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Technically, it's not a problem with the blower motor exceeding its voltage spec, but instead the current draw.
When the motor gets old, or gets obstructed in some way, preventing it from spinning freely, it draws excessive current for the same voltage input. This causes a greater proportional voltage drop across the fan speed controller (whether it's a resistor in the older cars or the ACC transistor controller in the newer ones). The larger voltage across the resistor translates into higher current, which means higher power being dissipated, which means higher heat, which is what takes out the thing.

Apparently they've sized the 30A fuse for the circuit a bit too large, because it really should blow before the other components get damaged; that's the purpose of the fuse. I'd consider changing the fuse to one of a 25A rating, which provides a greater safety margin before you get to blow out the controller. It seems we've only heard of the controller going south, not the fuses popping, which isn't the way it should be.

Until I rig up a replacement for the blown blower motor controller I've got, I temporarily put in two parallelled 50W 1 ohm resistors (about $3.50 each) in line with the connection at the fuse for the blower along with a relay to switch it on and off and select full speed (direct connection) and low speed (through the 0.5 ohm resistance). I connected the other end of the blower motor return to ground, bypassing the controller. It draws about 10A on low, which is about 50W being dissipated by the resistors (25W each). Lots of heat, that 25W; gets up to about 460 degrees, cause it's not a light bulb; all the energy goes to generate heat...
I'm going to try rigging up a PWM circuit replacement for the blower motor controller, which should be much more efficient than the factory setup, since it metes out the power as needed rather than dumping it as heat; we'll see just how well it'll work.

posted by 12.4.27...

• 9000 Heater resistor problem, Mike Bloor, Fri, 4 Feb 2005 08:21:21
  • Uh, wrong. It ain't no capacitor, Ari , Fri, 4 Feb 2005 09:59:40
    • In the case of an ACC car...., David Ingram, Fri, 4 Feb 2005 10:28:58
  • Root cause: bad blower motor, David Ingram, Fri, 4 Feb 2005 09:15:01
    • Blower motor OK, Mike Bloor, Fri, 4 Feb 2005 09:40:05
      • Check the current draw on the motor, Wingerr, Fri, 4 Feb 2005 14:28:04
      • One more thing...., David Ingram, Fri, 4 Feb 2005 10:39:38
      • Blower motor (probably not) OK, David Ingram, Fri, 4 Feb 2005 10:35:00
        • Current spec, Wingerr, Fri, 4 Feb 2005 14:22:23 <-- Viewing This Message
          • Re: Current spec, Wayne Garvin, Wed, 1 Aug 2007 20:30:23

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