Transmission Controlled Spark: Operation

When transmission is shifted into 4th gear, the top gear switch on the transmission sends an electrical signal to the electromagnetic switching valve. This signal activates the switching valve, closing the vent passage and allowing full vacuum to the advance side of the distributor vacuum unit. In all other gear positions, the switching valve interrupts the vacuum normally directed to the vacuum advance unit and, at the same time, opens the vent passage so that vacuum can be released.

On 320, a thermo valve is added to this system to cut-off vacuum to the switching valve until engine coolant temperature reaches approximately 91°F (33°C).