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.

[jorgensensc]

That is an absolutely perfect explanation! Thank you! Outstanding!
Now the only other question is how do we know/verify that the ECM wants to see ground to know that it can tell the Tcm to engage the TCC? How do we verify that it doesn't want to see 12 volts to know that it can engage the TCC. How do we know we don't have it backwards on the newer 6l80?

[dayj1]

Originally Posted by jorgensensc

That is an absolutely perfect explanation! Thank you! Outstanding!
Now the only other question is how do we know/verify that the ECM wants to see ground to know that it can tell the Tcm to engage the TCC? How do we verify that it doesn't want to see 12 volts to know that it can engage the TCC. How do we know we don't have it backwards on the newer 6l80?

Here's a schematic for a 2011 Escalade 6.2L with a 6L80. You can see that the brake switch is a normally open switch that connects 12 volts from fuse 43 in the underhood fuse box to pin 9 of ECM connector x1 (or pin 7 for RPO HP2). I was to lazy to look up what RPO HP2 is . So, in this instance, splice the light blue/white wire from the ECM into the white wire coming off the stock brake switch. No relay is needed and the stock 2 pin brake switch will work fine. Also, no resistor is needed as long as you don't have LED tail lights.

[BR3W CITY]

Originally Posted by dayj1

It is important to note here that 0 volts and ground are NOT the same thing.

This is something useful to keep in the back of ANYONE's mind when considering problems on electric-heavy cars. Things like electric switches, and body controls sometimes have rheostats or potentiometers with a voltage or resistance range. There is a tendancy to think that a reading of 0 means ground, when this is not true.

Just a good talking point, thanks for bringin it up.

[solidaxel]

Originally Posted by dayj1

In your case, the ECM wants 12V to ECM connector X1 (pin 9) when the brake is pressed.

The easiest way to hook everything up is to connect the light blue/white wire going to pin 9 of ECM connector X1 to the same terminal of your stock brake switch that goes to the brake lights (it's usually a white wire in a GM harness and actually goes to the turn signal switch before heading out to the brake lights).

That way you don't need an extra relay at all. You also don't need a resistor in circuit at all unless you have LED tail lights as the filament in the bulb will work as a pull-down resistor to ground the light blue/white wire when the brake pedal is not pressed.

THANK YOU, this will be very helpful on my next "adventure" into TCC and Cruise Control wiring on a 2004 4L60E, no LED's

[Hart_Rod]

Originally Posted by dayj1

Here's a schematic for a 2011 Escalade 6.2L with a 6L80. You can see that the brake switch is a normally open switch that connects 12 volts from fuse 43 in the underhood fuse box to pin 9 of ECM connector x1 (or pin 7 for RPO HP2). I was to lazy to look up what RPO HP2 is . So, in this instance, splice the light blue/white wire from the ECM into the white wire coming off the stock brake switch. No relay is needed and the stock 2 pin brake switch will work fine. Also, no resistor is needed as long as you don't have LED tail lights.

Dayj1 - thanks for the great circuit breakdown and your insight. I have a 2013 Camaro SS 6L80 and assume it is the same as the diagram you posted. I do have LED tailights and plan on using a relay and a 1K ohm/1 watt resistor for the pull down circuit. Do you see any problem with the setup? Thanks,

Rob

[dayj1]

Originally Posted by Hart_Rod

Dayj1 - thanks for the great circuit breakdown and your insight. I have a 2013 Camaro SS 6L80 and assume it is the same as the diagram you posted. I do have LED tailights and plan on using a relay and a 1K ohm/1 watt resistor for the pull down circuit. Do you see any problem with the setup? Thanks,

Rob

I think your plan is good. 1 Kohm should work fine as a pull down. The power dissipation would actually be just under 1/4 watt, so you could use a 1/2 watt resistor and still have plenty of margin for error. The formula is P=V^2/R if you want to check the math.

The schematic for the Camaro brake light control is below. It's actually quite a bit different than the Escalade diagram above. The Camaro uses a variable resistance pedal sensor instead of a conventional switch. The brake pedal sensor outputs to the BCM and the BCM "talks" to the ECM over the same pin as the Escalade (7 or 9 depending on RPO code). From there, the ECM communicates with the TCM over the CAN bus just like the Escalade.

I'm not sure why the Camaro needs a sensor to determine the exact position of the brake pedal. Maybe it's used by the traction control system? For the purpose of TCC lockup, though it looks like you just need to provide 12 volts to the ECM with the brake pedal applied and ground when not applied just like the Escalade.

I hope all that helped.

[jorgensensc]

Dayj1, can I just say that YOU DA MAN!!! Thank you so much for your help and your invaluable insight! You have cleared up what has been a very confusing issue!! Thank you again!!!
Shawn

[Houston Ben]

WOW!! Lots of good info the past day! I just got back an email from 150tunes stating all I had to do for the 2011 5.3 w/6L80 was hook the harness brake wire to the vehicle brake wire so that the harness wire receives power when the peddle is pressed, just like jday1 stated. I guess I can get rid of the relay now.

Thanks for all the help everyone, good learning lesson on this one!