The TCS system alters the distributor advance curve under certain operating conditions, thereby reducing emissions of oxide of nitrogen (NOx).
When the system is operational, the computer closes the vacuum switching valve (VSV) ground circuit. The valve in the VSV closes the passage between the distributor advance side and the carburetor advancer port. In this manner, advance of the engine timing is stopped.
The computer receives its messages from the speed sensor unit and the thermal sensor which only operates when the coolant temperature is between 140-210°F. 1975 and 1976 trucks use a TVSV (thermostatic vacuum switching valve) in addition to the VSV, computer, and speed sensor, to provide normal vacuum advance at engine temperatures below 122°F.