Anyone who has any experience at driving a 4WD vehicle will tell you that it is NOT a good idea to leave the 4WD system engaged, even on an ice/snow covered roadbed, once underway.

"..What good is 4WD if it only works below 25MPH????..."

Insofar as I can read, research, all of the vehicles making use of the relatively new electromagnetic rear engagement clutch system only have it engaged for low speed acceleration, or low speed "tight" turns wherein directional control might otherwise be threatened. The desire, obviously, is to put the system into use, pre-emptively in use, at the most likely times that wheelspin/slip is likely to result from engine torque application.

Note that the above procedure is used regardless of actual roadbed conditions. It should be obvious to all that were this procedure used once underway on a highly tractive surface the driveline components would then be put under continuous stress due to minor imperfections in tire circumference and/or inflation level, not to mention the F/R tire rotational disparately resulting in driveline windup and/or tire scrubbing in a turn or on a curving roadbed.

One other point, should the driver use the brakes or if Trac or VSC activation is required the F/R drive coupling will be INSTANTLY disabled.

This is a MAJOR short-coming of ACTUAL 4WD systems wherein a dog(spline)-clutch is used to lock the center diff'l. With the center diff'l locked the functionality of ABS, VSC, and Trac will be compromised, in some cases so severely that the function is automatically disabled.

Think/consider, for a moment, how the Porsche Cayenne makes use of this very same electromagnetic clutch system, but to dynamically engage/disengage the front drive in the same "linear" fashion.

Like the F/awd systems, both front and rear diff'l are driven during low speed acceleration. The first real difference arises with turning, the tighter the turn and/or the higher the acceleration level in a turn, the lower the torque level coupled to the front diff'l becomes.

So, unlike an equivalent F/awd system, loss of directional control arising from the application of engine drive torque to the front is NEVER an issue for the Cayenne.

Whereas with the very same implementation for a F/awd system wherein the front drive is ALWAYS engaged the HAZARDS, FWD HAZARDS, are ever present.

 

"...Ford's works at highway speeds and has done so for years.."

Would you be so very kind as to point me to, or provide a link to, the documentation taht supports this statement.

I would agree that Ford's F/awd system does not NOT work at highway speeds, but I would not agree that it is even ever enabled at those speeds. To do so would not only be detrimental to the service life of the driveline components but would also be patently UNSAFE.

Think about why the rear drive coupling is automatically disengaged when you use the brakes.....to allow for ABS functionality...

The use of Trac braking has the very same requirement, if the F/R rear coupling remains "locked" then the functionality of Trac and/or Stability control will be compromised.

 

How about a link to documentation that Ford's doesn't work above 25MPH.

 

I've owned four 4WD's since the mid-seventies before my Escape and don't agree with anything in your post which reads like a riddle anyway.

 

That would be ANY FEH AWD owners manual

 

I have access to all manuals and have read the 4WD section several times. There is nothing about 25MPH operation limit. Your statement is totally incorrect.

 

It's more true if you assume that all 4WD systems have locked differentials like they used to before the 1980s.
If you can't comprehend a full time AWD system with open diffs like Ford's i-4WD, then you can make all kinds of assumptions to fit your understanding of what's a 'real' 4wd.

 

I had full locking 4WD in my '97 Ranger, it had 4Hi/4Low range.

There is a speed limit factor for Traction Control but that isn't 25MPH either.

 

The current Escape "4WD" system uses simple open diff'ls front and rear but there is NO center diff'l. Instead there is a simple ring and pinion PTO, Power-Take-Off. Mounted within the front of the rear diff'l housing is an electromagnetically controlled clutch assembly. This clutch is operated "open loop" using PWM, duty cycle modulation of electrical power.

"Open Loop": There is NO feedback to the rear clutch controlling ECU as to the actual position, coefficient of coupling, of the clutch disks resulting from a change in duty cycle.

Using this method the level of coupling to the rear diff'l can be made to decline somewhat linearly as road speed rises during low speed acceleration. The second use is also somewhat linearly increase torque coupling to the rear when in a low speed turn. The tighter the turn and/or the more acceleration in a turn the more torque is routed to the rear so as to leave more front roadbed traction to the task of maintaining directional control/stability.

"...If you can't comprehend a full time AWD system.."

There is NOT, cannot be, a FULL-TIME AWD system in the sense commonly recognized by the public, the statement is TOTALLY misleading.

But you can have an AUTOMATIC "PART-TIME" AWD system, i-4WD.

An AWD system that is ENABLED FULL-TIME but only activates the AWD functionality PART-TIME. While it is, would be, highly desirable to have the AWD functionality be only when traction is actually "threatened", no method, no "pre-emptive" method has been found for doing that.

So the next best thing would be, is, engaging the "part-time" AWD system automatically but only at times when low roadbed traction would most likely be an issue.

The trade off, of course, compromise, is that driveline windup, tire scrubbing, must also be kept to an absolute minimum, given that the clear majority of the time the vehicle will be operating on a highly tractive surface.

So why would i-AWD engage the rear drive at all once the vehicle is traveling at a reasonable road speed when the probability of wheelspin/slip, absent a lead-foot, would be minimal.

 

Since ABS/VSC/Trac cannot be functional with a locked center diff'l they are always automatically disabled. The implementation of traction control on a RWD vehicle is an entirely different matter vs FWD and/or most F/awd systems.

I believe, or suspect, that all RWD vehicles with traction control have the ability to turn it off. Only recently have the manufacturer's of F/awd systems come to the realization that the "off" function is a necessary EVIL.

Operating a FWD or FWD torque bias vehicle on a slippery surface with no traction control can be a pretty hazardous situation.

I have not said that the traction control braking and engine dethrottling functions are disabled at any speed, only the rear clutch coupling capability.

And take a moment to think about why ABS/VSC/Trac is automatically disabled with a locked center diff'l/transfer case in a "true" 4WD system.

Wouldn't the engagement of the rear drive in the Escape with traction control activation suffer from the very same problem.

My guess would be that the i-AWD system will disable any level of rear drive coupling the instant the brakes are applied, stability control, or traction control activates.