See Figures 1, 2 and 3
In 1970, the Distributor Modulator System-also known as Dist-O-Vac-was first used as a further extension of the IMCO system of emissions control. This system supersedes the distributor vacuum advance control valve, but retains the temperature-sensing valve to prevent overheating. The temperature-sensing valve-when used with the Distributor Modulator System is also called a Ported Vacuum Switch (PVS).
The three components of the Dist-O-Vac system are the speed sensor, the thermal switch, and the electronic control module. The electronic control module consists of two sub-assemblies; the electronic control amplifier and the three-way solenoid valve.
The speed sensor, a small unit mounted in the speedometer cable contains a rotating magnet and a stationary winding which is insulated from ground. The magnet, which rotates with the speedometer cable, generates a small amount of voltage which increases in direct proportion to speed. This voltage is generated to the electronic control amplifier.
The thermal switch consists of a bimetallic-element switch which is mounted in the right door pillar and senses the temperatures above 58°F. This switch is also closed at 58°F or lower, and open at temperatures above 58°F. This switch is also connected to the electronic control amplifier.
Within the electronic control module case, there is a printed circuit board and an electronic amplifier. As previously mentioned, the speed sensor and thermal switch are connected to this assembly. The thermal switch is the dominant circuit. When the temperature of the outside air is 58° or lower, the circuit is closed, so that regardless of speed, the electronic control amplifier will not trigger the three-way solenoid valve. At temperatures above 58°, the thermal switch circuit is open, allowing the circuit from the speed sensor to control the action of the solenoid valve.
The three-way solenoid valve is located within the electronic control module and below the printed circuit board of the amplifier. It is vented to the atmosphere at the top and connected at the bottom to the carburetor spark port (small hose) and the primary-or advance-side of the dual-diaphragm distributor (large hose).The large hose is also channeled through the temperature-sensing valve. The small hose is equipped with an air bleed to provide a positive air flow in the direction of the carburetor. The air bleed purges the hose of vacuum, thus assuring that gasoline will not be drawn through the hose and into the distributor diaphragm.
When the thermal switch is closed (outside air temperature of 58° or lower), or when it is open and the speed sensor is not sending out a strong enough voltage signal (speeds below approximately 35 mph), the amplifier will not activate the solenoid valve and the valve is in the closed position, blocking the passage of air from the small tube through the large tube. With the valve in this position, the larger hose is vented to the atmosphere through the top opening in the three-way valve assembly. Consequently, there is no vacuum being supplied to the primary diaphragm on the distributor-hence no vacuum advance.
When the outside air temperature is above 58° and/or the speed of the car is sufficient to generate the required voltage (35 mph or faster), the valve opens, blocking the vent to the atmosphere while opening the vacuum line from the carburetor spark port to the primary diaphragm of the distributor.