A driveshaft connects the rear differential mechanism to the transmission or transfer case, transferring the engine’s power to the rear wheels. The rear differential’s role is simple: change the power’s direction from longitudinal (along the car) and direct it to the wheels.
Gears inside the differential accomplish this purpose, providing a 90-degree direction change along the rear axles. The power is transmitted from the rear differential along axle shafts that connect to the wheels and rotate to propel the vehicle forward.
The gears inside the rear differential are bathed in gear oil, which lubricates the gears and bearings and cools them to prevent overheating. The fluid breaks down over time, and metal filings from the gears and bearings collect in the fluid. Every once in a while rear differential fluid replacement must be performed. The gear oil must be changed to prevent damage to the gears inside, known as the ring gear and the pinion gear.
A rear differential service consists of removing the rear differential cover, cleaning any old fluid from inside the differential case, resealing the cover, and adding clean fluid. After a rear differential fluid change, most vehicles will go 20,000 to 40,000 miles before it’s due again.