GM Service Manual Online
For 1990-2009 cars only

Thermostat General

A pellet-type thermostat in the coolant outlet passage controls the flow of engine coolant in order to provide fast engine warm up and regulate the coolant temperatures. A wax pellet element in the thermostat expands when heated and contracts when cooled. The pellet is connected through a piston to a valve. When the pellet is heated, pressure is exerted against a rubber diaphragm which forces the valve to open. As the pellet cools, the contraction allows a spring to close the valve. The valve remains closed while the coolant is cold, preventing circulation of the coolant through the radiator. At this point, the coolant circulates throughout the engine in order to help warm it quickly and evenly.

As the engine warms, the pellet expands and the thermostat valve opens, permitting coolant to flow through the radiator where heat is dissipated to the air through the radiator walls and the cooling fins. This opening and closing of the thermostat permits enough coolant to enter the radiator to keep the engine within operating limits.

Thermostat Switch

This device is a normally open, temperature sensitive switch. When the ignition is ON, the toggle switch is in the AUTO position and the coolant temperature rises to a certain point, the thermostat provides a ground path which will activate the solenoid in order to engage the clutch. When the toggle switch is moved to ON, the thermostat is by-passed and the fan clutch is engaged.

Thermostat Alarmstat

This device is a normally closed, temperature sensitive switch. When the ignition is ON, the Alarmstat® provides a ground path which will activate the pressure regulator and the solenoid valve to disengage the clutch. When the coolant temperature rises to a certain point, the switch opens and breaks the ground path. At this time, the springs in the clutch engage and the fan is driven by the belt to draw air through the radiator.