See Figure 1
The Transmission-Regulated Spark Control System differs from the Dist-O-Vac and Electronic Spark Control Systems in that the speed sensor and amplifier are replaced by a valve located on the transmission. On manual transmissions, the valve is activated by a mechanical linkage which opens the switch when the transmission is shifted into high gear. On automatic transmissions, the valve is opened by fluid pressure when the vehicle is in high range or reverse. Either valve, when opened, causes the opening of the vacuum lines to the distributor, thus providing vacuum advance.
The PVS (ported vacuum switch) may be used with either spark control system. For 1972, this switch has been modified to react to coolant temperatures of 230°F rather than 220°F for pre-1972 cars. Temperature sensors for 1972 feature a critical temperature range of 49-65°F instead of the 50-58°F range of earlier models.