It's a 79 f-100 w/460 and c6 tranny. I just rebuilt the motor it runs great. The problem is the transmission. The truck probably sat for about 5 years before I bought it. The reason it was sitting was because the no. 8 connecting rod bent. When I bought the truck the guy said that the only thing wrong with it was the motor and that the transmission shifted great. The truck has only rolled over one time and it now registers 45,000 which means 145,000 miles. Now that I have the motor back in the truck It won't shift. By saying that I mean it doesn't shift automatically. If I put it in reverse it feels like a real positive engagement. But when I put it in drive it feels sluggish and I can barely feel it engage. When I'm driving it down the road it wont shift out of first gear. I can manually shift from first to second, but when it try to put it in d next it just shifts back into first. And also when I'm in D going down the road if I let off of the gas and put my foot back on the gas it feels like it's slipping and then it will pick back up in first gear. And all of this happen's whether the motor it is cold of hot, it doesn't matter. Things that I have done to the transmission: changed fluid and filter, fluid it dextron 3 and is at the correct level, I also changed the modulator valve with the purple ring around it, the old one had the purple ring around it. I just attempted a band adjustment today on the one on the drivers side. I'm not sure that I did it correctly (loosen nut, tighten 10 ft/lbs back off 1.5 turns and retighten nut) but it didn't seem to change anything. I know this is long and I'm sorry but I just feel like I need to list everything I can about the problem and maybe someone that has had a similar problem can enlighten me a little. Thanks in advance.

 

Are you sure your vacuum line is hooked up to the modulator and that it isn't pinched or have a hole in it somewhere?

 

yes I'm positive, I can take the line off at the modulator while the truck is running and it has great vacuum.

 

It will take a tranny shop to measure pressures and tell, but you may have a bad regulator or a valve body plugged up with crud.

Sounds like rebuild time.
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