Does anybody know what the factory ECU in a 2001 silverado uses to make the RPM output? I have a Wiring diagram but can't figure out the inputs the stock computer needs to generate its output signal.

I am splicing in the factory ECU with my Big Stuff 3 wiring to mantain the factory computer control of the trans, and also still have the factory Tach working. I just need to know what wires in the factory harnes to tap into the big stuff harness to get that signal.