[Jason7121]

When the tcm doesn't see speed signal often it will default to third and reverse only. The ecm and tcm share data for speed signal via data communication high speed gm lan. I wonder if ecm and tcm are programmed as a pair? Weird there are no codes associated with vss. You could measure resistance across vss and see what it reads then plug connector back in and measure resistance at other end at tcm and resistance should be about the same if it is then you know vss wiring and sensor resistance is good all the way to tcm. Hope that may help you Vic.