I decided to change the transmission gear oil in my smart fortwo (model 451). Although the factory fill is purported to be good for a lifetime of service, I figured it can’t hurt and may extend the life of the transmission by removing debris cast off during break-in. Following are the steps I used to change the gear oil. If you have other suggestions or notice that I've missed anything, please let me know so that these instructions can be updated.
Although I have provided some photos and step-by-step instructions, I want to acknowledge the smart owners that led the way by being the first to change the transmission oil, experimented with different brands of gear oil, tested the stock oil, and shared their experiences.
1. Ensure that you have the correct tools and supplies. You will need roughly 2.5 quarts of a GL-4 rated 75W-90 gear oil. I used RedLine MT-90 synthetic oil, but there are other quality choices available from various manufacturers. You will also need an 8mm Allen socket or wrench, a shallow pan to catch the old oil, and a funnel with a length of hose to refill. If you do not have an Allen socket (I didn’t) or a set of Allen wrenches that will work, you can purchase an inexpensive set with common sizes at Harbor Freight for less than $10 (Item #67891).
2. Although I had my car on ramps in these photos for other work, it is easy to access the transmission drain and fill plugs without ramps. If the car is on ramps, make sure you take it off the ramps onto a level surface before topping off or you will overfill the transmission
(ask me how I know).
3. Using the 8mm Allen socket, remove the fill plug on the left side of the transmission housing (as viewed from the rear of the car). You will want to ensure that you can remove the fill plug before attempting to drain the gear oil.
4. Once the fill plug is out, position the drain pan on the floor under the drain plug on the lower right side of the transmission housing and remove the drain plug being careful not to drop it into the drain pan.
5. Once all the oil has drained, replace the drain plug and tighten with the 8mm Allen socket.
6. Lower the tailgate and remove the engine cover (see owner’s manual if you have questions) being careful not to misplace the twist bolt that secures the engine cover. I am using an intake hose in place of the stock resonator box, so there is a bit more room in my engine bay for the funnel. Lower the funnel with hose through the engine bay and put the end of the hose in the fill port. My funnel hose was only about 18" and it was barely adequate even with the greater clearance provided by the missing resonator. A longer length would be advisable, especially if you have the stock resonator. 30" should be plenty.
If your 75W-90 oil bottles have a nipple top that will fit into the fill port, others have reported that the transmission can be refilled with oil directly without the need of a funnel/hose. The Redline bottles were not so equipped.
7. Ensure that the funnel is stable and proceed to slowly empty the quarts of gear oil into the funnel until gear oil begins to overflow from the fill port – roughly 2.5 quarts. As mentioned above, make sure you are on a level surface!
8. Once full, reinstall the fill plug with the 8mm Allen socket and clean up any excess oil around the fill or drain plugs with a rag.
9. Take the car for a test drive and recheck both plugs to make sure that there is no oil leaking from either.
10. Make a note of the date and the number of miles at which the transmission gear oil was changed for your records.