For some reason my transmission shifts real hard it sorta knocks when I put it into reverse and when it shifts to thir. What could be the problem, could it be because my car idols so high?
Even with a high idle you shouldn't get a knock or clunk on shifting. Check your tranny fluid level and top off if it's low.
If it's not, you could try getting a fluid flush - it might give you a lot more miles, or it might kill your tranny dead within a few hundred miles (in some cases flushing a neglected transmission can do more harm than good).
Your high idle is another problem - that car has fuel injection, so it's probably a bad sensor someplace, or a bad idle valve (if that car has one). That would be worth checking out if you have the time and money - or if you have time and no money try finding a Mercedes owner forum and reading old posts and maybe asking for guidance - the amount of knowledge on automotive forums is tremendous, and if you find a friendly group they can help you diagnose your own problems.
with automatic transmissions the problem could be fixed by the electronicson the transmission being needing a calibration. the belts could need tighting inside the transmission. the transmission has small internal ports that may be blocked not allowing the correct flow of transmission fluid. or the gears could be damaged so they do not mesh correctly not shifting properly. the transmissions internal clutches maybe going too.
if the car does not up shift it will have to rev higher to go the same speed.