Is This Really How a Car's Fan Works?

Dear Car Talk

Dear Car Talk | Mar 16, 2017

Dear Car Talk:

I have a 2002 Dodge Ram Wagon. The fan blower stopped working on anything other than the "high" setting. Poking around the internet, I found what is possibly the world's easiest car repair: Open the hood, unplug the resistor, remove two screws, wiggle out the old resistor and replace it. I did that, and it works. Life is good! Now for the problem. That resistor is enormous, replete with big coils that look like heating coils.

Is that how this whole thing works -- the resistor slows down the fan by diverting (and wasting) some of the power to heat? I always thought I was saving power, and improving my fuel economy, by running the fan on lower settings. But if this is how it works, the fan is always drawing maximum power no matter what setting I use. Say it ain't so! -- Dale

Well, let's start with the good news, Dale: You fixed your car. Hooray! 

The bad news is that you discovered the dirty secret of how the fan switch works.

When you set the fan on high, electricity goes straight through the fan switch and continues on to the fan motor. If you turn down the fan speed, the same amount of electricity goes to the fan switch, but the switch engages one or more of its resistors and diverts some of that electricity, turning it into heat. That heat is just electricity going to waste -- unless you happen to be cooking a 40-pound pheasant under your hood. But because less electricity ultimately makes it to the fan motor (because some is diverted as heat), the motor turns more slowly. I should add that when you cause the fan to turn more slowly, the fan motor probably will last longer. So running the fan more slowly isn't all bad or wasteful.

But if your only goal is to avoid wasting electricity and fuel, you'd have to run your fan on high speed all the time. And install a windmill between the seats to recapture some of that power.

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