Marvin if you find out whats wrong with your tranny please let me know. I have the same identical problem with a high mileage 1985 500 SEC. I was driving on the highway and the temp gauge started going up as if it was going to overheat. I pulled over and checked the fluids. They looked ok but when I tried to drive off the tranny refused to shift from 1st gear to any higher gears. I had to get it towed. Had the fluids and filter/screen changed and still it wont upshift!!!