Hi guys new member here - am currently in the process of a frame up restoration on my 76 f100. It has a c6 that i rebuilt with some goodies including a b&m converter with a stall speed at 2000 rpm i believe. I have enough of the truck together to drive it short distances - and it is giving me some shifting trouble. Manual 1st gear seems fine, manual 2nd is fine, but drive almost feels like a mix between 1st and 2nd. If i start moving in drive and downshift to 1st, it will rev up.. but if i only move it to 2nd the rpms will decrease. You can feel the shift each time. If i start in 2nd it is noticably more sluggish on take off, but if i throw it up to drive you feel the shift again, but it revs up like youre actually lowering the gear... not really sure where to start troubleshooting this so looking for some guidance.. it is a new rebuild so has all new fluid filters etc.

 

Bump anybody have any ideas?

 

The manual shifting sounds about right for a c6, this is how mine is except drive actually feels like drive and not a mixture of 1st and 2nd. Are you certain the fluid level is correct?

Also, have you attempted to adjust the vacuum modulator on the transmission? This can have an impact on when the transmission shifts, and how firm it shifts into each gear.

That said, it sounds like the transmission may be slipping, and you need to verify it is full of fluid and everything is working correctly with it. I'm not a transmission mechanic, but these are the few things I always check when dealing with an older vehicle.

 

I am fairly certain the level is correct. It is difficult to get an exact level but i believe it is right. Also i have tried adjusting the modulator valve. I began with backing it out 2 turns, which seemed to have no impact. Is there a recommended "factory" setting for these valves?

 

Since you have been inside it I really have no idea on what to suggest other than are you 100% sure if got back together right or maybe this info below will help.

Here is some automatic transmission troubleshooting from fellow FTE members.

Check the vacuum modulator and the line that goes to it.
Sounds like either the modulator is bad or maybe the vacuum line has come off?
Check the line for kinks or other problems, make sure it hasn't come off.
If the line checks out, take the line off where it connects to the modulator. If trans fluid leaks out, its bad. If no fluid, it still could be bad.
They're simple to replace, disconnect the vacuum line, remove one bolt that holds the clamp in place and the modulator comes out. It has an o-ring seal.

Modulators are identified by the color of the stripe. Pretty much the "go to" one is the black stripe. If they don't show that one in the catalog, go with the green stripe. Avoid the pink stripe. That one is a dual diaphragm version that has two vacuum line going to it. If by chance you have one of these, replace it with one of the other two and use the vacuum line that goes to direct manifold vacuum, cap the other line. When you pull the modulator out, be careful. There is a pin that goes in the modulator. Sometimes it comes out with the modulator, sometimes it stays in the transmission. If it comes out, don't lose it.

 

In order to firm up the shifts and get it to shift on its own, this screw needs to be adjusted inward. There is no magic number of turns as far as I'm aware of, but you should be able to find what works for you by trial and error.

 

I forgot to mention in the original post that i did put a vacuum gauge on it and had a pretty steady reading of 20" + at the line right where it goes into the modulator. Also no fluid came out either. I plan to replace it anyways before i move on to other potential causes

 

Thank you i will try adjusting the screw in once i get home. Its killing me not to be able to get it moving very far. Im currently lacking any lights, a windshield, and a bed..

 

Sound like fun to take down the road. Hope you get it figure out.