dd564 said:
So tell me,
If you have your truck in 4 wheel drive, and you don't have the electroni differential, aren't all 4 wheels going to turn anyway?
this may help:
What is four wheel drive?
As part of our guide for the beginner in the world of off roading we examine some of the basic concepts.
Everyone is aware of four wheel drive vehicles and they certainly have been the boom vehicle of the nineties, but how many people actually understand how four wheel drive works? Four wheel drives are generally characterised by a plethora of extra gear levers and switches many of which are a complete mystery to many drivers. How many people have seen expensive four wheel drive vehicles floundering in wet grass fields? This is usually due to a complete lack of understanding on how the vehicle works. This article will aim to resolve many questions and facilitate the reader to make an informed decision on four wheel dive systems. Most four wheel drive vehicles are in their road format actually only one wheel drive. This is because all road vehicles are fitted with a device called a differential (or diff for short). Diffs are essential in any vehicle for going round corners. When a vehicle goes round a corner the outside wheel must travel much further than the inside wheel, the diff allows this to happen. However the diff is a very lazy piece of engineering and generally hates working.
Effectively the diff will transmit the engine power to the easiest way possible. So if for example in a two wheel drive car one driving wheel is on dry tarmac and the other is on sheet ice. The power to the wheels will take the path of least resistance and the car will be left stationary with one wheel spinning on the ice. Not generally useful if you are trying to get somewhere
Most Japanese four wheel drives and early Land Rovers (pre 1983) utilise two wheel drive for the road and the driver can select four wheel drive for off road conditions. In these vehicles when the four wheel drive lever is engaged the front and rear axles are joined together and thus make power available to all four wheels, hence four wheel drive (astounding!!). Unfortunately Mr. Differential can be found in both front and rear axles and, as previously discussed, is still a very lazy person.
Consequently if we take the previous example of ice and tarmac and in this case both wheels on the left hand side of the car are on the ice while the right hand side is on good tractive tarmac. Four wheel drive is selected yet all that happens is that the left hand wheels spin and the vehicle remains where it is.
This is because the diff in the front is sending the power to the easiest route as is the one in the back axle. Thus our four wheel drive is in fact only two wheel drive.
Now if our situation had improved a little and only one of rear wheels was on the ice things would be a little different. In two wheel drive mode we would be sat with one wheel spinning ineffectually, so being bright people we select four wheel drive and as if by magic our vehicle would be propelled forward. But how?
Effectively what is happening is that the front axle is dragging the car along as this has adequate traction to both wheels. Thus all four wheel drive means is that you can afford to lose traction to only one wheel - disappointing isn't it.
Consequently Mr. Differential also has to be present in the middle. Now we have three diffs in the vehicle and all of them are idle and lazy. Each one wants to transmit the power the easiest way. Each axle wants to transmit the power to the easiest wheel and the middle or centre diff wants to transmit the power to the easiest axle. Consequently we end up with one wheel drive. Back to our previous ice and tarmac example and this time we assume that one rear wheel is on ice and the other three are on tarmac. The rear axle diff wants to send all of its power to the wheel on the ice and the centre diff wants to transmit all the power to the rear axle, thus the rear wheel on the ice spins and the vehicle goes nowhere. At this point we introduce the Differential Lock. Effectively the Diff Lock eliminates the diff from the equation, the centre diff ceases to work and becomes a solid device that transmits equal power to both front and rear axles. We are now in the same situation as described above with four wheel drive.
borrowed from here: www.worldoffroad.com/begin/beg4x4.asp
Scooter421 · updated 
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