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One of the easiest tasks to perform on your BMW is to change the transmission and differential fluid. The differential and transmission both use the same lubricating fluid. It’s very important that the fluid in your transmission is at the proper level, or your transmission will experience significant wear. The synchro rings and sliders depend on a slick surface to match speeds when shifting. If your transmission is low on oil, the wear on these components will accelerate significantly and shifting the car will be more difficult. If your BMW is having problems shifting, check the level of the transmission oil. In addition, keeping the differential and its associated gears well lubricated should help increase your fuel mileage.
The transmission oil also keeps temperatures down inside the transmission. The engine is a primary source of heat for the transmission, as the heat conducts and radiates through and around the points where the engine and transmission are mounted. The transmission creates heat itself as the gears and synchros turn within its case. Keeping the transmission fluid at its proper level helps to mitigate heat problems. Note that on some higher-performance BMW transmissions, there is an external transmission cooler that operates similarly to the engine cooler.
Change the transmission fluid every 30,000 miles or about once every two years. Check your owner’s manual for more details on the scheduled requirements for your BMW. This number is a rough estimate, and may vary depending upon the use of your 3 Series (track vs. street). There are many moving parts in the transmission, and they tend to drop microscopic metal particles into the tranny oil. Specifically, the synchro rings wear down each time you shift. While transmission bearings are not as sensitive as engine bearings, they can exhibit wear from these particles in the oil.
The 3 Series manual transmission has two plugs for filling and emptying the transmission oil, located on the side and bottom of the transmission case. To check the level of the transmission oil, remove the top filler plug on the side of the transmission (where you usually add fluid). When you have the plug removed, stick your finger inside the hole, angle it toward the ground, and see if you can feel any fluid. Do this when the car is cold and parked on level ground. If you can feel the fluid level with your finger, your fluid level is about right, or perhaps will need only a little topping off.
If you cannot feel the fluid level, add transmission oil to the case. If you plan to change the oil, remove the small plug on the bottom of the transmission case. Empty the transmission oil when the car is still warm, as it will drain easier. For this task, have a drain pan capable of handling at least 5 quarts of transmission oil. As you go, check the fluid in the pan for any unusual metal pieces or grit in the oil.
While the fluid is emptying, clean out the drain plugs. Using a cotton swab or a paper towel, carefully clean out any black debris and particles present.
Replace the bottom plug on the transmission, but don’t tighten it too much (50 N-m or 37 ft-lbs maximum). Fortunately, this plug does not tend to leak (transmission oil is thicker than engine oil). If the plug does leak later on, however, you can always tighten it a little more. Now, add transmission oil to the case with a hand-operated oil pump. These pumps are available from most auto parts stores, and attach to the top of the plastic transmission-oil bottle. They work similarly to liquid soap dispensers. Pump the transmission case full of fluid until it just starts to run out the filler hole. It should take a little more than 1 quart to fill. Replace the filler plug and clean up the few drips that might have come out of the hole. Tighten the filler plug in a similar manner to the drain plug.
Replacement of the differential fluid is nearly identical, except you will need a 14-millimeter Allen wrench for the plugs. If you have one of those semi-rare, four-wheel-drive 3 Series cars (like the 325ix), don’t forget to change the fluid in the forward differential and center transfer case as well.
In many cases, generic transmission gear oil will suffice. However, for those BMW 3 Series owners wishing to have the best of everything for their cars, there is Swepco 201 multipurpose gear lube. This gear oil is excellent for transmissions, and many of our loyal Pelican Parts BMW and Porsche customers swear by it. Rumor has it that adding Swepco 201 will prolong transmission life and help to postpone a costly rebuild. While this can hardly be proven, a lot of our customers agree that Swepco 201 creates a difference they can feel while shifting.
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