SUCCESS!
I got it to work by grounding the TCC ENABLE wire through the switch and left the PWM wire cut. I found this article that clued me in to the fact that the solenoid gets grounded to apply the TCC.
http://www.motor.com/article_pdf_dow...rticle_ID=1039
So I now have a toggle switch next to the shifter which simply grounds the TCC ENABLE wire from the tranny to the engine block. I guess this wouldn't have been such a problem had I known that the solenoid had to be grounded instead of energized through that wire.
I can now definitely see how the auto tranny likes to upshift early, which usually results in rumbling, which I'm guessing is the TCC slipping due to low hydraulic fluid pressure. Strangely enough, I can flip the switch on at a stop with the vehicle in gear and it doesn't stall, in fact I don't think it even tries to engage. I'm guessing there must be a pressure switch or check valve somewhere in there that prevents it from engaging at low pressure. Now I just have to re-train myself how to drive this thing.
