If you are curious as to the internal workings of 4 wheel drive auto and how it differs from full-time 4 wheel drive 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 like snow. We specify when it’s best suited to use the 4-Wheel Drive Auto option over 2H or 4H.
How does 4-Wheel Drive Auto work? 4-wheel drive auto functions by sending power only to the rear wheels until traction is lost. It engages the front wheels by activating an electric motor mounted on the transfer case which engages a metal fork that applies pressure to a set of friction plates inside the transfer case. This synchronizes the front and rear driveshafts by sending power to the front wheels converting it to 4H mode automatically until traction is regained.
Now that we have a much-summarised explanation on how 4WD auto functions let us now delve deeper into the actual driving scenarios that will require you to engage 4A as well as the similarities in functionality it has to a part-time All-Wheel drive system. Later we will demonstrate how the 4 Wheel drive auto setting is superior to 2WD and more convenient than full-time 4-Wheel Drive.
How 4A (4WD Auto) works
So we now have a very high-level explanation and understanding of how the 4WD (A) system works, and yes, if you guessed it works very similar to an AWD then you guessed right. To recap, the 4-Wheel Drive Auto system when activated will always be rear-wheel biased. This means all the power will be sent 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. Let’s look at the inner workings of the 4WD-auto drivetrain.
4WD Auto Electronic Sensors
So there are speed sensors on the transfer case that measures and calculate the speed of the output shaft as well as 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, one or more wheels will spin faster, it immediately engages 4-Wheel Drive. This is achieved in a matter of split seconds and is hardly noticeable to the driver.
The wheel(s) that loses traction will spin faster because power will always follow the line of least resistance. The sensors will then send a signal to the electric motor to engage the fork inside the transfer box activating 4H mode. Now power is sent to both front and rear wheels for improved traction and stability on slippery low traction surfaces.
Electric Motor
Once the signal is received from the sensors that traction has been lost, the electric motor is called to action to activate a fork inside the transfer case. The electric motor moves the fork onto the metal plates applying pressure and begins to synchronize the rotational speeds of the two driveshafts. The fork applies immense pressure on the set of tightly packed friction and metal discs inside the transfer box that are connected to the front driveshaft. Once enough force is applied to the discs it synchronizes the front and rear driveshafts rotational speeds in an attempt to regain traction that has been lost on either the front or rear wheels.
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 done in split seconds and not even noticed by the driver. There is zero driver interaction required either. This process is controlled by a computerized system and negates the need for the driver to engage and disengage 4-Wheel drive 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 4 wheel drive.
If you want to get a more visual explanation of how the internals of a 4-wheel drive auto works, watch the video below.
How does a Non-Permanent 4-Wheel Drive system differ?
Below is an extract from an article you can read here explaining the difference between Permanent 4WD and part-time 4WD.
Part-time or temporary 4-wheel drives 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 convert the vehicle to a 4-wheel drive. This is achieved by selecting 4H from a short lever or 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. Failure to engage the 4WD system on a low-traction surface will result in serious drivetrain component damage and eventually drivetrain “binding”
Extract from article: Difference Between 4WD and AWD
The part-time 4 Wheel drive almost seems dated compared to a vehicle equipped with 4-Wheel drive auto, since you have the robust ladder frame, off-road capability as well as all the advantages of an AWD (All-Wheel Drive) in the sense that you can utilize the vehicle 4 wheel drive system on high traction surfaces as well as low traction surfaces, very similar to an AWD. So it basically functions like an AWD, just with low range, Impressive!!!
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
This is actually a brilliant design since it requires no driver input, delivers the best mpg the engine can offer yet also gives you peace of mind that when traction is lost, everything is taken care of automatically. Pretty ingenious if you ask me, plus you have the advantage of low range and the diff lockers for challenging off-road driving. I recon someday all 4 wheel drives will be built this way. I fail to identify any flaws in this design. All the weaknesses of a semi-permanent 4 wheel drive have been resolved by incorporating all the advantages an AWD had over a traditional 4 wheel drive plus the added benefit of improved fuel consumption even over a permanent 4 wheel drive.
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:
- Wet roads
- Snow roads
- Sleek road surfaces
- Partially Icy roads
- 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 cause 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.
Next, let’s look at how 4-wheel drive auto compares to a 2-Wheel drive in snow conditions.
4-Wheel Drive Auto vs Full-Time 4WD in Snow
If you would like to see the advantages of 4-wheel drive auto in action, do yourself a favor and watch the below video.
The video clearly shows a major difference in traction and acceleration speed between 4WD auto and 2-wheel drive mode. In 2-wheel drive mode, the SUV could barely gain any traction from pull-off and struggled to get to the third pole. Severe fishtailing was also experienced when accelerating hard in 2H mode on the snow. In 4WD-Auto the vehicle was much more manageable and covered the distance quicker by a good few seconds. There was a slight lag on pull-off as the system engaged.
What was interesting was how full-time 4WD mode was faster than the 4WD-auto albeit by only a second or two. This is because the full-time 4WD or 4H option sends power to all 4 wheels all the time, irrespective if there is traction or not. The 4-wheel drive Auto first has to lose traction before it engages the front wheels for added traction and that is what cost it a few valuable seconds. The minuscule difference in time between 4WD and 4WD-A clearly demonstrates how quickly the system kicks in.
4WD Auto vs AWD
So there are many similarities between the 4-wheel drive Auto and the part-time AWD system. Let us now look at a few common denominators between the two.
| Semi-Permanent AWD | 4WD Auto |
| – Functions in 2WD Mainly Rear-wheel biased | – Functions in 2WD Mainly Rear-wheel biased |
| – Activates 4WD only until traction is lost | – Activates 4WD only until traction is lost |
| – Computerised systems monitor the wheel traction by means of driveshaft sensors | – Computerised systems monitor the wheel traction by means of driveshaft sensors |
| – Engages the center transfer box when traction is lost | – Engages the center transfer box when traction is lost |
| – Uses a set of plates to activate and synchronize drivetrains | – Uses a set of plates to activate and synchronize drivetrains |
| – Uses an electric motor or thick fluid to activate the 4WD system | – Uses an electric motor to engage a fork against tightly stacked metal plates |
Now that we know some of the similarities, let us look at the differences between the AWD systems and the 4WD Auto system.
The traditional permanent AWD 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.
Below is an extract of the article titled 4Wvs AWD which you can read here.
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 AWD’s. A multi-plate clutch system, on the other hand only activates once traction is lost in either front or rear wheels then deactivates to allow the vehicle to operate as a 2WD vehicle again.
This has a fuel efficiency advantage since there are fewer drivetrain components that are functioning to propel the vehicle forward.
Extract from article: Difference Between 4WD and AWD
Conclusion
If you live in a part of the world that experiences heavy snowfall and blizzards 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 is minimal, perhaps a second or 2. It is quite unnoticeable except for some minor torque steer. 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.
If you are getting heavy snowfall at night, then melting during the day, then freezing at night, It’s convenient to simply engage 4WD-A and leave it in auto and let the system do its thing when you hit a slippery surface. There have been a few owners on 4-wheel drive auto SUVs that have complained about a slight increase in road noise and light drivetrain binding sensations when making sharp turns.