See Figure 1
All 1975-78 models using the 7.5L engine are equipped with the Cold Start Spark Advance (CSSA) system. It is a modification of the existing spark control system to aid in cold start driveability. The system uses a coolant temperature sensing vacuum switch located on the thermostat housing. When the engine is cold, below 125°F (52°C), it permits full manifold vacuum to the distributor advance diaphragm. After the engine warms up, normal spark control (retard) resumes.