Thanks to everyone who has responded to my mysterious root cause issue with my PTU.

To answer your reponses:

1. Here is an actual Ford hotline response to a PTU issue that a Ford dealer was grappling with:

"*Comment from: *Hotline *Comment Date:* 2/2/2011 11:46:41 AM

Andrew,

Please measure the circumference of all four tires, as they should be within 1/2" of each other. "

So....apparently there can be a PTU issue if the tires' rotating circumference is more than 1/2" of each other.

2. The flat tire occurred overnight while the FEH was in the garage. I apparently picked up the screw in the front left tire while turning around on a nearby gravel road. I did not drive on the flat although the car certainly was resting on a flat tire when I saw it the next day.

I am reading as many hotlines and FEH blogs as I can and will report anything new on this issue. It is still interesting that my PTU broke down precisely after a brand new mfg spare tire (with a new tread) was put on a FEH with one original, 72000 mile Continental tire (front right tire) and two newer rear tires - three different treads altogether.

I don't think this is a bogus problem although I agree that it is bogus, should the spare tire be an issue here, that Ford Engineering would allow that spare to be mounted in the first place or not fix something as ridiculous as having all four tires within 1/2" of rotating circumference.

 

It's not a bogus problem. I just think Ford hasn't put knowledgeable FEH people on your problem yet.

What baffles me about Ford's response is the tire circumference difference measurement thing just doesn't seem related to the PTU in any way. As I mentioned the PTU is a straightforward, simple right angle drive.

So we'll just have to see where Ford goes with this one.

Note that a difference of 1/2" in circumference is less than a 0.2" difference in diameter of the tire. I think Ford is going down a blind alley on this circumference issue.

Just doesn't make sense. But who knows, maybe they'll pull the rabbit out of the hat yet!

 

My son-in-law inadvertently put the wrong size tire/wheels on the front of their Chrysler T&C F/awd. Wheels he fitted had adapters so tire chains could be installed quickly and easily. They drove less than 50 miles before the PTO/PTU began shedding gear teeth.

The T&C used a standard right angle PTO with a VC in line with the rear drive shaft. I would have NEVER considered that a VC could tighten up the rear drive enough to have this result but the PTO got so HOT that it started coming apart and the gear oil stank so badly that someone called the gas company to check for leaks within a day of our discarding it in our trash bin at work.

When you're driving an "unbalanced" AWD or 4WD on solid, highly tractive, surfaces everything in the driveline is under undue stress, EVERYTHING. And lets not forget that the PTO is also a ~3:1 torque "multiplier".

 

When you put different size tires on the front versus rear axles on a 4wd...
One that doesn't have Full time 4WD capability...

No kidding it'll kill something in the power train. Without differentiation the windup kills something.

That point is irrelevant to the question on the table.

And I still think Ford is going down a blind alley on the tire circumference issue.

I have a Mustang. Lost a tire. Put on the do-nut spare. Drove it 200+ miles to get to where I could get it fixed. Was careful & didn't go over 55 MPH. No, repeat no, subsequent problems with that axle.

Inadvertently left my Suburban in 4WD and drove 80 some miles that way. Noticed some "smoke" coming out from under it on the road. Stopped and realized I was in 4WD. Corrected that dumb error. The "smoke" was the front differential fluid overheating. Caught things before any damage done & refilled axle. Still using that axle at 436,000 miles

 

Assuming that I understand correctly...You seem to be saying that since the Escape has "full time 4WD capability" it is not subject to failures due to disparate tire circumference, differing wheel rotation rates.

But in a very real, base/core, sense the Escape DOES NOT HAVE FULL TIME 4WD CAPABILITY.

The rear drive "link" is ALWAYS engaged during low speed acceleration, and/or starting out from a stop. So in comparison it might take an extended period of driving with disparate tire rotation rates as the drive train would be under just as much stress, throughout, during the periods when the rear drive is engaged.

 

 

The rear wheel link is always engaged under high speed acceleration.

 

I think what was meant was that the rear wheels are always engaged when pulling out from a stop. I have verified this with my SG.

 

Your thought is incorrect. Go on the interstate, press the accelerator when traveling at 65 mph and watch your scanguage change numbers.

 

My understanding is that any time a high level of torque(***) would otherwise be applied ONLY to the front drive wheels the rear link is engaged to distribute that torque more evenly over ALL roadbed traction surfaces.

I suspect that the answer as to whether it does that at higher speeds or not has to do with the level of "downshifting" the CVT/PSD must do in response to the level of accelerator depression along with road conditions, uphill, etc.

*** The rear link is also engaged if/when a large portion of the front tires' roadbed traction coefficient must be allocated to lateral, directional control, forces. While this may seem counter-intuitive to anyone familier with traditional 4WD vehicles there is no choice when dealing with a base FWD vehicle.