Coolant Surge Tank
WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
The coolant surge tank is made of molded plastic. It incorporates a low coolant sensor. This sensor is designed to signal low coolant level within the reservoir through a light on the dash. A magnetic float assembly moves up and down with the coolant. An electrical switch is held in the closed position by the magnetic float. When the coolant level drops below a pre-determined point, the float assembly drops and opens the switch which triggers the low coolant light. When properly filled, the coolant level on a cold engine should be level with the surge tank seam.