I'm not familiar with these 2-speed motors at all. But, that 'covered bump' on top that you refer to is just the starting capacitor. Looking at the external wiring on the B979 drawing from here
http://www.aosmithmotors.com/calcula...ult.aspx?id=57
it looks like you connect to either the high or low speed windings via an external single pole double throw switch. Could be that switch is bad in the high speed position OR something else is open in the high speed circuit. Does it make any sound at all when in the high speed mode?
Al