I have an 88 f350 7.5 / c6 vac controlled. It was shifting very late so I replace the vac modulator with a new black marked one. I just went for a test drive and noticed that it was shifting at about 2800 to 3000 rpm while I was driving VERY light.

It was also draging the engine when I get off the gas. The rpm went down very slow and the truck was slowing down as if it was a manual and in gear.

I tried puting it into N and that dropped the RPM, but back into D and the rpms jumped back up.

I was expecting the shift point to be lower, I guess I could adjust the modulator or the bands?

what about the tranny not letting the engine go back to idle when I get off the gas? is that the way it's supposed to work ??

thanks, KarlJay.

 

I have an 89 f250 with 460 and c6 and it shifts really nice going up. It shifts into 2nd at like 15 mph and 3rd at around 35-38 mph and will upshift earlier if I feather the gas and let up on the gas a bit. I don't have a tach but I know I am no where near those rpms. It also idles right down when I get off the gas to coast. There should be a screw adjustment in the modulator. Also check to make sure you have good vacuum.