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On one of the many frigid days this past winter, my husband and I had a discussion about using (versus not using) the rear defrost in our Buick Century. He rarely uses it and tried to convince me that the rear defrost is one of the biggest energy guzzlers in the car (as compared with the radio, heater, etc.), so he tends to turn it off as soon as he can see out a little. I still contest that visibility should be more important than energy use anyhow. I'm just wondering if indeed the rear defrost actually uses any more power/energy than most of the other standard features in the car. -- Kari

RAY: It uses a small amount of electricity, Kari. And anything that uses electricity does impose an additional load on the engine, which costs you some fuel.

TOM: We don't know what your particular rear defroster actually uses, in terms of amps. But it's probably somewhere between the radio and the headlights. At worst, let's say it's comparable to your headlights. Using your headlights may decrease your fuel economy by about 1 percent.

RAY: But your headlights are on for the entire length of your drive at night. The rear defroster runs for about 10 minutes, then automatically shuts itself off. So they're not really equivalent.

TOM: Nonetheless, he's risking your safety to get 20.5 miles per gallon instead of 20.3. This generally is not something one does with the woman he loves.

RAY: Especially if that woman also has an easy outlet for revenge because she prepares his food. You may want to remind him of that next time you slide a bowl of congealing gruel in front of him.

TOM: He's half a taco short of a combination plate, Kari. And he needs you to point this out to him. I suggest you do it in the form of a helpful suggestion.

RAY: Tell him that you've been thinking about other ways to save money. And you realized that the rear defroster is small potatoes, and that if he really wants to save money, he needs to stop using those energy-hogging headlights at night. You have our best wishes, Kari. And our deepest sympathies.

Dear Tom and Ray:

Our 2000 Saturn sedan has died twice in the same exact spot (stopping at the end of a highway exit ramp). The first time, it took 10 minutes or so to get going again; the second time, it took 30-40 minutes. Then it happened again on a straight stretch when slowing down at a stoplight. An engine scan showed a torque converter solenoid problem, but the mechanic does not think this is the problem. Could it be a fuel pressure regulator? Please help! My wife is stranded at home with two kids. Thanks! -- Jim

TOM: Well, I hate to disagree with your mechanic, but I think it probably IS the torque converter solenoid.

RAY: Modern automatic transmissions have a device called a lock-up torque converter that improves your fuel economy. Normally, in an automatic transmission, power is transmitted through the (how's this for an inventive name?) "automatic transmission fluid."

TOM: By using a viscous fluid to transmit power from the engine to the wheels, the transmission allows for some "slippage." That way, the engine can keep running even while the wheels are stopped (like when you're at a red light).

RAY: If you had a rigid mechanical connection, like in a manual transmission, you'd need to push in a clutch to keep the car from stalling when you stopped.

TOM: But that slippage, which is necessary in stop-and-go driving, is undesirable at sustained high speeds. When you're going over 30 mph or so, you don't want any slippage, because it just wastes energy and lowers your gas mileage.

RAY: So, modern automatic transmissions have a solenoid (which is an electronic switch) that activates something called a lock-up torque converter. That locks up the two halves of the torque converter, giving you a temporary solid mechanical connection, just like you'd have with a clutch on a manual transmission.

TOM: But if the lock-up converter happens to stick in the "on" position, the car will stall when you try to stop it, just like a manual-transmission car will stall if you're in gear and you come to a stop without pushing in the clutch pedal.

RAY: And the car will continue to stall when you put it in gear until the lock-up torque converter disengages.

TOM: Since the scan showed a code for the lock-up torque converter, and the symptoms fit, too, I'm going to go way out on a limb here and suggest that your mechanic check it out. It's easy to do. He can simply unplug it for you.

RAY: Then you can try it out and see if the problem recurs. If the problem disappears, then have your guy replace the solenoid. And if the problem comes back, and your wife and kids scream at you, blame us! Good luck, Jim.

(c) 2009 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate Inc.

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