Gregory Cooper

I have a 1988 GMC G3500 Vandura Box truck with 350 5.7L TBI. I am renovating it to be an RV. bought new motor and had a mechanic I know and trust do that and a bunch of other stuff. It drives great. Runs great and shifts great; EXCEPT when I need to down shift to pass another vehicle or going up a mountain here in West Virginia (and there are allot of them, lol). My son is a mechanic but in Alabama. I texted back and forth and he mentioned the things I put as tags. TPS, modulator and adjusting the TV cable. I have the service manual and two other manuals. Anyone else have the same or similar problems? I downshift by using the shifter on the column. any help is greatly appreciated. email is: [email protected].