I have a '78 F250 4x4 with the 400 engine and C6 automatic tranny in it. All of the sudden, it won't shift as I'm driving. It will go into reverse, drive, 2, 3, when I'm shifting, stays in the same gear all the time while I'm driving.

I was looking around and I found a hose that I think goes from the top of the tranny to...nothing. A friend said it's a vacuum hose that should go from the shift modulator to the carb. Well, I can't figure out where it should connect to the carburetor. There is a hose towards the front of the carb that was cut off and is plugged with a bolt, but that's been like that since I bought the truck 3 years ago. So if that's where the hose SHOULD go, then something else must have caused the tranny to give up on me.

Any suggestions? Tranny has been acting fine until now. And I'm sure that hose should go SOMEWHERE!

 

Look down near a rear corner of the trans pan. You should see the modulator hanging on there, attached to the case. It has a nipple for a hose on it. Follow that hose forward. It may/will become a steel line for a bit, before it probably becomes a rubber hose again under the hood.

If you find the modulator vacuum line, and it is indeed still hooked up, then the other hose you found might well be the trans case vent. Years ago on C6 cars, the trans case vent was a little doohickey up on top of the case, to let air in/out, but not let water spray in. On trucks, it was common to remote the vent tube opening a lot higher up, so fording some water would not submerge the vent. Water getting into a auto trans is bad news, as can be expected. Yet, provisions must be made for ATF expansion and air expansion over temperature, to avoid stressing/blowing trans seals, therefore the vent.

Time to get back under the truck! Hey, at least it's shady under there