I've been researching heaters. I have two different ones to choose from, a 2 speed and a 3 speed and was looking for schematics. I found lots of word descriptions which were mostly correct but poorly phrased so they were confusing. I finally found the two diagrams I've attached. They should help anyone to identify and/or trace any problems with the early heater fans. The obvious differences are the 2 speed switch is hot and powers the motor while the 3 speed switch supplies a ground through different resistors to complete the circuit.