Wouldn'tcha know it, the brand-new heater-fan switch is already failing!

Yesterday in the full-on position it was running about half-speed, which is actually faster than the other settings, and if I jiggled the knob just-right it would do full-speed.

Today, no full-speed. Oddly, I'd been thinking it'd be nice to change those resistors such that the distribution was more logical, specifically I wanted a setting close to 50%: e.g. before the new switch-fail, its speed settings seemed around 10%, 15%, 20%, and 100%. 100's too fast, sometimes, but 20's too slow.

So, was thinking different resistors to make it more like 10%, 20%, 50%, 100%.

Then, the next day, after thinking this, the new switch[?] started failing, giving me basically 50% where 100% used to be.

No joke.

Weird failure.

50%'s nice, though. So, I'm not particularly complaining, but it leaves some thoughts, aside from the obvious, which I'll ignore for the sake of science. [Especially being that this is easily the third time the heating system's failed-to-/almost/-desired-functionality within a day of my thinking it].

So, main concern: if it's running at, say, 50% where it was 100%, and 50% is not an option with the available resistors [presumably, the replacement fan motor has a lower winding-resistance than the original], that must mean there's resistance [or voltage-drop] somewhere in the 100% path. This resistance is new, AND jiggling the switch did, for a while, give nearer 100%, as well. So, logically, this resistance must be in the switch. No?

So, as we found with the last switch, even a tiny resistance in the contacts [as I couldn't tell much difference from 100% fan-blowage] was enough to produce so much heat as to melt the switch's plastic.

So, then, an even bigger resistance should cause even more heat, no?

P=VI, V=IR, P=IRI=I^2*R... hmm, maybe.

I dunno. And no guess as to what'd cause said resistance, anyhow... near-perfectly-distributed oxidation across a sliding/rotating switch-wiper's path? [Or on the wiper itself?]

Well, through what'd appear to be sheer dumb-luck, I now have that 50% fan-blowage and recirculation without AC running, just as I'd planned on implementing but didn't actually implement.

What when I want 100%, or AC, again? I dunno. The third case was bad-luck, I'd rather not recall it, though repair turned out to be quite cheaper than I expected, which, after two weeks in the cold, was quite nice "luck" indeed. 50-50, I guess.