Quick Info
Model
450, 452
Estimated Cost
€30 – €50
Difficulty
Intermediate
Time Required
35-45 Min
Safety Notes
The Golden Rule: Always loosen and remove the FILL plug (the upper one) before touching the drain plug. If you drain the oil and then realize the fill plug is seized or stripped, you will be unable to drive the car or refill it.
Level Ground: For an accurate fill, the car must be perfectly level. If the car is tilted on jack stands, you may under-fill or over-fill the gearbox, leading to poor shifting or seal failure.
Burn Hazard: Gear oil can be extremely hot after a drive. Let the car sit for at least 30-45 minutes before starting this procedure.
Environmental Protection: Gear oil has a strong, pungent odor and is highly toxic. Always use a proper catch pan and dispose of the old fluid at a certified recycling center.
Seal Inspection: Check the threads on your plugs. If the internal 8mm hex is rounded, replace the plugs with new ones immediately to avoid future issues.
Step-by-Step Guide
Introduction
The Smart Roadster’s 6-speed automated manual transmission is often neglected due to the manufacturer's "sealed for life" claims. However, anyone who has seen the blackened, metallic-tinted oil that comes out of a 50,000km gearbox knows that "lifetime" is a myth. Over time, the gear oil loses its viscosity and accumulates microscopic metal shavings from normal gear wear.
This degradation leads to "notchy" gear changes, slower shift times, and increased stress on the electric shift actuators. Changing the transmission oil is one of the most effective ways to "refresh" the driving experience of a 452. By introducing fresh, high-quality synthetic Dexron III fluid, you reduce internal friction and heat, resulting in a significantly smoother and quieter gearbox. This guide walk you through the "Fill to Spill" method to ensure your transmission is perfectly lubricated.
Leveling & Fill Plug Access
Ensure the vehicle is parked on a level surface. If you are jacking the car up, use four jack stands to keep the chassis horizontal. Locate the Fill Plug on the side of the gearbox. Using an 8mm Hex (Allen) key, loosen and remove the fill plug. This ensures that you can definitely refill the car once it is empty.
Draining the Gearbox
Place your drain pan directly under the Drain Plug at the bottom of the transmission. Use the 8mm Hex key to remove the plug. The oil will flow out quickly; be prepared for the strong smell characteristic of used gear oil. Allow it to drain until it slows to a very occasional drip.
Plug Inspection & Cleaning
While the oil is draining, inspect the magnetic tips of both plugs. It is normal to find a small amount of fine "sludge" (microscopic metal shavings). Use a clean cloth and brake cleaner to wipe the magnets until they are shiny and free of debris. If you find large metal chunks, it may indicate internal gearbox damage.
The "Fill to Spill" Process
Reinstall the bottom drain plug and tighten it firmly (approx. 35Nm). Using a fluid pump or a long funnel with a flexible hose, begin adding fresh Dextron III ATF oil. The Smart Roadster takes approximately 1.4 Liters. Continue filling until the oil begins to slowly drip back out of the fill hole—this is the signal that the level is perfect.
Final Sealing & Clean Up
Once the oil has finished "spilling" and settled at the lip of the hole, reinstall the fill plug. Tighten it to 43Nm. Use brake cleaner to wipe down the entire gearbox housing; this is crucial so you can easily spot any leaks during your next inspection. Lower the car and take it for a short drive, cycling through all gears (including reverse) to circulate the new fluid.
