Gear ratio question

[dayj1]

Originally Posted by jorgensensc

This sounds great, but please help me to understand. Why would not having any voltage ( or possibly a ground through the lights) tell the ECM to tell the Tcm to flip the ground in the Tcm to engage the TCC solenoid?
Conversely why would giving the ECM 12 volts while braking tell the ECM to disengage the TCC?
Not trying to be difficult. I totally understand why it worked that way on the older 6l80, but the newer ones communicate differently with the Tcm.
Are most of the signals the ECM receives grounds or voltage? Why does the ECM need both voltage and a ground to send a signal to the Tcm via canbus?it seems like you would just need one or the other.
Can anyone elaborate any more?
Shawn

In the digital world, things are either "on" or "off", "high" or "low", zero or one, or, in the case of the ECM, 12V or ground. It is important to note here that 0 volts and ground are NOT the same thing. For example, a voltage meter with one lead on the "+" battery post and one lead on the orange wire at the brake switch will read 0 volts, but neither one of them is at ground potential.

If an input (such as the brake signal input) is left disconnected, it is said to be "floating" and is in an undefined state that is neither high nor low.

The brake switch only has two states, it's either on or off. For the ECM to verify that the switch is working, it needs to see a change between the two states (remember that a floating input is not a valid state). So, the ECM can not verify that the switch is working unless it "sees" the state flip from ground to 12V (low to high).

Now, you may be saying that's good and all, but why does the ECM care that the switch is working? The answer is simple. It's a safety measure. While your life doesn't depend on your torque converter locking up, The Gen IV ECM uses the same switch for turning off cruise control. It's pretty important to verify that the brake switch is working in both the "on" and "off" states before locking in that fancy DBW throttle body at 75 MPH on the freeway.

I hope that helps. Let me know if it still doesn't make sense.