1975-76 280 Models

When coolant temperature is above 149°F (65°C) and transmission is in Drive, the temperature switch is closed and the EGR switchover valve is de-energized. See Figure. The EGR valve is then controlled by venturi vacuum through the vacuum booster. The EGR valve cannot be activated by the vacuum booster when transmission is in Park or Neutral, or when coolant temperature is below 149°F (65°C).