just installed a transmission out of a wrecked truck in my 2000 chevy z71,it does not go in first gear at all it takes off in 2nd never goes to first,when driving down the road it shifts through the gears as long as you manually shift it but never runs in first at all. when running down highway in overdrive and gas on it it downshifts fine by itself and then shifts back to overdrive. does anyone know what could be causing this? thanks