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

Not every light-duty fleet vehicle needs all-wheel or four-wheel drive, but depending on what conditions your drivers are moving through, they could be an absolute must.

While 4WD systems generally call for the driver to manually switch between two-wheel mode for street and highway driving and four-wheel mode in low-traction conditions like ice, mud or gravel, AWD systems are sometimes called “full-time 4WD.”

The difference is AWD can be used on dry pavement because it uses a center differential, which allows each tire to rotate at a different speed, eliminating driveline binding, wheel hop and other related issues associated with the use of 4WD on dry pavement. AWD systems are more sophisticated, and the computer will transfer grip to the tires with the best traction, diverting power where necessary

Dave Williams, fleet operations area manager for Verizon Communications, is responsible for maintaining around 100 four-wheel drive vehicles that go through some of the most snowy, mountainous terrain in the country—upstate New York. He said while the fleet has no AWD vehicles, the four-wheelers are a necessity—typically GM or Chrysler pick-up trucks and SUVs.

“Most of the (trucks) that service the cell towers have four-wheel drive because they’re typically on a higher elevation, and these four-wheel drives help them get up the hills, which are roads but are not typically paved,” he says.

When trying to figure out whether to equip your vehicles with AWD or 4WD, Williams says the first, and most important step, is justification.

“If most fleets are like we are, if you ask them they all need four-wheel drives, so you really have to look at the job requirement—does (the job) really need it?” he says. “Typically we replace like for like—if the job requirements are the same, then we give them the same kind of vehicle back—but when they’re asking for it initially, we look to qualify it to what they really need it for.”


Picking the optimal truck for the job is a crucial first step, says Chrysler, LLC product engineer Frank Thompson.

“Down South, maybe they just pick the two-wheel drive because there isn’t the need for four wheel, but in the Midwest, especially, there are days where you would not have been able to get through with a two-wheel drive, but you don’t necessarily want the mileage hit of a full-time four-wheel drive truck.”

Toyota spokesperson Bill Kwong says cost-wise, since it’s cheaper for fleets to own a part-time four-wheel drive system versus a full-time system, four-wheel drives should only be used if needed.

“Maintenance-wise, that’s one less differential to service, and you don’t have to service the transfer case,” he says. “You’re not dragging around all those extra components. Even when you’re in two-wheel drive mode with a part-time four-wheel drive, you’re still dragging all that weight—any little edge you can get.”

Tim Cavanaugh, marketing product manager for GM’s fleet and commercial operations, says fleet officials have varied opinions on choosing 4WD or AWD for their needs.

“People that are picking the four-wheel drive are usually picking it because of the payload and what they’re hauling, and also your four-wheel drive, because it’s in trucks or utilities, would give you a lot more clearance up off the road,” he says. “I know in cases where we have long linemen that have to go out and check lines where they have to drive up to a major power pole—they might have to drive up the side of the mountain, and literally the rocks that are there, because it’s not generally a road that gets you there, it’s driving off-road.

“They need to be a lot higher up, they need on/off road tires so they have puncture-resistance sidewalls, and it’s got to be a really robust suspension to keep them above the rocks so they don’t poke a hole in the gas tank or oil pan, to make it to the top of the hill,” he says. “Your all-wheel drive would be good to some extent, until it comes to the bigger rocks, where it doesn’t have the clearance off the bottom of the vehicle.”

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.”


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.


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.

“What’s really nice about a full-time four-wheel drive system is that what allows it to do that is the center differential,” he says. “Unfortunately in a real severe heavy-duty situation, that’s the weak link, because now you have a third differential in the link that could possibly fail if you’re carrying a lot of capacity; where in your typical four-wheel drive system, the center differential is replaced by a transfer case, which is a much more heavy-duty gearbox.”


Williams says there is not too much extra for technicians to do when troubleshooting and maintaining 4WD systems.

“It’s just a little bit more when you’re doing your (preventative maintenance), that you’re checking the front axle differential level, you’re looking at the u-joints, you’re looking for leaks,” he says. “So you spend a few more minutes looking at the front end, but the maintenance isn’t much more different on them.”

Thompson says the biggest maintenance concern is—surprise—diagnostics.

“(Technicians) need to understand what circuits to look at and when,” he says. “We try to establish the fault codes in the controlling module so that if the ‘service four-wheel drive’ light comes up on the dash, we try to narrow it down for the techs. The quick response sometimes is to replace the module rather than find out why it set a fault.”

“Diagnosing these electronic systems does demand a more adequate diagnostic procedure to be written down for them to follow,” Thompson says. “Back in the days, they could just wing it or it was in the capacity of most technicians to just look at the mechanical linkage and say, ‘Oh, this just isn’t adjusted right’ But now the demand is more on us to get them the information and documentation they need to thoroughly analyze a faulty system.”

Toyota’s Kwong says since they did away with manual-locking hubs, everything is now automatic.

“The only thing you need to do every 30,000 miles is inspect and replace, if necessary, the transfer case and differential fluid, unless you’re doing some really heavy-duty stuff on a regular basis,” he says.

Knowledge of electronics and sensors is very important, says Cavanaugh.

“If they’ve got a broken part and they don’t really understand that it’s broken, they can look at a sensor all day and may not understand that it’s not working properly,” he says. “Just replacing things doesn’t necessarily do it. If you have a good technician, he should be able to read and understand that the actual sensor is not functioning if he follows his checklist, and being able to do that through an OED2 connector and actually going in and reading what’s going on and analyzing; that would help him out a lot and save him a lot of time.”


All-wheel and four-wheel drive vehicles are a necessity for fleets in some regions and conditions, but first you need to make sure you’re not wasting money buying expensive systems your drivers may want but you don’t need.

And since much of the maintenance these days is electronics-based, if you make sure your technicians are up to speed on their diagnostics, your vehicles will have much smoother sailing ahead, no matter how rough the roads they travel.