every tooth that breaks increases the length of the skip and the violence of engagement, resulting in ever more severe noise and vibration. This noise may alternately be described as either a popping, banging or binding, depending upon the diff and the observer.

Broken teeth on a single pinion gear may not disable the differential completely, but enough busted teeth on a ring or pinion will. On the axle gear, broken teeth will result in a complete loss of power transfer when turning in one direction or the other. Eventually, the gaps between the teeth will get so wide that the gear fails to engage at all, and the differential has officially failed.

Limited slip differentials tend to lose their slip-limiting qualities when they wear out. A traditional clutch-type differential will lose its ability to transfer power to the slower spinning wheel, essentially turning it back into a standard "open" differential. The viscous-coupling differentials preferred by some manufacturers use a compression-sensitive viscous fluid to transfer power. Once the fluid wears out, the differential will progressively fail to transfer power while cruising or turning.