~Basic Driveline Maintenance Part 1~
Manual transmissions found in modern passenger cars basically consist of an input shaft, layshaft (also called a countershaft); drive gears, dog gears and an output shaft. The input shaft spins the layshaft, and the layshaft spins all of the drive gears simultaneously. The dog gears are attached to the shift lever and output shaft. Moving the shift lever causes the dog gears to lock the output shaft to first through high gear and reverse. The dog gears and synchros have a cone-shaped engagement face machined into them that allows frictional contact to equalize the speed of the drive gear and dog gear as they engage. That "farting" noise your car makes when misshifted is the noise of the dog gear grinding against the drive gear. It doesn't take a PhD in automotive technology to realize this all needs copious amounts of a tough lubricant.

You change your oil every 3,000 miles with Mobil-1, keep exactly 25.0 psi of pressure in all four tires and flush your cooling system every year. But how about your transmission and differential? Sure, gears and gearboxes are pretty tough customers, but they still need lubrication for wear protection and to help reduce shock loads during drag launches and hard shifts. Although the driveline is considered a sealed system, lubricants do leak out and break down, albeit slowly. And a dry gearbox will spell disaster almost instantly. In Part I of our series, we're going to take a look at a front-engine, rear-drive (FR) vehicle to see what the grassroots enthusiast can do to keep their driveline in tip-top condition

The vehicle we're using for this demonstration is a 1990 Nissan 240SX. (It has an SR20DET engine swap and 180SX front end in case you were curious.) The logistics of driveline maintenance will be nearly identical on other FR vehicles including the AE86 Corolla, Mazda Miata, most BMWs and Lexus IS/GS/LS, et al. The differential is located in a housing in the rear-center of the vehicle and has two plugs; one for draining fluid and one for filling fluid. Usually, the plugs are not your typical hex-head bolts; instead, they'll have a half-inch square cut into the center, which will fit a half-inch drive ratchet. When loosening the plugs, it's a good idea to start with the fill plug. You wouldn't want to drain all of the oil to find that the fill plug cannot be removed because it is stripped or seized!

Draining the differential housing is just like changing engine oil. Most differential drain plugs have a magnet riveted to them (in lieu of a filter) that will capture metal shavings. A small amount of metal on the plug is normal and is easily wiped away. Be sure to wipe any dirt and grease from the area of the drain plug before you reinstall it. (Dirt in the threads of the plug will virtually guarantee a leak.)

Most import cars take about a quart of gear lubricant in the differential housing; usually it will be 75W-85W, 75W-90 or 75W-140 "Hypoid". This particular vehicle has a KAAZ limited-slip differential, so we're using KAAZ fluid that has special friction modifiers blended into it that keep the LSD's clutch packs happy. Since the fill plug will invariably be located on the side of the differential housing, you'll also need a lubricant pump to transfer lubricant from the can to the diff. Don't rely on those cute ketchup-bottle squirters that come with gear oil bottles, unless you like wallowing around in gear oil. We're using a three-dollar pump manufactured by Castrol.

Generally speaking, the bigger pumps will move a larger volume of fluid in a shorter amount of time. This can be advantageous if you're lying underneath your car with a greasy bottle of lube slipping out of your hands and gooey gear oil dripping in your hair. When snugging the drain plug, use a torque wrench so you don't run the risk of stripping the drain or fill plugs or cracking the differential cover. (They're usually made of cast aluminum.)

The transmission oil is changed much the same was as the differential oil. Drain plug on the bottom, fill plug on the side. The transmissions on rear-wheel drive cars tend to hold a relatively large amount of lubricant, sometimes over a gallon but usually closer to 2.5 to 3.5 quarts.