All wheel drive explained

Why is it important to know how your all wheel drive works? First, it may appear that your all wheel drive system is not meant to be used on-road. For example, part-time all wheel drive cannot be used in non-slippery conditions - you'll have to drive this car in rear-wheel drive mode, even when it is raining or snowing - in the weather conditions where all wheel drive might be needed. Second, depending on the type of all wheel drive, your car behaves differently when driving and cornering in slippery conditions. You might want to know what to expect.

Don't get confused by the abbreviations the manufacturers use: "AWD" is not necessary a full-time all wheel drive, "4WD" is not just for off-road vehicles. There is a dozen of brands the car manufacturers are using to distinguish their four-wheel drive vehicles - "quattro", "4motion", and so on. None of these actually represent the type of all wheel drive system used on the particular vehicle.

Note: On this web site, when we describe details of the all wheel drive system used on a particular vehicle, we use the definitions that are listed here.

Part-time all wheel drive

This is a "temporary" all wheel drive system. In normal driving conditions, just one axle (the rear axle normally) is driven. In slippery conditions, another axle is engaged by the driver, whether by a lever or a button. This type of all wheel drive does not have a center differential - when all wheel drive is engaged, the front and rear driveshafts are mechanically connected and rotate at the same speed.

When a vehicle is turning, the front wheels travel greater distance than the rear wheels. Because the part-time all wheel drive system does not have a center differential, the front wheels cannot go faster than the rear wheels. This type of all wheel drive cannot be used on pavement. Turning on pavement (even on a wet pavement) with all wheel drive engaged causes transmission windup and increases the chances of the transmission breakdown. When all wheel drive is engaged, the vehicle heavily understeers and this can lead to an accident.

The all wheel drive mode shoud only be used on surfaces with low traction (mud, snow, ice, sand), for short periods, and at low speeds. In these conditions the transmission windup is eliminated by slipping of the wheels.

Note: "Part-time 4wd" mode of the Jeep Cherokee's SelecTrac transmission means "locking of center differential". Jeep's SelecTrac is a selectable all wheel drive system.

Full-time all wheel drive

This is a permanent all wheel drive or permanently engaged all wheel drive system. All wheels are powered at all times. The vehicles with full-time all wheel drive are equipped with a center differential that lets all wheels travel different distances while turning. This type of all wheel drive can be used both on and off road. In slippery conditions, the center differential can be locked, whether manually or automatically, depending on the vehicle.

When a manual center differential lock (available on off-road vehicles and some SUVs) is engaged, the transmission's behavior is similar to part-time all wheel drive, i.e. the front and rear driveshafts rotate at the same speed. The use of full-time all wheel drive with locked center differential is limited to surfaces with low traction.

In case of an automatic lock, a Torsen differential, viscous coupling, multi-plate hydraulic clutch, or similar traction device is used in conjunction with the center differential. When a wheel slip occurs (one driveshaft rotates faster than the other) the device locks the center differential and the torque is transferred from the axle that slips to the other axle that has traction. As soon as the wheel slip is eliminated, the device unlocks.

Some vehicles (Land Rover Discovery II, pre-xDrive BMW X5) do not have a locking center differential, but are equipped with an electronic traction control system (known as Electronic Differential Lock - EDL) on all four wheels. This electronic system detects slipping wheels by reading ABS sensors, then it applies brakes to the slipping wheels and the torque gets transferred to the wheels that have traction. While it performs well on slippery roads, the system cannot compete with a real mechanically locking differential when driving off-road.

Automatic all wheel drive

This is an "on-demand" all wheel drive system. Under normal driving conditions, only one axle is powered. When wheel slipping occurs (the driving driveshaft rotates faster than the driven driveshaft), a multiplate hydraulic clutch, viscous coupling, or other similar traction device locks and engages another axle. The torque gets transferred to another axle. As soon as the difference in the front and rear axle speeds is eliminated, the device unlocks and the vehicle goes back to the two-wheel drive mode.

The difference between the traction devices that are used in full-time all wheel drive and automatic all wheel drive systems is that the device used in automatic all wheel drive system replaces the center differential.

Advanced electronically controlled all wheel drive systems can be pro-active and lock the traction device even before wheels start to slip - the need of all wheel drive is determined in real-time, based on the information that is collected from different sensors (i.e. g-force sensor, accelerator pedal position, etc.).

Some vehicles let the driver to lock the multiplate hydraulic clutch manually when the driver feels that he needs all wheel drive engaged permanently and before wheels start to slip. For example, in Nissan X-Trail, this is accomplished by pressing a button on the dashboard console. In Subaru Legacy, the clutch is locked when the automatic transmission gear shift lever is at the position "1".

Selectable all wheel drive

In this category fall Mitsubishi Pajero(Montero) with its Super Select transmission, Jeep Grand Cherokee with SelecTrac transmission, and a few other off-road vehicles. Mitsubishi, for example, has in fact a full-time all wheel drive transmission with two wheel drive possibility. In Mitsubishi, the driver can choose between the 2wd mode, 4wd mode with automatic distribution of torque via viscous coupling (acts like the full-time all wheel drive ), 4wd with locked differential (acts like the part-time all wheel drive) and 4wd with low gearing (low range part-time all wheel drive).

1-15 of 18 Comments

Nzali Mwambuma

May 19, 2010 - 22:35

Subject: Questions

I have Lite ace Noah full time 4WD,how to use in mud road,the gear shifted in L or 2?

JOSE

March 05, 2010 - 00:12

Subject: jeep cherokee 1992

tengo un problema con mi carro, al conectar la doble traccion, al momento de que camina el carro se escucha un tromido en el tranfer. mi pregunta es se puede reparar el tranfer o lo tengo que cambiar por otro ?

Reply to JOSE

awd.ee

March 21, 2010 - 16:13

Subject: Re: jeep cherokee 1992

I don't know, please talk to a car repair specialist.

JetJet

February 15, 2010 - 06:28

Subject: Chevrolet van astro AWD

tengo una van chevrolet astro 2000 y se desconecto el AWD, (se encendio el anuncio de SERVICE AWD) quien me prodra orientar donde me la pueden revisar, o donde puedo conseguir informacion para su reparacion, agradecere sus comentarios
SALUDOS

Reply to JetJet

awd.ee

March 21, 2010 - 16:12

Subject: Re: Chevrolet van astro AWD

Hi, please contact your Chevrolet dealer.

mike

January 10, 2010 - 07:26

Subject: four wheel drive

2001 suzuki grand vitara xl7 have four low, but four high wont work , not even the indicator light?

Darren

January 04, 2010 - 00:29

Subject: chain driven

Are all 2.5 v6 x type jaguars chain driven engines, or cam belt ? I am refering to a 52 reg

Reply to Darren

awd.ee

March 21, 2010 - 16:15

Subject: Re: chain driven

I don't know.

rae

November 28, 2009 - 22:17

Subject: all wheel drive

on my gt 2.8 it seems as if the awd is locked in all the time when i try to turn the car wants to go straight i have to almost idle round curve it has automatic trans

Reply to rae

awd.ee

November 30, 2009 - 10:41

Subject: Re: all wheel drive

What car are you talking about?

bahram zangeneh

July 01, 2009 - 07:58

Subject: inquiry

with an automatic transmition , when driving upan steepy hill, do you need to engage 2 or 1 depending on the power you might need ? or it work when you already engage D? for the downward driving it is understood to shift to 2 or 1. Thank you

Reply to bahram zangeneh

awd.ee

July 01, 2009 - 12:06

Subject: Re: inquiry

From my personal experience, modern automatic transmissions are smart enough to decide on their own which gear is needed when going up the hill. You don't have to engage 2 or 1 on just any hill. You can engage 2 or 1 if you know the hill is too steep and if you are towing something, or driving off-road and you don't want the gearbox to switch to the 3rd gear while driving uphill.

For example, you drive off-road, up a steep hill with gearshift lever at D, and at the end of the hill the electronics decide that it can switch to the 3rd gear. But after that maybe you lose some power, and the gearbox switches to the 2nd gear again. If you feel that you don't need that switching back and forth and you loose speed because of that, so you can put your gearbox lever at 2. This prevents the 3rd gear from being engaged.

Well, it all depends on the road conditions and on the car you are driving.

Tommy Clark

June 07, 2009 - 23:21

Subject: ford excape 4 wheel drive

Would this type or drive work on muddy dirt roads
I am looking for a 4 wheel drive to be used on farm and occasionally hunting and 90% of the time on hard surface roads.

Reply to Tommy Clark

awd.ee

July 02, 2009 - 15:17

Subject: Re: ford excape 4 wheel drive

I don't have any information about this car right now.
Anyone can answer that?

ahmad z

March 25, 2009 - 11:59

Subject: Subaru AWD type

Does the Subaru AWD system be in the 'Full time AWD' rather than the 'Automatic AWD'? Reason being it uses the viscous coupling on some models too.