Transmission Controlled Vacuum Advance System (TCVAS) is an emission control system used starting with 1972 USA models. This system provides the vacuum advance only when the gear is in the top (4th) position, and retarded spark timing (with no vacuum advance) at other positions for complete combustion. For satisfactory performance during engine warm-up, TCVAS is disabled until the temperature warms up.
The thermo switch is located inside the passenger compartment:
Location: Under dash, right side of car
Tip: To prevent advance cut-out, leave heater off and roll down the windows.
Schematic
Both vacuum and electricity are used.