swamp cooler problem

I have a swamp cooler that only works on high speed, but not low speed. I checked the belt and it's has about 3/4 of inch loose which is normal. I oiled the places where your supposed to oil. The brand of the swamp cooler is Champion. It is 4 years old and is a window unit. I have fixed swamp cooler's before, but I have never seen this problem.

Dont know if this applies, but in a car, thats a dead giveway for a blower motor resistor.
Maybe it has one....

My guess is that the switch is faulty. Test the voltage at the switch with a voltmeter. Any decent hardware store would carry a two speed switch for it.

I'll test the voltage. Thanks