Learning how to add transmission fluid to your car or truck is not all that hard of a DIY task. Some vehicle manufacturers require the engine to be running to check the transmission fluid. Check your vehicle owner's manual to be sure. You'll need to check what your transmission fluid level is on the dipstick before adding transmission fluid to the transmission. Automatic tranmission fluid (ATF) is added through the dipstick, usually located at the rear of the engine next to the firewall. Remove the dipstick and place a long funnel in the dipstick hole. Slowly add the correct transmission fluid through the funnel a little at a time until you feel that the transmission fluid is at the correct level. Check your transmission fluid level on the dipstick to make sure your fluid is at the correct level as noted on the dipstick. Now that you know how to add transmission fluid, you can move on to some of our other DIY transmission guides below.