Look at the diode when it is damaged blocks the first three speeds and the fourth is direct must work even without putting (connect) the resistance, if you have a current or test tester as you call it, disconnect the resistance you put the speed control in the one and in the resistance connector in a terminal it will arrive positive (+), then the control you put in the two and should arrive positive (+) in another terminal And the fourth as I said is direct if everything arrives,阻力发生了变化,因为正如我以前理解的那样,如果它不起作用,那是因为在阻力中损坏了灯丝,问候