Tuesday, February 1, 2011

How to engage 4WD - example Jeep Grand Cherokee but this explanation is valid for all vehicles with part time four wheel drive

4WD and AWD systems explained

A word of caution first. Manufacturers tend to obscure the true nature of their 4WD/AWD systems behind phantasy names like "Active 4WD", "Quadra-Drive", etc. - others might call their automatic AWD "Real Time 4WD".

There is plenty of confusion about what is what. Does it matter? Can't you just call "four wheel drive" "all wheel drive"? Yes you could. If all eight wheels of your big truck are driven, isn't it all wheel drive? Yes it is.
However, there are so many different 4WD systems on the market now that it is important to be precise and specific. It is important to call AWD when it is AWD and full time 4WD when it is full time 4WD. Just to say 4x4 is not sufficient any longer.
Wrong terms lead to misinformation - intentional or not. Wrong terms might make you buy something you neither want nor need! Do your homework before buying a 4WD - to get what you really need. If necessary, ask me.

There is no sanctioning body who ever established the definition of terms regarding 4x4. The terms I use below are the ones used internationally by engineers and competent magazines.
4WD has been invented a very long time ago and various concepts have been tried. Here is a simplified synopsis of what is what. Links within this page provide detailed insight.

#1 Part time 4WD is a system that can only be used part of the time in four wheel drive.
This 4WD system was created to provide a vehicle with more traction to either carry higher loads and/or to travel in adverse terrain conditions. Clearly purpose built to do hard work. It can only be used for adverse terrain conditions - not for dry pavement.
Typical lever settings are 2WD, 4WD Hi, 4WD Lo (4WD Lo is missing on some newer cars like Dodge Nitro).
Very good off-road. Most competent when combined with axle differential locks.
• 2WD setting must be used on dry pavement.
• If 4WD is selected, all 4 wheels are permanently powered.
Prominent examples: 1942 Willys, Jeep Wrangler, Toyota Tundra, etc etc.

#2 Full time 4WD - also called permanent 4WD, can be used full time on all surfaces including pavement.
Full time 4WD
was created to provide a vehicle with more traction and to make 4WD more useful for everyday life. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time.
2WD is no longer available. Can still be a strong workhorse. Some rough terrain competence is retained - the priority is added stability as a safety gain for everyday driving.
Typical lever or switch settings are 4WD Hi, 4WD Lo. Very good off-road when center diff is lockable. Even better when combined with axle differential locks.
• All 4 wheels are permanently powered.
Prominent examples: pre 2006 Mercedes M-Class, Mercedes G500, LandRover, RangeRover, Toyota Prado, Lexus GX470.

#3 Full time AWD is similar to full time 4WD - only it lacks the slow speed torque enhancing low range feature. Can be used full time on all surfaces including dry pavement.
AWD was created as a safety feature for modern day vehicles. Not designed for hard work or off-road use. Rough terrain competence has almost vanished - focus is on added on-road stability and performance.
Limited use for off-road.
• All 4 wheels are permanently powered.
Prominent examples: 2006 and later Mercedes M-Class, Audi Quattro, most Subaru, pre 2006 RAV4

Eventually you will break parts - expensive parts.
All caused by driveline windup

broken transfer case due to drive line bind - axle wind up - driveline

The terms used differ from driveline wind up to axle bind and drive line bind, binding hubs, etc. - they relate to stress created by the use of part time 4 wheel drive on dry pavement. Fact is, that you can not use part time 4WD on dry high traction surfaces. Period. You'll get drive line windup. This has been known since the invention of 4WD more than 100 years ago and center differentials have been employed since then to correct the problem. Here is more about how it occurs.

Unfortunately many owners of part time 4WD vehicles don't know yet.

Then there are some "smart" guys claiming their truck being strong enough to handle driving on dry pavement with part time 4WD and the resulting driveline windup/bind. And it is true, some trucks are stronger than others - they break later than the weaker ones.

If you know about the stresses created by part time 4WD on pavement, you should never expose your truck/car to that kind of abuse.

The extreme tensions and pressures caused by the absence of a center differential (or caused by a center diff that has been locked) will break down the protective properties of all lubricants, eventually leading to expensive component failure.

The image above is the transfer case of my Jeep Grand Cherokee after accidentally driving for about 150 miles in part time 4WD on the freeway. It exploded at about 70 mph.

#4 Automatic AWD was created solely as a stability enhancing system. Auto AWD can be used full time on all surfaces including pavement. AWD only briefly activates automatically when stability threatening conditions arise. Essentially a 2WD car with 2WD handling characteristics. Absolutely no adverse terrain capabilities. Clearly built for added road stability and safety.
Not recommended for off-road beyond graded dirt roads.
• 2 wheels are powered most of the time
4 wheels are only temporarily powered.
Prominent examples: Volvo AWD, 2006 and later RAV4, Honda CRV.

A more detailed explanation of the differences between 4WD and AWD - read more..,.

So, is "real time 4WD" really 4WD? No, it is a sophisticated 2WD car with automatic asymmetric AWD. There is no commonly accepted standard how to name the different 4WD systems - companies and especially their PR departments use terms for 4WD systems very loosely - transparent consumer information is not their priority.

Many vehicles are offered with a combination of 4WD systems outlined above. Very confusing for consumers.

Here is more about why part time 4WD should not be used on pavement

part time 4WD should never be used on dry pavement:

You bought a Grand Cherokee , it is missing the owners manual and you are wondering how to shift into 4WD. And when. And why.

You can shift between 2WD, ft 4WD and pt 4WD at any speed back and forth while driving (since the Grand Cherokee does not have manual or automatic hubs you can always "shift on the fly"). There is no advantage in stopping. In order to shift into part time 4WD Low you must be at a standstill - tranny in neutral of course.

Since there are many other 4WD systems than the Jeep Grand Cherokee around - the shifting procdure can be different. I'll add more versions as time allows.

Which four wheel drive setting should you chose?
You can use the full time setting at all times on all surfaces. On snow and ice you can use either full time or part time. Part time has better climbing abilities on steep slippery driveways but it has slightly negative effects on the steering. So, to get to your house on the hill, use part time - to drive around town and on the freeway, use the full time setting. If snow gets really deep and you are going slow anyway, use part time 4WD - but be very gentle on the throttle, as the higher torque of low range makes the tires spin more easily.

Part time low is an exclusive off-road setting - don't use it on anything slippery (too much torque - not enough traction). And don't use it on dry pavement either. Same is valid for part time high. Do not use it on dry pavement! The damage will be very expensive.

I accidentally drove my Grand Cherokee in part time 4WD once. It made the transfer case explode. Including towing, a $2,000 goof.

Hubs (in case you stille have them) - to lock or not to lock?

Now, when to lock the diff(s) - to lock or not to lock?