In this continuing series of maintenance article for the BMW E30 3 Series models, we will focus on changing the differential oil. This article is written with the 325is model in mind, however this article applies to all BMW’s in general.
Why change the differential oil? The differential is just like any other mechanism with moving parts. It generates heat and friction while in operation. Just like an engine, regular oil changes help to keep parts such as gears well lubricated and clean. Over time, the differential wears just like any other component. The differences between the differential and an engine is that the differential has no internal filter to capture small particles of metal as the gears wear. Also, the differential uses oil that is much thicker than engine oil. Usually differential oil is good for up to 80,000 miles without changing. However it is commonly overlooked in the wide spectrum of vehicle maintenance. Failure to change the fluid at regular intervals can cause a variety of problems, eventually causing the bearings in the diff to fail. In this article I will go over the simple steps to changing the oil yourself and avoid a costly trip to your mechanic for something you can do in under an hour in your driveway.
First, drive the car around the block a few times to get the oil inside the differential warmed up,
Now, let’s jack the car up. First, chock the front wheels to keep them from rolling while you have the rear wheels jacked up. Be sure to jack the car up on a re-inforced part of the body, such as a crossmember or chassis beam. Be sure not to jack the car up on any other part, as you could put a hole right through the bottom of the car. I have seen Porsches in the past where jacks have punctured the floorboards. A handy reference is Wayne’s article on jacking up your BMW.
Now jack up the front wheels of the car as well. We will want to get the car as level as possible in order to drain all the old fluid out of the differential.
Now crawl under the car and look at the differential. On the very rear of the differential towards the bottom, you will see two 17mm internal Allen head plugs (NOTE: the size can vary with the year and model of your car). These are the drain and filler plugs for the oil. Put a suitable drain tray under the plugs and first open the top plug to relieve any possible vacuum inside the diff and help the oil drain out. Now loosen the lower plug and thread it out by hand. Keep in mind that the old oil will start to flow out as soon as you remove the plug, so make sure the drain pan is directly below. Also, gear oil is some of the worst smelling stuff on earth. If you get any on your clothes, you will never get the smell off again, so be sure to wear your worst work clothes or just work naked! Well, maybe that’s not such a good idea.
Anyway, let the old fluid drain out of the car completely just watch it, it will go from a full stream to a small trickle then it will stop flowing. Once it has stopped, thread the drain plug back in and torque it to 41 ft./lbs. Now, get the new fluid and using a siphon, pump the new fluid into the filer hole at the top. When the fluid begins to flow out of the top hole, it is full. Now re-install the filler plug and torque it to 41 ft./lbs.
Be sure to check your owner’s manual as to what oil to run in the differential. I always use Swepco oil. This stuff is great, it keeps the diff cool, lubricated, and most of all, quiet!
Now lower the car off the jack stands and take your car for a spin to work the new oil into the gears.
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