Why? Keep loseing fan Resister.

Tiny
TFARNHAM
  • MEMBER
  • 2004 DODGE CARAVAN
  • 120,000 MILES
Heater fan stuck on high speed.
On 3rd module.
Works for a day or two then gose back to high speed.
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Tuesday, December 6th, 2011 AT 10:05 PM

3 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Chances are the blower motor is drawing too many amps and burning up the resister.
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Tuesday, December 6th, 2011 AT 10:21 PM
Tiny
RASMATAZ
  • MEMBER
Could be the blower motor resistor circuit shorting out or the blower switch
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Tuesday, December 6th, 2011 AT 10:22 PM
Tiny
CARADIODOC
  • EXPERT
Two things commonly cause repeat failures, especially when complicated, unreliable computers are added to do things computers were never needed for before. In this case, suspect the fan motor is tight and drawing high current. That will overheat the high-power switching transistor in the module. Heat is the deadly enemy of transistors. In this circuit they are turning on and off hundreds of times per second. The ratio of "on-time" to "off-time" is varied to vary fan speed. When the transistor overheats it will short and remain turned on all the time. That's why the fan would run at high speed.

A second common problem is something in the wiring or a corroded connector pin. Unplugging a connector will often make it scratch a clean spot that will let it work when it's reconnected, ... Until it gets corroded again. That corrosion is usually just a very light coating and can be hard to see.

The place to start is by measuring the current flow to the fan motor. I don't know what "normal" is but I'd expect to see around six to eight amps on the highest speed. An experienced AC mechanic would know for sure what's normal.
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Tuesday, December 6th, 2011 AT 10:35 PM

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