That's unusual. Normally the 3 lower speeds fail and you end up with maximum only. Find the black/tan wire on the back of the switch assembly and short it to ground. This should make the fan run at max speed. If it does check the connection to the switch, if this is ok, the switch could be faulty. If the fan still won't run, check the black/tan connections at the resistor pack. If it all looks ok, short between the two terminals on the resistor pack with black/tan wires. If it works now, the resistor pack is probably faulty,