Furnace problem run only on low speed

[patbucks]

Hi,
I have a electric furnace problem.

The furnace is heating the element are turning on when I put the temperature up on the thermostat but the fan won't start blowing by it self.
There is a switch to make it blow on high speed, that will not work, there is an other switch to make it blow at low speed and that works perfectly.
I Have no idea has of what piece could be defective.
Please someone help.