Microwave Fan Runs at HIGH when in cook

Manual states that the fan should run at LOW when the microwave is in use and should switch to HIGH only when the temperature around rises to a certain level. Is this fan a cooling fan separate from the VENT FAN? The Vent Fan runs normal at LOW and HIGH with the button on the touch pad. Which component could be defective? How can I get hold of a schematic/wiring diagram?