swamp cooler problem

[glock74]

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.