is it neccesary to change the transmission fluid on any scheduled basis? or change if fluid becomes smelly or discolored only?
Your owners manual should tell you. Many manufactures recommend every 30k miles. Some indication of a tranny change is the color of the fluid, pink is generally o.k.,but brown and smelly is bad.
On automatics, drop pan, change filter and refill every 25k.
About every 30K is recommended for most vehicles. Filter kits come with new pan gaskets. Be sure to use it! Also ensure that you use new fluid of the proper type recommended by the manufacturer. On an unfamiliar vehicle, I drain the fluid into a container that I can monitor the amount of old fluid. I also remove the pan gasket and filter. Pay attention as to how the old filter comes out. The new one has to go in the same way. Some filters are just press-fit on–others have a screw near the center of them. Check it out first instead of just trying to rip the old filter off its tube. Take the old cleaned gasket and filter with you. (Counter people don’t like old, drippy fluids all over their counters!). Some replacement filters are not EXACTLY the same as the original, but your parts persons will check their books for proper replacement filters. You need to ensure that any little curves or ‘dents’ in the new filter match up to the same curves/dents in the old filter. Also carefully check the connector off the old filter to ensure that the new filter is spaced the same. A different type of material (plastic vs. metal, for instance), used as a spacer or insert to connect the filter onto the trans. tube does not necessarily negate the new filter as being appropriate. Again, your parts person can verify that he/she is selling you the correct replacement. Whatever you measured as the drained fluid, buy one extra quart of fluid. You’ll use it somewhere down the line. When done, ensure that the hehicle is on a flat surface. Start 'er up, let it get to operating temp., then check the fluid level in whatever gear the manufacturer says. Remember that it’s always easier to add fluid than to drain fluid. DO NOT OVERFILL! Some transmission dip sticks have the type of fluid recommended stamped on them. But your parts person also has this info. “Oil is cheaper than metal. Preventive maintenance is always cheaper than repair maintenance”. (Just something that was planted in my head 50+ years ago).