my 420 has been driving superbly and no indication of any tranny problems .
Without warning ,2 days ago ,it would not go into reverse ,so i put it in to N and flinstoned it in order to get home .
It drives just fine going forward .
Next morning ,reverse is now N gear and going forward it whines and kinda grinds
and does appear to be going in to top gear ...I did not want to force it by going at 60mph .

I am not a mechanic ,but it does appear as if it is serious .

I am always reticent about going to a dealer with such a problem so if have some idea as to what it might be i may be able to get a savvy mechanic to fix it