Older article about PIU AWD system

[KayGee]

http://www.hendonpub.com/resources/article_archive/results/details?id=5091

"The AWD may distribute up to 100 percent of the torque to any one wheel, front or rear, left or right. For example, the left front tire may be the only one of the four with traction, so it will get all of the torque. Or it may be the right rear."

"While new to law enforcement vehicles, the AWD system used on the Sedan Police Interceptor and Utility Police Interceptor is not new. This AWD system has been used on the entire Volvo line since 1998: Volvo S60 sedan, Volvo V70 wagon, Volvo XC90 SUV. The AWD system from Haldex Traction is currently in its Fourth Generation."

"Tire Wear Tolerant

Some AWD drivetrains are so sensitive that you must have exactly the same tread depth/tire wear on all four times. If your times are half worn and you must replace one tire with a new one, you must actually replace all four tires so the trend depth (overall tire height-diameter) stays the same.

Not so with the Ford AWD system. The AWD controllers are tolerant of any tread depth differences. The PI Sedan and PI Utility can accommodate two new tires on the front and two half-worn tries on the rear, or one new one or three new ones."

Raises some interesting questions:

Is the AWD system really as sensitive as quite a few around here keep saying (i.e. must stay within 3% tire diameter variance, must not mix worn and new tires on different or same axle, etc...)? Article seems to imply it's not sensitive at all and can handle worn and new tires on same axle or different axles. Who's right?

How much of this system crosses over to the retail explorer (none, some, all)?

Sometimes AWD Isn't Enough - Thinking back, I kind of wonder how that PIU was unable to get "unstuck" if the AWD system can truly transfer 100% of torque to any one wheel.

 

[MarioMatos]

Ford Explorer Used Intelligent Torque Controlled Coupling (ITCC) Almost All The Time Is Working (FWD) And When It Detects Demand Locking Center Coupling F 50;50 R Its Not Full Time

It May Send 100% Of The Torque To A Tire Via Abs And Traction Control

 

[kobresia]

I think all manufacturers are prone to overstating the performance of these AWD systems. Although Ford's is good, it's not magical.

In the predicament that particular car is in, it's probably the weight distribution wreaking havoc with the AWD's ability to read the conditions. That also assumes it is an AWD as implied, and not a FWD from the earlier PIU years. I don't think it matters how intelligent a system is, it's going to struggle with a wheel up in the air and the vehicle close to teetering. The only thing that's going to work to move a vehicle under its own power without assistance or jacking is either a manual override for the tires you want to have traction, or just having all the diffs locked-- both axles and the center diff (if applicable because few vehicles seem to have a proper transfer cases anymore).

To discuss the AWD testing as outlined in the article, the most brutal traction test is climbing a 17% grade in a split of traction & soft or slippery surface with all wheels having weight on the ground. While weight distribution will factor in more than on a perfectly flat surface, that isn't that big of a deal; that's an 8.5 degree slope, or the front tires of the vehicle being about 1.5' higher than the rear tires over the Explorer's wheelbase.

Part of my driveway is much steeper than that, even when it gets iced-over, I generally have no problems climbing it. I sometimes have difficulty stopping when going down the grade when there's ice or deeper snow. When there's snow deeper than about 10", that's when things get interesting even trying to climb the hill, and that's primarily due to poor ground clearance and me not wanting to risk ripping-off the bumper cover while thrashing around in deeper snow.

I think the manufacturer is always going to recommend using the same size tire all around an AWD so that cheap individuals or fleet shops don't test the limits of tolerances. Even if it doesn't damage the drivetrain, tires with different diameters are going to degrade the vehicle's handling.

I agree that it would be nice to know how much is shared between the PI vehicles and the civilian market AWD systems, I'd guess they're mostly the same from a physical standpoint, the PI versions probably just have some programming tweaks.

 

[Odrapnew]

I question the ability to transfer 100% of the power to one wheel(particularly a rear wheel on the Ex).
I haven't been able to find a diagram, but in order to transfer 100% power to the rear, it would require a central differential of some sort.
Does the Explorer have a central diff(maybe built into the transmission)?
How is the power transferred to the front wheels vs. the PTU to the rear?

 

[peterk9]

I question the ability to transfer 100% of the power to one wheel(particularly a rear wheel on the Ex).

I agree. I don't believe it is possible with the 5th generation Explorer. There is a video in this Forum that shows an Explorer in a position with one wheel suspended and the vehicle in a rough spot but the Explorer was able to extricate itself. I believe it is in one of the threads on off road capability.

Peter

 

[kobresia]

The transaxle looks like a normal FWD one which has the PTU bolted onto the right side. The PTU almost certainly has a tiny differential in it, one which may disengage electronically, which allows the AWD Explorers to be FWD until additional traction is needed.

What I think the claim is based-on is the automated braking component of the traction control system. Torque will follow the path of least resistance. Take the most simple 3-differential AWD system, one with no limited slips or locking of any sort, and you have a vehicle which will be unable to move if any one wheel has lost traction, since that one wheel will be the path of least resistance so all torque will go into making it spin. Add the automated braking system which has the ability to lock any wheel instantly, and it makes the weakest AWD system become the most effective as long as the computer is able to to keep up with blocking all the paths of low resistance.

 

[omar302]

As far as I know, Ford's AWD system is a simple open diff behind the transmission, and the an electronically controlled clutch operated coupler just ahead of the rear differential which controls how much power is given to the rear wheels.