How to Add Transmission Fluid to a Mercedes 300D

1. Locate the transmission fluid dipstick tube, which is located on the passenger's side near the firewall. It should be near the rear left side of the engine and just to the right of the battery compartment. At the top of the tube, you will either see a ring, a T-shaped dipstick handle or a red plastic cap. If you see a dipstick handle, skip to Step 3. If you see a red cap, proceed to the next Step.
2. Buy a transmission fluid dipstick sized to your model year of Mercedes 300D. You can purchase these directly from your local Mercedes dealer or find a used or aftermarket equivalent from a junkyard or parts dealer. Unlike many car manufacturers, Mercedes did not make a transmission dipstick part of the standard equipment on many of its models.
3. Drive the car for a bit to get it warmed up, and take your transmission fluid, funnel, rag and dipstick with you. Drive about 10 miles or 20 minutes, whichever comes first. If possible, spend part of the drive on a high-speed road where you can cycle through all of the gears.
4. Park the car on a level surface and put the parking brake on. Pull the hood release lever and leave the engine running.
5. Insert the dipstick all the way into the dipstick tube. If you already have a dipstick in there, pull it out, wipe it clean and reinsert it. Then pull the dipstick all the way out without touching the fluid on the end. Look at the fluid line; it should be on or very near the line marked 'Warm Full.' You should only add transmission fluid if it is low.
6. Set the dipstick to the side and pour a very small amount, about an ounce, into the dipstick tube using the plastic funnel. Wait for a couple of minutes while the fluid cycles through the system, and keep the engine running the entire time.
7. Wipe the dipstick clean and re-check the fluid level using the same procedure as before. If it is still low, add another small dose of fluid using the funnel, then give it a minute to cycle through before testing again. You must keep up this process of adding a little fluid, waiting and re-checking the level to be sure you don't overfill it.
8. Replace either the dipstick or plastic cap on the end of the transmission fluid dipstick tube when your transmission fluid level is full.