By Mike Bumbeck,
Estimated time: 180 minutes
As much as you try to ignore it, that pesky spot of oil on the driveway isn't getting any smaller. Sure, you can put a tray of some sort under the car, or use cat litter or some type of space age polymers to sop up the drips. You can even try parking in a different spot, but the oil will keep on dripping. While oil spots on the driveway are certainly a nuisance, the fact that the oil is no longer in the engine crankcase or transmission can pose a real problem for your transmission and your budget.
The automobile contains a number of fluids that should all stay where they're supposed to be. Oil spots, coolant leaks, mysterious drips, and other signs of fluids leaving their rightful places inside the automobile are telltale signs that you should pay attention to. One of these parts crying for care by leaking gear oil all over the driveway or garage floor is an axle seal. Sometimes, oil on the outside of the case where the axles enter will be the only indication of a problem. A surefire way to determine if the leaking oil in question is engine or gear oil is to follow your nose. Gear oil has a unique, rotten egg type of smell.
Gear oil has this unique smell for a reason. The inside of a manual transmission, transaxle, transfer case or differential of an automobile is a truly harsh environment. Gear oil is manufactured specifically to handle the gnashing and spinning of gears as you shift from First to Second, from Third to Fourth, and so on.
Gear oil is different from motor oil in a number of ways. One is that it is much thicker, possessing a higher viscosity. This viscosity helps the oil stick to the gears as they gnash and spin. The other difference is that gear oil is designed to provide lubrication in the high shock conditions that accompany the meshing and occasional mashing or grinding of gears. The chemical compounds in the oil often contain sulfur, which also give the gear oil its unique, expired egg smell. The thickness, combined with the special chemical compounds of the gear oil, lubricates and cushions the gears as they shift. The gear oil is crucial to achieving the ultimate goal of the transmission, transaxle, differential or transfer case – to transmit the power of the engine through the axles and the wheels and then down to the ground – and managing to do it all again, day after day.
Since the ends of these spinning axles that extend into the transmission must share the same gear oil with the transmission, transaxle, transfer case or differential, the axle seal has two tasks. One is to keep the gear oil inside the case that contains the gears and the ends of the axles. The other is to keep crud, road grime and rocks out of the transmission, ruining the whole deal by converting the gears into rock crushers.
Since the axles going into the transmission, and the transmission itself, are made of metal, the axle seal houses a flexible seal that rides on the axle and keeps gear oil in, and crud out. The flexible seal uses a small amount of the gear oil in conjunction with the seal itself to achieve this task. The flexible seal is housed in a metal carrier, which allows it to be pressed into the transmission or differential semi-permanently.
Axle seals can simply wear out, but the most common cause of failure is improper axle removal or installation. While the metal carrier of the seal is sturdy, the flexible seal is fragile and can be easily torn by a ham-handed installation of an incoming axle. Since installing new axle seals requires removal of the axles, a great time to inspect and replace them is during axle service.