Hi Phil,
The 4WD system you have is what Ford calls Control Trac. The Control Trac system uses an electrically controlled clutch in the transfer case to engage the front wheels. Normally in a typical part time 4 wheel drive system in 2-High mode, only the rears would be mechanically connected to the engine/transmission, with the front drive train freewheeling.
Now, keep in mind that when you turn, the front wheels must travel farther than the rear ones, since the actual turning circle for the outside front wheel is larger than for the inside rear (actually, each wheel travels at a different speed through a turn, and the difference depends on the tightness of the turn). This is important on 4WD vehicles. Keep it in mind throughout the rest of this description.
In 4-High, the clutch in the transfer case is fully engaged, and the front drive train is locked "in step" with the rear, that is, the front axle turns at the same speed as the rear. If you try to turn on solid ground such as wet or dry road surfaces, there is binding as the front wheels try to turn faster than the rear but cannot. Something has to give, and it could be the tires sliding and jumping across the pavement or it could be a broken axle shaft, drive shaft, or other part. This is why you shouldn't use 4-High on the road unless it is quite slippery.
The same holds true for 4-Low. A different gear is used in 4-Low to allow for steep hill climbing and descending in low range, but like 4-High, the front and rear drive trains are locked together through the electric clutch in the transfer case.
Now, in the Auto mode, things get more complicated. While the rear axle is the primary drive axle, the fronts do have some power applied as well. This is controlled electronically. The GEM module (I think that stands for generic electronic module) actually continually pulses the electric clutch such that it is engaged for short bursts, long enough to allow some drive forces to go through the front axle, but short enough to relieve the binding that can occur when turning. These pulses can occur several times a second.
Now, when traction is good, these short periods of engagement occur such that the front axle is engaged approximately 5% of the time. That is where Ford gets the 95% rear/ 5% front power transfer levels.
When things get a little more slippery, the system reacts. The wheel speed sensors that are part of the ABS system (anti-lock brakes) are used to detect rear wheel spin. If the computer senses that the rear wheels are turning faster than the front wheels, the computer automatically tells the clutch to be engaged for a longer portion of the time. This is done in 10% increments, up to a duty cycle of 100%. (It will go to 15% front/ 85% rear and so on until traction is regained.) The change begins to occur within about 1/5 of a turn of the loose or spinning rear wheel. By increasing engaged time in small increments, the system can apply just enough torque to the front axle to give you added traction, while allowing periods of relief for the drive train to prevent binding. These engage/disengage pulses occur several times per second.
Once traction is regained and the wheels are turning at the same speed (the rears are no longer turning faster than the front wheels), the procedure reverses itself, once again in 10% increments. This continues over a period of a couple of minutes until the power distribution is back to the 5% front/ 95% rear ratio.
I hope that I explained this reasonably clearly. I'm no expert, but I have been reading up on the system.
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Tom
[This message has been edited by Tom Wilk (edited 10-09-1999).]
