Speed Controlled Spark: Operation: Notes

Distributor vacuum advance is controlled by solenoid vacuum valve which is, in turn, controlled by speed sensor in speedometer cable. On some vehicles with model T-407 transmission, speed sensor unit is mounted on transmission. At vehicle speeds below vacuum advance cut-in speed (34±2 MPH), speed sensor switch is closed, allowing electrical current from ignition switch to activate solenoid and open valve. Air then passes from air cleaner through valve into intake manifold. Flow of air bleeds (weakens) vacuum in distributor vacuum advance unit, preventing vacuum advance ignition timing. As vehicle accelerates past cut-in speed, speed sensor switch opens. This shuts off electrical current to solenoid valve and allows valve to close, stopping flow of bleed air. This permits manifold vacuum to advance ignition in normal manner. When vehicle decelerates through cut-in speed 25±3 MPH, speed sensor switch closes. This activates solenoid, opening valve to bleed vacuum from distributor and cancels vacuum advance.