Shift Resistance

The $300 shift box and cupholder is just $100 if you skip the cupholders.

You get an on-off switch and two momentary switches.

If you had the 9C3 RPO, you wouldn't need this box because the shifter has it from the factory.

To the right of 'D' is a detent you can move the shifter into and that puts the trans in "sport mode".  Pushing the shifter forwards and back manually shifts the trans up and down...

No.  Not quite.

Pushing the shifter forward and back sends a request to the Body Control Module (BCM) to send a command to the Transmission Control Module (TCM) to shift up or down.

One pin from the BCM sends 12v up to the shifter and another receives a voltage signal back to a different pin.

13 to 11v and the TCM runs the transmission automatically.

3.1 to 2.7v and it's in sport/manual mode.

2.3 to 1.9v is a tap-up signal.

1.56 to 1.15v is a tap-down signal.

We've seen this paradigm of doing things before in how the cruise control operates.  Steering wheel controls as well.

Pushing the switch routes the current through a resistor and the voltage drop tells the controller what you asked for.

Marv's calculating the resistance values the box is probably using.

I would not be shocked to find that GM uses the same values on all of its cars.

The shift box taps into the 12v wire that's already on my BCM and adds a wire to an empty pin; plus a separate ground that's not present in the factory diagram.  The factory circuit grounds through a resistor in the BCM on the same pin the shift box is adding a wire to.

Because Harvey wants cupholders, we're going to end up with the $300 box.  But we're going to be measuring resistances and voltage drops when it gets here!