See Figures 1 and 2
All 1971 models equipped with automatic transmission and the 5.0L, or 6.4L engines, along with 1971 models equipped with automatic transmission and the 6.4L 2, or 7.0L engines are equipped with a Dist-O-Vac spark control system.
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 the ground. The magnet, which rotates with the speedometer cable, generates a small voltage which increases directly with speed. This voltage is directed 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 temperature, of the air. The switch is closed at 58ºF (14ºC) or lower, and open at temperatures above 58ºF (14ºC). 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. 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 58ºF (14ºC) 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 58ºF (14ºC), however, the thermal switch circuit is open, allowing the circuit from the speed sensor to take over and 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 of the carburetor spark port (small hose) and the primary (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 airflow in the direction of the carburetor. The air bleed purges the hose of vacuum, thus assuring that raw gasoline will not be drawn through the hose and into the distributor diaphragm.
When the thermal switch is closed, air temperature 58ºF (14ºC) or lower, or when it is open and the speed sensor is not sending out a strong enough voltage signal, usually at speeds below approximately 35 mph (56 km), 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, no vacuum is being supplied to the primary diaphragm on the distributor, and, therefore, no vacuum advance.
When the air temperature is above 58ºF (14ºC) and/or the speed of the car is sufficient to generate the required voltage of 35 mph (56 km) 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.