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, the resistance changes because as I understood the diode before, if it did not work it is because within the resistance a filament is broken, greetings