Light Duty: Maintaining All-Wheel and Four-Wheel Drive

Whether your fleet is built for rocky off-roads or slick winter highways, AWDs and 4WDs both need TLC.


Williams suggests coming up with specifications that would qualify certain drivers for four-wheel drive to reduce any confusion.

“Just so you don’t hand them out without any kind of a program where everybody will say, ‘Well, where’s mine?’” he says. “So if you put some requirements in there where it has to be in the snow belt area or typically gets more than 20 inches of snow a year or something like that, some kind of guidelines and measurements you can use as a policy so you can kind of control that policy.”

NEW PRODUCTS

Thompson says in 2006, the Dodge light-duty trucks included a new AWD wrinkle: an electronic on-demand transfer case (the 246) and electronic front axle disconnect.

“When the customer selects two-wheel drive, they get front axle disconnect to give them better fuel mileage, and the 246 provided an automatic mode so the front axle would be engaged so you’re ready but primarily it would act as if you’re a two-wheel drive truck for most situations,” he says. “Then it would send power to the front wheels if there was slipping going on. You can be not engaged on a dry, paved road, yet if it’s a mixed surface where you hit an occasional slippery patch, the system can react to that and give you four-wheel drive capability. If there is a mixed surface or if they weren’t quite sure what mode they should be in, they can put it in this auto mode and the system would react accordingly.”

The front axle disconnect allows front driveline components to stop spinning, which creates drag and decreases fuel mileage up to a mile per gallon. It’s the “best of both worlds,” Thompson says.

“When you’re in the two-wheel drive mode, you cause a disconnect to occur in the front axle so you can allow some of the rotating components to come to a stop, therefore there’s less rolling resistance, so you get better mileage,” he says. “Yet when you need four-wheel drive, you can engage that. It’s the most economical way to go, especially if you get a one mile per gallon increase and you multiply that by the number of vehicles and the number of miles they drive.

If you can allow those heavy components to stop spinning, that’s where your savings is,” he says.

Cavanaugh says GM’s vehicles have several available modes, depending on traction needs.

“In our four-wheel drive units, you can opt to either (engage) the two-wheel drive, or the four wheel drive or in our current vehicles you can put it in automatic mode, which will pick up on whether or not you’ve got any spin that’s going on with the regular two-wheel drive, which would automatically kick in the four-wheel drive,” he says. “(On) an all-wheel drive, you now have sensors that are picking up on all four corners of the vehicle, and today our systems are going from an all-wheel drive system that manages the power to all four wheels to including the stability control system that we have put into our vehicles.

“So it’s getting ever more complex,” he says.

Cavanaugh says the all-wheel system has a computer which helps keep the vehicle moving along smoothly in a variety of conditions.

“Let’s say you’re driving down the road, 70 miles an hour on dry pavement,” he says. “You‘re probably going to have (around) 90 percent of the power going to the front wheels, 10 percent going to the rear.

“Now if you go from either concrete or asphalt into a gravelly or muddy-type situation where each one of the tires is measuring its spin and the EVCM picks up on what’s going on with each one of those tires, then it will determine where the extra traction needs to be put,” he says.

AWD ON DEMAND

Toyota developed an “all-wheel drive on demand” system back in 2006 for their light-duty vehicles, says Kwong, but otherwise, AWD and 4WD systems have generally not changed too much in the past few years—why switch something that’s worked well?

“The advantage of all-wheel drive is that it’s constantly on,” he says. “If the weather’s nice (but) you head up a hill and it gets a little icy, a little snow, you don’t have to get out of the vehicle and put chains on, you don’t have to engage anything, it’s already in full-time four wheel drive, and power will vary front and back and left and right according to condition and wheel slip.

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