If you are shopping for a vehicle, or already own a vehicle, that is either 4WD or AWD you may not even understand the real difference. In all the marketing material it sounds like the main difference is 4WD allows you to control when power is applied to just 2 wheels or to all 4, and AWD vehicles apply power to all 4 wheels all the time, or at least under automatic control. Well, that’s true but the mechanical systems used to create 4WD vs AWD are very different, and therefore have different repair and maintenance requirements.
Your vehicle’s four-wheel and all-wheel drive is much more than just a safety feature. When you have better traction and control on slippery roads along with more grip on rough terrain, then even the roughest or most slippery roads become a more drivable place. A sudden storm or snow shouldn’t keep you inside.
However, with greater power comes greater responsibility. The additional mechanical components, and the complexity required to run an AWD or 4WD system, require more complex electrical and mechanical repair and maintenance. And, being able to drive in harsher conditions means you’ll also be putting additional wear and tear on the suspension, drivetrain, steering components, brakes, and chassis.
4WD vs All-Wheel Drive
Mechanically, the key difference between 4WD and AWD is in the method used to direct power to the dominant wheels and the auxiliary wheels. Most 4WD vehicles, especially pickup trucks and large SUVs, use the rear wheels as dominant (the ones the motor turns in 2WD settings) whereas most AWD vehicles are front wheel dominant. 4WD vehicles utilize a transfer case to switch between 2WD and 4WD. Instead, most AWD vehicles have a mechanical device called a transaxle. This is essentially a transmission and differentials all in one. The transmission portion of the transaxle functions the same as a stand-alone transmission, and a built in differential directs power to each of the front wheels. An additional built-in differential directs power to the rear driveshaft, which in turn powers the rear wheels.
Advantages of FWD and AWD
4WD vehicles generally offer the driver more control over traction. Engaging 4WD provides great traction when needed, but it adds wear and tear to the drivetrain, and reduces fuel economy. It’s not a good idea to operate a 4WD vehicle in four wheel mode all the time, only when necessary for extra traction. If you sometimes drive your vehicle off road and want direct driver control over your traction, then you will greatly benefit from a good 4WD system.
The AWD transaxles in many vehicles use mechanical and electrical controls to regulate when and how much power to apply to the auxiliary wheels. These systems generally automate the traction vs fuel economy function so the driver doesn’t have to worry about it although some models offer an override button that allows the driver to select a snow mode to ensure all four wheels get power the motor.
AWD is perfect for driving in situations where the road may be slippery, and you may lose traction. For example, if you frequently drive in rain or snow, or variable road conditions where traction could change at any minute.
Intelligent All-Wheel Drive Systems
The most sophisticated AWD vehicles use various sensors and control modules to intelligently direct power the motor to the wheel, or wheels, where power is most needed. We have already discussed the advantages of power to just 2 wheels for improved fuel economy and reduced drivetrain wear, and the advantages of power to all 4 wheels for improved traction. But what about when one wheel is slipping, or the two wheels on one side of the vehicle are slipping while the other side has traction? Directing power to wheels that are slipping takes power away the wheels with traction. This is not a good situation because the wheels with traction are the only ones that can move the vehicle.
Another benefit of intelligently directing power to specific wheels is for improved handling and performance. This system is not just used for off-road vehicles but frequently for high performance vehicles as well. It was actually developed for rally racing where off road traction at high speeds is critical. And that combination of performance and safety translates into excellent, safe drivability for your family SUV. For example, when going around a tight corner at high speed there is a definite advantage to directing power to the 2 wheels on the outside of the corner. This effectively pulls the car around the corner. At other times it is important to have the majority of power directed to the rear wheels so that oversteer is reduced, or to the front wheels to reduce understeer.
4WD and AWD Service
It is unfortunate that some vehicle manufacturers suggest in their maintenance schedules that automatic transmissions and transaxles do not need to be serviced. They sometimes call the automatic transmission fluid “fill for life”. They even design their vehicles to make it difficult or impossible for the owner to measure the level of the automatic transmission fluid. This strategy is ok if the objective is just to get the vehicle to the end of its warranty period without the added cost of transmission maintenance. But sadly it isn’t great for getting the longest durability out of your vehicle.
The ATF in a transmission or transaxle must be kept clean or the minute particles in the fluid act like a grinding compound and deteriorate all the components. To accomplish this, a fluid filter is used. But the filter is located inside the transmission or transaxle. If the fluid is never changed, and the filter is never replaced, the normal wear of the internal components will accelerate until something fails. With good maintenance the service life can be extended a lot longer.
For many AWD vehicles, servicing the transaxle takes care of the transmission, front differential, and transfer case functions. In a 4WD vehicle there is typically a separate Transfer Case that must be serviced in addition to the transmission. The T-Case usually has no fluid filter at all, so regular fluid change maintenance is critical. Both front and rear differentials are separate components as well. These components use gear oil to ensure smooth operation, lubrication of internal components, and cooling. They do not have filters for this fluid so replacing the fluid is again critical to long term durability.
It is important to have a driveline specialty shop perform all of the drivetrain services. The reason is that opening up transmission, transaxles, transfer cases and differentials should only be done by knowledgeable experts. If fluid is simply drained and refilled, much of the contaminant material remains inside the unit. This can cause damage long before the next service interval comes up. In addition, ensuring the correct fluid type and level setting is critically important. Too much fluid can be as bad as not enough. Over-filling can lead to aerated fluid (filled with air bubbles) which completely changes the viscosity and lubrication properties.
4WD and AWD Repair
In a 4WD and AWD vehicle, the drivetrain delivers torque to all four wheels as directed by the driver or the automated drive control systems. This enables the front and rear drive axles to operate at changing speeds as required, and helps increase torque to the wheels with the most traction. When everything is working as designed it provides excellent advantages over common 2WD vehicles.
If the system isn’t working as designed, however, repairs should be performed as soon as possible to avoid additional damage.
Signs that driveline system needs repairing
- Vibration or chattering when accelerating and turning
- Difficulty or unusual motor revs while shifting gears
- Vehicle feels sluggish
- High-pitched whistle or a clunking sound near the axles
- Vehicle shifts back and forth between 2WD and 4WD
- 4WD system doesn’t engage properly
- Flashing or inaccurate indicator light
- Vehicle gets stuck in 4WD
In addition to the components and systems mentioned above, a driveline or drivetrain shop will diagnose and provide any necessary universal joint repair and service, as well as cv axles repair. With these items taken care of, your vehicle will be running and driving smoothly as designed, and ready for your next family outing or off-road adventure.