The early ones had a vacuum-actuated switch, mounted on the firewall, and a governor pressure switch on the valve body, wired in series. It took a combination of moderate to high manifold vacuum and enough governor pressure(speed) to close both switches. Either stepping down on the gas, or losing speed would disengage the lockup clutch. Later ones used a signal from the powertrain computer. Governor pressure corresponded roughly to MPH, 45 PSI=45 MPH, etc. We used to change pressure switches to keep from dropping in and out of lockup because of speed limits, etc.
Jerry
