Ok, there's three wires going to the 700R4 trans. the outside two goto 12v and ground. whats the third wire for?

 

Light blue (usually) 4th gear

 

The purple goes via an extra brake switch and then to 12V. Internally it goes to the lockup solenoid. It loses power when you touch the brake pedal to ensure the engine doesn't stall upon panic braking (wheel lock).
The dark-green/white goes to the ECM and is grounded by a switch in the tranny as long as it's not in 4th gear.
The tan/black goes to the ECM where it's grounded when it commands lockup. This is the other side of the lockup solenoid in the tranny.

 

Gonna threadjack for a sec, sorry. I'm installing a megasquirt system in my car and I want it to control the fourth gear lockup. So theoretically, I should be able to route the tan/black through a relay controlled by the dark green/white, correct? That should effectively stop the ability to lock up the tranny unless it's in fourth gear. Then I just have to figure out the megasquirt part, but this isn't the best forum for that.

thanks!

M