Vacuum Advance System
When engine coolant temperature is less than 113°F (45°C), No. 1 and No. 2 outlets of the thermal switch are connected. Intake manifold vacuum then flows to the distributor vacuum advance unit. Applying vacuum advance helps cold engine driveability and cold engine emissions.
After coolant reaches 113°F (45°C), No. 2 and No. 3 outlets of the thermal switch are connected. Vacuum then flows through the solenoid valve (vacuum control valve on California models), to the advance mechanism, according to transmission gear position. Only when 4th gear is selected, will the transmission switch activate the solenoid to apply vacuum to the vacuum advance unit.
On California models, a Blue and White delay valve, slowly releases vacuum inside vacuum advance unit. Vacuum between distributor and thermal switch goes to atmosphere through a calibrated orifice. See Fig 1.