GM Full-Size Trucks 1970-1979 Repair Guide

Trapped Vacuum Spark


This system is used to prevent a drop in vacuum to the distributor vacuum advance during cold engine operation, when the engine is accelerating. A thermal vacuum switch (TVS) is used to sense engine coolant temperature. A check valve is installed in the vacuum line to the distributor. The other side of the check valve has two connections: one to manifold vacuum (at the carburetor base), and the other to the thermal vacuum switch.

When the engine is cold, the TVSA vacuum ports are closed. Manifold vacuum is routed through the check valve to the distributor. The check valve keeps the vacuum to the distributor at a high vacuum level, so that when the engine is accelerated, the vacuum to the distributor does not drop. This results in a constant spark advance.

When the engine temperature reaches a predetermined value, the TVS ports open to allow manifold vacuum to the distributor, and the check valve operates only as a connector.