Distributor Modulator: Operation

Current is supplied to the control module when the ignition is turned on. The current then triggers the solenoid valve inside the control module to shut off carburetor vacuum to the distributor vacuum advance unit. As vehicle speed increases, the speed sensor, in line with the speedometer cable, builds up a voltage frequency proportional to vehicle speed. This voltage is sent to the amplifier inside the control module. The solenoid valve is then triggered and it opens, allowing vacuum advance at about 23 miles per hour. As the vehicle decelerates and the speed drops to about 18 miles per hour, the reduced voltage from the speed sensor causes the solenoid to close. Now no vacuum is applied to the distributor vacuum advance unit.