Distributor Thermal Vacuum Switch (DTVS)

Engines With Ported Spark
This switch is used to advance the ignition timing when engine coolant temperature is high. At temperatures above the calibration value, a valve within the TVS moves, and if not equipped with a Vacuum Reducer Valve (VRV), a full manifold vacuum is directed to the distributor vacuum advance. On engines with a VRV, the vacuum will be 1 1/2 inches less than manifold vacuum. The timing is then advanced allowing the engine to run cooler. This is the only time when vacuum is directed to the vacuum advance at idle.

Engines With Full Manifold Vacuum
Some engines use a DTVS to allow increased ignition timing at normal operating temperature. At temperatures above the calibration value, the DTVS switches the vacuum supplied to the distributor from manifold vacuum to vacuum which has been reduced by 1 1/2 inches to prevent detonation at higher operating temperatures.