Some use seperate circuits for the low and high. On low it runs through a resistor which may be fused. If you only have high, the low circuit or resistor is not getting current. Check fuses. Check continuity in resistor. Some have a thermal protection internally that quits. If resistor is bad motor may be going causing high current. Check amperage draw at motor on high. Typically 50% of fuse rating. A slow motor raises current.If motor is binding more current has been going through low circuit. Make sense?