TRAPPED VACUUM SPARK
Thermal vacuum switch (TVS) is mounted in intake manifold and is used to sense engine coolant temperature. Vacuum check valve is mounted between manifold vacuum, distributor and thermal vacuum switch. System maintains high vacuum levels to distributor during cold engine operation and cold engine acceleration.
SPARK VACUUM DELAY
Spark vacuum delay is used on 5.7L V8 engines with Heavy Duty emissions. Spark vacuum delay is installed between TVS check valve and distributor.
When engine temperature is below pre-set specified value, manifold vacuum signal is routed through check valve to distributor. Ports on TVS are blocked. Check valve will keep distributor vacuum at levels higher than manifold depression during vehicle acceleration.
Small sintered iron bleed orifice is provided in check valve to allow for leak-down to enable engine to be restarted if it stalls. This applies to all models except Light Duty California and High Altitude emissions, 5.7L V8 with Heavy Duty emissions, and all 7.4L V8 engines.
When engine temperature is above pre-set value, TVS ports will be open to allow manifold vacuum to distributor. During this mode of operation, check valve will act as a connector.
If manifold vacuum increases, check valve opens and allows distributor vacuum to increase to same level. When vacuum decreases during vehicle acceleration, check valve closes and distributor vacuum will decrease at a rate controlled by internal bleed.
COMPONENT TESTING
Note. Testing information not available.