Audi and VW vehicles equipped with the quattro all-wheel drive system have a rear differential. The job of the differential is to takes power from the driveshaft extending rearward from the transmission and deliver it to the two rear wheels.
The rear differential is filled with lubricating fluid that should be changed periodically. Yes, apparently Audi does not specify a replacement interval for this fluid, but that shouldn’t keep you from changing it. It’s not too difficult a job, and it will help keep your Audi’s quattro system running smoothly, particularly if you’re increasing the car’s performance or have a higher mileage vehicle.
The vehicle needs to be level as you fill the differential in order to ensure that the correct amount of new fluid is added. So, rather than simply jacking up the rear of the car, you should elevate the whole vehicle and secure it on jackstands.
Somewhere under that caked-on grease is the differential fill plug. It is located on the right-hand side of the differential, just behind and below the passenger-side rear axle. Thus, in this photo, the front of the car is to the right.
This photo shows more of the differential. The fill plug is still visible to the upper left, and the drain plug is right on the bottom of the diff housing, toward the front of the car. Judging by the pattern of the grease deposited on the diff, axle, and fuel tank, I may have a bad axle seal or CV joint.
The plug has been removed. If your differential is grungy like mine, clean off the diff beforehand, or at least be careful not to get any debris inside the differential. Despite the evidence of a leak in the area, the level of oil in the differential was up to spec.
This is the view of the differential fill opening as seen from the right rear wheel well. The best way to fill the differential is through a tube the runs from the wheel well into this opening. Move the drain pan underneath the fill hole before starting the refill process.
Here is the hose, emerging from the wheel well. I could have used another foot of hose or so, but this worked fine too. I simply snipped the bottle end to the proper size, inserted it into the hose, and let the fluid drain into the hose. Keep the outside end of the hose elevated so that the fluid runs downhill into the differential.