Ford Mustang/Capri 1979-1988 Repair Guide

Dual Signal Spark Advance (DSSA) System



See Figure 1

The 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 port vacuum to the distributor advance diaphragm during mild acceleration. During steady speed or cruise conditions, EGR port vacuum is applied to the EGR valve, while spark port vacuum is applied to the distributor advance diaphragm.

Click image to see an enlarged view

Fig. Fig. 1: DSSA system schematic


Since the DSSA System is essentially the standard spark control system with an added spark delay valve and a vacuum check valve, component testing is done in the same manner as on other emission control systems. If the system is malfunctioning, simply isolate and test each specific component. Be sure to first check the vacuum lines for any signs of leaking or deteriorated hoses, and replace as necessary.

Spark Delay Valve
  1. Disconnect the vacuum lines which run to the spark delay valve and connect a hand vacuum pump.
  3. Operate the pump and note if vacuum can be achieved.
    1. A valve with one black or white side and one colored side is good if vacuum can be built up in one direction, but not in the other direction, and if the vacuum slowly decreases.
    3. A valve with both sides the same color is good if vacuum can be built up in both directions before visibly decreasing.


Vacuum Check Valve

  1. Connect a hand vacuum pump to the vacuum (black) side of the valve.
  3. Apply 16 in. Hg (54 kpa) of vacuum.
  5. If vacuum remains above 15 in. Hg (50.7 kpa) for 10 seconds, the valve is acceptable.