Improved Combustion System (With Transmission Controlled Spark): Operation

When vehicle speed exceeds 11 MPH, switching valve is actuated by speed marker and allows atmospheric pressure to throttle positioner diaphragm. As accelerator is depressed, throttle positioner is pulled away from diaphragm by a return spring. Its lower end moves downward to a set position. When accelerator is let off, throttle valve is held open by throttle positioner at an angle slightly larger than that of idle speed.
When speed decreases below 11 MPH, circuit to switching valve is open. This movement changes passage of vacuum and pressure, intake manifold vacuum acts on positioner diaphragm through switching valve. Throttle positioner is pulled closer to diaphragm unit and permits throttle valve to close to idle speed position.
T.C.S. system delays ignition timing when vehicle speed drops below 35 MPH (4th gear engaged on 8R-C with manual transmission) and coolant temperature is between 140° and 203°F. When decelerating, however, T.C.S. will not operate until vehicle speed decreases to 13 MPH (8R-C with automatic transmission), or 30 MPH (2T-C engine).

Fig 1: Shift Point Sensor
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
T.C.S. system does not operate at speeds above 35 MPH (automatic transmission or when 4th gear is engaged on 8R-C with manual transmission). At coolant temperature below 140°F or above 203°F, T.C.S. system does not operate.
When T.C.S. is in off state, vacuum at carburetor advance post permits vacuum advance. Mixture control valve (2T-C engine) prevents mixture from becoming too rich upon decelerating from speeds above 35 MPH.