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4x4 Truck Problems Disengaging From 4WD - Willies Workbench

Posted in How To: Tech Qa on September 1, 2002
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Over the last year I've received several letters from people who have had difficulty getting their vehicles out of four-wheel drive after extended usage. The greatest number of complaints seem to come from people whose trucks have electronically controlled transfer cases. One person had to have his vehicle hauled to the dealership on three different occasions to get it out of four-wheel drive. The first time the dealer's service folks replaced the switch, the second time they replaced the motor that did the shifting, and the last time, they put it on the lift, ran the vehicle while suspended and it came out of four-wheel drive. One person wanted to know just what good four-wheel drive was if he couldn't use it on partly snow-covered roads. Shifting in and out every time he came to a snowy section was a real pain.

What's happening here I think is that most people don't fully understand the concept behind a part-time four-wheel-drive system. What you have to remember is that when in four-wheel drive with a part-time system the front axle is locked to the rear axle. Therefore, the front end must operate at the same exact speed as the rear axle, something that in a practical sense can't happen. First off, any time you turn, the front tires turn at different radii than the rear tires, hence different speeds. Another thing that a lot of people don't take into account is that tire size front to rear has to be very nearly the same. In the real world, this also doesn't happen. Even though the sidewall numbers on the front and rear tires may be the same, that doesn't mean the overall diameter is. There are small variations between brands, tread designs, and even within the same brand, depending on the mold the tire came out of. Equal tread depth front to rear is also an important consideration

When one end tries to turn at a different speed than the other, any slack in the drivetrain is taken up and the parts begin to bind. Sometimes this binding gets so bad that you can't manually move the shift lever, or the shift motor isn't powerful enough to do so. Normally, one only uses four-wheel drive when some tire slippage can take place, such as on an icy road or a dirt trail. This tire slippage prevents the binding from building. But there are times, such as on a partly snow-covered road, where it isn't practical to keep taking it in and out of four-wheel drive to match road conditions. I know that in Montana, where I live, this is often the case. What I do is periodically put one tire into the loose stuff so it can slip just enough to release any tension.

This is where the advantages of a full-time system become important. Special differential gears, viscous-coupling clutches, or other means within the transfer case allow for some internal slippage and/or transfer power to the axle with the most traction.

So how do you prevent gear bind? Well, like I said, find a place to occasionally let a tire slip-be sure you're going slowly, and remain in complete control when you do this. Most likely, you won't even feel the slippage. Make sure all tires are of the same brand and design, and have the same tread depth. This means replacing all four tires at the same time.

But what to do the next time your rig's transfer case won't disengage from four-wheel drive? Back up, maybe as far as 100 yards. What you're trying to do is relieve the tension buildup by running the drivetrain in the opposite direction. Sometimes this doesn't even do it, so find some place you can slip one or more tires when backing.

One more thing: What's the disadvantage of running on a hard surface with four-wheel drive? Well, besides difficult steering and using more fuel, just a few little things like more tire wear, and a very good chance that you could break axle U-joints, axleshafts, differential gears, driveshaft U-joints or transfer-case gears. The thought of those horrors makes shifting into and out of four-wheel drive a little easier and not such a pain, right?

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