I have a 1984 F150 step side / automatic. I just had a rebuilt 1979 351W put in with a performance cam. Even with the old 302 I was having a problem shifting into high gear. When I get around 45 Mph the truck begins running a little rough. It had been doing this for a while so I installed an aftermarket tach to check things out. It continues running rough until I get around 55 Mph (approx. 3200 RPMs) and will keep running this way until I just ever so slightly remove my foot from the accelerator. At that time it will shift into high gear and run like a dream. I have found that even if I get up to around 65 or 70 Mph unless I ease off of the accelerator just for a second it will not shift into high gear. When my speed decreases after successfully shifting into high gear as described above, say from exiting off the interstate, and I get around 35 Mph I can feel a very strong "kick back" as it is shifting down. There isn't any "bad" noises that I can hear during either of these processes. ANY ideas as why this might be happening would be gratefully appreciated. The lower gears appear to be shifting with no problem.

 

My guess is that the vacuum modulator is dying, or that the engine is not making enough vacuum to operate the modulator. If the modulator doesn't see enough vacuum it thinks you are at WOT and makes the shifts at higher RPM, which is what you want at WOT.

If the modulator holds vacuum you'll need to find an adjustable one that you may be able to adjust to get it to work.

 

Is there any way to test the vacuum modulator to know for sure? Could it have anything to do with the throttle linkage from the Carb?

 

Which transmission do you have? The C6 or the AOD?

 

I have the C6. I also read last night in my Chilton's manual that it could have something to do with the belts?

 

Not belts - bands. It should tell you how to adjust them. If not, buy a Haynes. Chilton's makes a good tire chock or headrest; that's about it.

 

Thanks for the advice Steve. Does my problem sound like it could have something to do with the bands?

 

Could be, or also a vacuum leak or a bad vacuum modulator.

Adrianspeeder

 

Where exactly should the vacuum modulator be located? If I understand there should be a hose that runs from the back of the intake to the vacuum modulator?

 

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