Ford Vans 1961-1988 Repair Guides

Distributor Modulator System (Dist-O-Vac)



This system is found on 1970-71 models equipped with the 6-240 (3.9L) and automatic transmission.

The system is used in conjunction with the IMCO system components with the exception of the deceleration valve.

The three components of the system are: the speed sensor, the thermal switch and the electronic control module.

The control module consists of two sub-assemblies: the electronic control amplifier and the 3-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 voltage which increases as the speed increases. The voltage is directed to the electronic control amplifier.

The thermal switch consists of a bi-metallic element switch which is mounted in the right door pillar and senses the outside air temperature. The switch is closed at 58°F or lower. The switch feeds the electronic control amplifier.

Within the electronic control module case is a printed circuit board and electronic amplifier. The speed sensor and thermal switch is the dominant circuit. When the temperature of the outside air is 58°F or lower, the circuit is closed, so that regardless of speed, the electronic amplifier will not trigger the 3-way solenoid valve. Above 58°F, 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 3-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 primary side of the distributor (large hose). The large hose is 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.

When the thermal switch is closed, or when it is open and the speed sensor is not sending a strong enough voltage signal (road speed below 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 large hose is vented to the atmosphere through the top opening in the 3-way valve. Consequently, no vacuum is being supplied to the primary diaphragm on the distributor, and therefore, no vacuum advance.

When the air temperature is above 58°F and.or the road speed is sufficient to generate the required voltage (35 mph or greater), the valve opens, blocking the vent and opening the vacuum line from the carburetor spark port to the distributor primary port.