How Does 4-Wheel Drive Auto Work? 4-A EXPLAINED

This article explains exactly how 4-wheel Drive Auto Works. If you are curious as to the internal workings of a 4WD auto and how it differs from the full-time 4WD and AWD (All-Wheel-Drive) then this article is for you. We will look at the performance differences between the modes on a low traction surface.

4WD auto operates by initially powering the rear wheels until traction diminishes. Activation of an electric motor on the transfer case engages a metal fork, applying pressure to friction plates. This syncs front and rear driveshafts, channeling power to the front wheels, and shifting to 4H mode automatically until traction is restored.

Now that we’ve briefly explained how 4WD auto works, let’s explore specific driving situations that necessitate engaging 4A. We’ll also examine its similarities to a part-time All-Wheel Drive system. Following that, we’ll showcase how the 4 4-wheel drive auto setting outperforms 2WD and is more convenient than full-time 4-Wheel Drive.

How 4A (4WD Auto) Works

The 4WD Auto system is always rear-wheel biased. This means most of the power will be distributed to the rear wheels. The driving surface is pretty much irrelevant since the automatic systems’ highly sophisticated computer will be taking care of business while you enjoy the ride.

4WD Auto Electronic Sensors

There are electric sensors that measure and calculate the speed of the front wheels versus that of the rear wheels. Once the sensors detect that there is a difference in rotational speed, usually when traction is lost, it immediately engages 4WD. This is achieved in a matter of split seconds and is hardly noticeable to the driver. No driver input is required.

Traction Regained

The available engine power will then be split between the front and rear axles sending power to all four wheels. This process is controlled by a computerized system and negates the need for the driver to engage and disengage 4WD when driving on a partially low traction surface such as a snow highway or cement pavement. This is a very sophisticated design that is more user-friendly and practical than say a non-permanent 4WD.

If you want to get a more visual explanation of how the internals of a 4-wheel drive auto work, watch the video below.

How Does A Non-Permanent 4-Wheel Drive System Differ?

Part-time or temporary 4WD are normally driven in 2H mode where the power is sent only to the rear axle of the vehicle. This 2H mode is designed to be used for daily driving on high-traction surfaces like tar roads and concrete pavements. The part-time 4-wheel drive’s drivetrain will function very similar to a rear-wheel-drive when driving on-road and offers decent fuel economy.

With a part-time 4-Wheel drive the input of the driver is needed to engage 4WD. This is achieved by selecting 4H from a short lever or electronic dial inside the cabin which locks the front and rear driveshaft to function as one unit. This is at the discretion of the driver and should only be done when the vehicle is driving on a low traction surface where enough wheel slippage is possible.

Next, let us look at when would be the ideal driving surface to engage 4-wheel Drive Auto mode.

When To Use 4-Wheel Drive Auto

So when would you typically engage 4-wheel drive Auto? Let us look at a few driving scenarios. These will typically comprise low-traction surfaces combined with high-traction surfaces on the same road or track. These can include but are not limited to:

1. Wet roads
2. Snow roads
3. Sleek road surfaces
4. Partially Icy roads
5. Any semi-slippery surface

So basically when you have a mixed driving surface that includes good traction and some slippery low traction areas, is an ideal time to engage 4A. On the other hand, with a part-time 4-wheel drive, you will need to assess the driving conditions and determine whether the surface is slippery enough to engage 4-wheel drive and then engage when needed. When traction is regained you will need to revert to 2-wheel drive mode immediately to avoid drivetrain binding. You can read more about drivetrain binding causes and effects here.

Alternatively, you could just leave your vehicle in 4-wheel drive Auto mode. There is no risk whatsoever of drivetrain binding or damage to your tires. Fuel consumption will be at its best since it will always be rear-wheel biased until traction is lost. So you have the added safety benefit too.

4WD Auto vs AWD

The traditional permanent 4WD makes use of a viscous coupling to compensate for rotational speeds when turning. The semi-permanent AWD system found on a Honda CRV uses an electric motor to engage the AWD system once traction is lost. The Subaru Forester uses a set of plates submerged in the thick fluid inside the transfer case to maintain traction on all 4 wheels. All very interesting designs with each having its pros and cons.

Multi-Plate Clutch System
It usually makes use of a multi-plate clutch system combined with a clever on-board computer and wheel sensors. The wheel sensors monitor front and rear wheel rotational speeds and send signals to the on-board computer. When traction is lost, the computer will attempt to synchronize the front and rear wheels by activating the multi-plate clutch system to bring them back to the same rotational speeds.

This is usually achieved by temporarily “connecting” the front and rear drive shafts. This is a temporary connection and once the computer recognizes that traction has been regained and the front and rear wheels rotational speeds are back in sync it will disengage and revert back to either front or rear-wheel-drive mode.

The multi-plate clutch system and the viscous coupling might sound as if they function the same, however, they are distinctly different in design and function. The Viscous coupling is always engaged and usually incorporated in permanent AWDs. A multi-plate clutch system, on the other hand only activates once traction is lost in either front or rear wheels and then deactivates to allow the vehicle to operate as a 2WD vehicle again.

This has a fuel efficiency advantage since fewer drivetrain components are functioning to propel the vehicle forward.

Final Thoughts

If you live in a part of the world that experiences heavy rain, icy roads, or snowfall in the winter season then driving a 4-wheel drive with the 4WD-Auto option is perfectly suited for this.

Drivers of 4-wheel drives with this option report the lag time between when traction is lost to when the auto system kicks in hardly noticeable. When snowfall gets really deep drivers simply engage 4H until driving conditions improve. This is perfectly fine since there is enough slippage on snow and ice to prevent drivetrain binding.