'74 F100 390/C6 trans.
Driving to work this morning and everything going fine and all of a sudden the trans shifts into 2nd gear for no reason at 55 mph.I pulled over and checked the fluid level and it was full.drove the last 5 miles in 2nd gear at 35 mph. I have first and 2nd gear but no 3rd.I tried moving the shift selector through the gears and it works like it supposed to but no Drive.Any ideas what could have gone wrong?
I changed fluid and filter back in March and have been driving it with no problems until today.
Anything I need to look for?It has no trans fluid burning smell and not leaking any fluid.
Thanks for the help guy's-Ken

 

Other than rebuild time, check the modulator and the vacuum line that goes to it.
Check and make sure the line hasn't become disconnected from the modulator or intake manifold. Pull the line off the modulator and see if any fluid leaks out of the modulator, if it does, replace the modulator.