Burning Insights of the Second Degree

Aug 24, 2009

RAY: This puzzler was sent in by Wade Satterfield. He writes:

"It was an early fall evening, when my wife drove off in our '94 Explorer. About three blocks from home, it started making a great screeching, screaming noise with smoke coming out from under the hood. My wife turned off the car and, of course, called me for help.

"I was home in my jammies, but I got in my car and drove over there. Being the guy, I was obliged to open the hood, and I tried to look for something wrong...even though it was dark. It was, after all, a dark and stormy night.

"On a hunch, I reached in and touched something that should have been cool. Instead of touching something cool, though, I got a second-degree burn on my finger. Wounded and whimpering I walked home, while my wife continued her trip in my car, leaving the Explorer smoking and smoldering by the roadside.

"After putting ice on my finger, the pain subsided enough so that I could think straight. I walked back to the Explorer. I adjusted something on the dashboard, started the engine, and drove home with no problems."

So, what did Wade do?

RAY: Here's the answer. Wade turned off the defroster because what was wrong was that the air-conditioner compressor had seized.

Most modern cars only have one belt that runs everything, and the air-conditioner pulley is turning all the time, whether the air conditioner is on or off. When you turn it on, you engage a magnetic clutch that makes the pulley that's spinning via the belt turn the compressor. But if the compressor is seized, the pulley can no longer turn and the belt will begin to screech and slip around that seized pulley-- and cause smoke and eventually fire.

If you turn off the air conditioner that compressor pulley will just go back to freewheeling because the magnetic clutch is disengaged.

The reason the thing was hot enough to burn his finger, was because the defroster was on and that belt was screeching over that seized pulley. Then, as soon as he turned off the defroster, the belt stopped screeching.

Don't forget, it was a dark and stormy night and the windows were probably getting fogged up, so you would turn on the defroster-- which on most cars also turns on the air conditioner. But, turning off the air conditioner would also be an acceptable answer.

Get the Car Talk Newsletter