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25A fuse works as variable resistor... Or Why my fan slowed down when running. 700

posted by  Will740turbo on  Thu Jul 10 18:38 CST 2008
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last visit: Wed Oct 22 16:09 CST 2014
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OK, so I was going through my fuse box and realized someone ran out of 30A fuses and replaced them with 25amp fuses instead.
I switched them all back to 30 amp fuses, noting that none of them had been blown or anything, but then I realized how hot the fuse (#6?) for my blower motor was.
I had posted some crazy theory that when the AC was on, it made the oil in the fan's bearings so cold, that it slowed the fan. Lo, and behold, I have a different, better, still somewhat crazy theory.

Given that there are NTC/PTC resistance vs. temp sensors and whatnot, it's safe to say that a 25amp fuse, while not blowing, heated up enough to change resistance and act as a PTC resistor. The hotter the fuse got, the higher the resistance. And sine there is a "resistor pack" for the fan to control the speeds, it makes sense.....
That's why "resting the fan" for a minute would make it work again (and it turned out to happen all the time, not only with AC use)
But is it wrong? can a 25amp fuse really do this? Seems like it's pushing it. I suppose I could see if a 20 amp fuse would blow in its place.

Interesting phenomenon! I can't wait to be an engineer.. lol.

-Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond






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