When should I be engaging the 4WD on my Ford Explorer?

Dear Car Talk

Dear Car Talk | Sep 01, 1995

Dear Tom and Ray:

With winter driving conditions upon us, I need to have a difference of opinion settled. I have a 1992, four wheel drive Ford Explorer. The four wheel drive should be a plus in hazardous driving conditions such as ice. However, whenever my husband is driving, and there's patchy ice on the road, he will take the car out of four wheel drive prior to making a turn. His reason is that going suddenly from dry road to ice could put stress on the drive train. My opinion is that sliding into a curb or into another car would be a lot more stressful. How should the four wheel drive be used?
Jo Anne

RAY: We're going to send you back to the book, Jo Anne... the owner's manual. Do not pass GO, and do not collect $200.

TOM: The manual should explain that the only time you should be using four wheel drive in your car (and any car that has driver-engaged, or "part-time" four wheel drive) is when you're already stuck and you need extra traction to get out. This kind of four wheel drive system is not designed to be used during normal driving.

RAY: Right. Part-time four wheel drive systems do not have "center differentials." Now, what differential does that make, you may ask? Well, it means that the vehicle can behave erratically on turns when the four wheel drive is engaged.

TOM: When you're going straight, all the wheels are turning at the same speed. But when you go around turns, the wheels turn at different speeds, because their travelling in different arcs. And without a center differential to make up for this imbalance, the car tries to push the front and rear wheels at the same speeds, and that can cause all kinds of weird things to happen. In fact, you may be even MORE likely to skid into a curb if you drive around with your four wheel drive engaged.

RAY: But don't get nervous Jo Anne. This doesn't mean your husband is right. Taking the car in and out of four wheel drive--especially if it's done while the car is turning--can cause the same kind of unpredictable handling. So that's not recommended either.

TOM: There IS a type of four wheel drive called "full-time four wheel drive" or "all wheel drive" that can be left on all the time. But those systems DO have center differentials. And some vehicles are even using "electronically controled" center differentials. Ford introduced a new system called "Control Trac" on the 1995 Explorer. That system is computer controlled, and automatically switches the four wheel drive on and off whenever road conditions dictate.

RAY: Your owner's manual should explain all of this, Jo Anne. I know it's not exactly a Michael Crighton thriller, but curl up with it in front of a fire some winter evening.

TOM: You know....make a romantic evening of it and read it aloud to your husband.

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