There is another issue to consider on this. On many manual transmissions you cannot tow more than 50 miles with the rear axle on the road without damaging the transmission.

The reason is that the rear bearing is lubricated by the flow of oil within the transmission. This flow is caused by the input gear driven by the engine. If the engine is off then nothing drives this flow and the rear bearing eventually dries out and damages itself if driven by the rear axle.

It could be that Ford designed the PTU's lubrication to be driven by the input gear rotation. Thus there is the possibility that pulling the fuse may prevent this flow and result in damage to the output bearing.

Don't know enough of the internal design of this unit to say for sure, but there may a good basis for Ford not installing a manual switch to disable the AWD feature when desired.

I'd be interested to know if PTU failures seem to follow those that pull the fuse to get better mileage.

 

The rear drive clutch that "the" fuse controls is downstream of the PTO/PTU, within the rear differential case, actually. So, provided you don't need the additional drive traction, only "good" can come of the fuse removal.

Without a serious level of public education a manual switch, electrical switch, would only enable the operation of the current system when what is really needed is some way to temporarily(***) engage the rear driveline.

My '94 R/awd Ford AeroStar normally has 30/70 F/R engine torque apportionment. Upon detection of disparate F/R driveline rotation, presumption is rear wheelspin/slip, it automatically switches to 50/50 mode for a short period of time. If wheelspin/slip thenre-occurs it runs in 50/50 for yet another time period. Repeat as needed/required.

*** That's also the way the "old" Ford Escape "automatic" F/awd worked, upon front wheelspin/slip detection it would automatically engage the rear drive for a short period of time.

 

wwest: RTP, you completely missed my point. You should refrain from such posts until you understand what was said.

Once upon a time I shuddered when teachers wrote RTQ for Read the Question on an exam answer.

I suppose that the functional equivalent is RTP, as in Read the Post.

 

The PTU is "always" powered! If not, you wouldn't have 4WD available all the time. Pulling one fuse, both fuses if possible "only" defeats the operation of the 4WD module and/or the clutch in the rearend, not the PTU.

 

Then I guess you need to SIOMP, "spell it out more plainly" as I have read and re-read your original post and still see my response as appropreate.

 

Hi Bill,

I assume that you created this post because the thread title is more approprate than "Changing my PTU Fluid". I'll try not to retype anything that I've already written in that thread. (the key word is try ).

Just so I understand your question correctly, you are saying that by disabling the PTU and rear wheels, you are essentually "towing" your rear drive train. If so, that is a very good point and then you for posting it. If not, could you please explain what you are asking?

Assuming that my understanding of your question is correct, this is how I see everything working and I'm sure there are some who will disagree with me but that is the reason for these message boards, to debate ideas.

With the fuse pulled, what is being done is removing torque from both the ICE (Internal Combustion Engine) and electric motor to the rear wheels. Everything still spins, the rear clutch is just never commanded to engage.

Now some may argue that because of the fact that everything is still spinning that there is nothing to gain from keeping torque from going to the rear wheels but there is a gain from the reduction of friction. Every joint and gear creates heat from friction as it spins. When the force is changed, (the torque from the engine) the friction is increased. When the work is increased (the inertial resistance caused by the weight of the vehicle, Newton's laws of motion) the friction is also increased. By keeping torque from going to the rear wheels, there is no work being done by these gears and u-joints of the rear drive train other than the work required to turn them because the rear wheels are turning. Therefore the work done by the engine is less due to less friction and heat being generated.

But, because everything is still spinning, all of the systems are still being lubricated. Removing the fuse and disengaging the PTU is the same as driving 1,000 miles at 55 mph over flat land. All of the torque would be applied to the front wheels and the rear wheels would essentually be towed holding a constant speed. I live in an area with a lot of hills and my PTU failed because torque is being applied 40% of the time to the rear wheels which caused the PTU to overheat (construction season didn't help either ).

I understand what you are saying with the lubrication but I believe everything is still rotating and getting its proper lubrication so it is nothing to be worried about but if others have greater knowledge of this, please feel free to correct me. (corrections seem to be a abundant on this message board )

 

The clutch is in the rearend not the PTU.

 

In general wwest is on my ignore list. His words here indicate why.

I don't know enough about the internals of the PTU to say whether or not things remain spinning to provide lube if the fuse is pulled. It would depend on just how it's geared, how the electric coupling is activated, and what stops turning, if anything does stop, when the fuse is pulled.

To assume that because the input gear is turning the lube is good, is an educated guess. But still a guess, maybe right, maybe wrong. I have the shop manuals and they just don't give enough info to tell.

A discussion of various operating modes of other transmissions and transfer cases is bedside the point and not the issue. The issue is whether or not the circulation of lube inside the FEH PTU continues to be driven when the fuse is pulled.

So, in the end I was asking for folks that had a PTU failure to say whether they were doing the fuse pulling drill to gain better mileage when the failure occurred.

For me I won't pull the fuse until it's clear that there is no effect on lubrication inside the PTU. I've now got 65k on my FEH without PTU problems. Just changed out PTU gear oil because of a rear end collision (minor). No evidence of any unusual debris in the drained oil.

 

crabby bob: yes, when the fuse is pulled it doesn't stop the turning of the differential. As I understand the drive train this then drives the propeller shaft which then drives the output side of the PTU. So if the rear wheels are turning the output gear of the PTU is turning.

BTW I just changed out the gear lube in my rear axle unit and it is a standard, open differential. No trac-lok or positrac feature in it.

It is an assumption that everything remains spinning with the fuse pulled. You don't know that. I accept that it is a good, educated guess, and I think you're right. But I'm not going to bet my PTU on its correctness.

So if folks with a PTU failure could respond, that would give confidence to your thoughts.

 

[quote=crabby_bob;226110]


I live in an area with a lot of hills and my PTU failed because torque is being applied 40% of the time to the rear wheels which caused the PTU to overheat (construction season didn't help either ).


I hate to be "nitpicking" but where does this 40% value come from?