i got a 77 f150 with a c6 automatic and according to the guy i bought it from it shifted fine and all the gears worked. but it sat for a few months and now it has no drive. reverse and 1st an2 second gear work good and dont slip at all but if you put it in drive and take off it will stay in first and not shift , if you hold it to the floor it will attemp to shift but it seems to hit neutral for a second until your foot comes off the pedal and then it goes right back into 1st gear. the only way to get second is to manually put the lever into 2nd gear. and when you do that 2nd seems good and strong. i was going to pull it but i figured id ask first and see if anyone has any suggestions in case its an easy fix. and yes the fluid is full and not terribly burnt. its a little dirty but should not be enough to cause an issue. the vacuum modulator is connected and is not leaking but i never pulled it out. and i checked the kickdown rod to make sure it is returning all the way and not sticking.

 

I too would like to know if this is just a vacuum problem or modulator. I'm supposed to look at a 78 that shifts in 1 and 2 just not drive. I'm thinking its just a vacuum leak. I hope....

 

I typed up a whole response to this last night, and when I submitted it, got a FTE database error, and forum was down, lost it all.

A short recap - Be absolutely sure that you have good vacuum down to the modulator, and that the modulator has the pin in it that connects it to the actual throttle valve inside the trans case, and that the diaphragm is good.

If that all checks out OK, it starts to get uglier. Like maybe a stuck governor. The governor output is needed for 1-2 and 2-3 upshifts and downshifts in Drive. A moving governor can be checked by pulling the valve body out, and applying approx. 25 PSI compressed air to the governor passage in the case. A working governor will make a rather rude sound
The governor itself is on the output shaft inside the extension housing. Not too terribly bad to get to on a 2WD, on a 4WD... yuck!

And beyond that, it gets a lot uglier. Like a sticking/plugged or whatever shift valves in the valve body. Touching a valve body must be done with great care. Lose a ball or a spring, or not know where to put one, or overtighten mounting screws etc. will screw it up royal. It really is an operation. Need clean area, need a place to carefully lay each spool and springs and ***** out in order and direction for each valve, etc. A piece of white cardboard at least 8 1/2 x 11" that is fan-folded back and forth to make many grooves to hold spool assemblies will keep things in order, if the cardboard is labelled first for each valve assembly. And each part needs to be lubed with ATF before reassembly. Cleaning all the valve body passages out with solvent, and then cleaning the solvent out completely is a must.
My personal opinion... is that the majority of people working on cars and trucks should not get down to this level, to avoid creating a mess. The chances for error are high.

 

iv re-built a couple of c6 trannys but its been about 15 yrs since i been in one. thinking i will order a rebuilt kit when it warms up.