1977-78 Models

When transmission is placed in high gear, the top gear detecting switch allows electrical current to flow to vacuum switching valve. This valve closes and cuts off outside air to distributor, permitting vacuum advance using intake manifold vacuum.

In all other gear positions, current is interrupted by top gear detecting switch. This opens the vacuum switching valve which, in turn, allows atmospheric pressure into the vacuum control unit of the distributor. With pressure equalized (no vacuum present), the advance mechanism cannot operate and spark timing cannot advance.