The Spark Hits the Fan

Feb 09, 2002

RAY: We had a GM car that was in the shop recently that had a whole bunch of problems, but none was as interesting as this one.

Most modern cars have an electric cooling fan that's thermostatically controlled, so that when the engine reaches a certain temperature and needs to be cooled off, the fan gets turned on by a switch and a relay.

One of the problems with this car was that the fan was running all the time, whether the engine was hot or ice cold. When you turned the key off, the fan would stop.

So, we opened the hood. It didn't take too long to figure out that someone had hotwired the fan. It turned out that the fan relay was bad, and rather than fix it, someone merely took a jumper wire, spliced it into the hot wire of the fan, and then ran the other end to one of the ignition wires. When the car was turned on and the ignition was energized, the fan ran.

I yanked off the jumper wire. As soon as I did that, the fan began to slow down.

I don't know what possessed me to do this, but I took the wire that had been supplying electricity to the fan, and I touched it to a grounded piece of metal under the hood.

I got a spark!

TOM: Whoa! That wire should be dead, because the only thing it's connected to after you pulled it out was the fan.

RAY: Right. By the way, I also solved another problem that the car had. If the car was running, and you turned the ignition off, the engine would continue to run for sometimes four, five or six seconds.

What the heck was going on?


RAY: Tommy's right. You didn't need to know a whole lot about, or anything maybe about, cars, but when I pulled that wire off --

TOM: All you needed to know was Faraday.

RAY: Yeah, and we're going to get letters from all the engineers in the group, who are going to say that this can't happen.

TOM: Oh yeah, we'll say. Oh yeah.

RAY: Just like they all said, you can't have a DC motor that runs backwards, if you change the poles, which is not true either. We know that. But what happened when I disconnected that wire is the fan didn't just stop turning it immediately, it was slowing down, but it was still, like any fan that's unplugged, it continues to turn. While it was turning, it was acting as a generator.

TOM: Of course.

RAY: And it was making electricity, hence the spark. And it also explained why when I turned the key off, the thing didn't shut off right away, because, believe it or not, the electric fan's spinning now. Even though the key is off and there's no power being supplied to the fan, the fan is supplying electricity, which is running the car's ignition system.

TOM: Isn't this sweet?

RAY: And as the fan slows down, the engine begins to stumble and miss and finally, the fan is going too slowly to supply enough current to keep the engine running, and the thing --

TOM: Wow.

RAY: -- stops running.

TOM: What a beautiful puzzler.

RAY: Do we have a winner this week?

TOM: Do we ever. The winner this week is Michael Faraday.

RAY: From jolly old England.

TOM: No, the winner is Christina Stolberg from Denver, Colorado.

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