There are actually four modes of operation- - - - -depending on whether the trans is in 3rd. or 4th. gear, and converter locked or unlocked. 3rd. is direct drive, same as on a Turbo 350 or 400, for instance. The torque converter clutch can be locked or unlocked here, depending on intake manifold vacuum and road speed. 4th. gear is overdrive, and again, the converter lockup can be engaged or disengaged.
Getting the converter locked in- - - -it's best to go with OEM parts, a firewall-mounted vacuum switch, and a pressure switch in the valve body that completes the electrical circuit to the converter clutch relay. There's also a brake pedal switch that drops the converter out of lockup when the brake is applied. Early 700's had a bad habit of keeping the converter clutch applied as the vehicle came to a stop, stalling the engine just like failing to disengage a manual clutch would cause. The normally-closed brake switch was added to positively drop the apply voltage off the lockup solenoid during a stop. Throttle pressure adjustments are critical, but the lockup system can be used or not, without damage to the transmission. It does result in slightly lower engine RPM anytime it's engaged by eliminating the built-in slip in the torque converter.
Jerry
