See Figure 1
The Dual Signal Spark Spark Advance (DSSA) system is used on many engines. It incorporates a Spark Delay Valve (SDV) and a one-way check valve to provide improved spark and EGR function during mild acceleration.
The check valve prevents spark port vacuum from reaching the EGR valve and causing excessive EGR valve flow. It also prevents EGR port vacuum, which could result in improper spark advance due to a weakened signal. The SDV permits application of full EGR vacuum to the distributor vacuum advance diaphragm during mild acceleration. During steady speed or cruise conditions, EGR port vacuum is applied to the EGR valve and spark port vacuum is applied to the distributor vacuum advance diaphragm.