Over the years, the wiring of the alternators available for the Stairmaster steppers has varied. The common issue that presents itself with a new alternator installment is no resistance on the machine; most commonly the result of a wiring issue.
If you have installed a new alternator onto your Stairmaster stepper and have no resistance, it is possible the blue (TACH) wire is in the wrong place.
If you have installed an alternator with a SILVER sticker, the blue wire should be connected to the AC TAP.
If you have installed an alternator with a YELLOW sticker, the blue wire should be on TACH.