I pulled the 4x4 fuse. No wrench, no "Stop Now - Safety" messages and no diagnostic codes. The one indication that I've pulled the fuse is when I start the engine, the info center displays "Service 4x4". Pressing the "Info" button and then the "Set" button clears this message (the display returns to "Miles Till Empty") and it doesn't come back until I turn off and start the engine again.

The driving characteristics are somewhat different. It reminds me of how it handled when my PTU was bad before it was replaced. Slamming on the brakes yeilded the same stopping ability and control. Slowing down for a stop sign, it seems like I need to press on the brake pedal further but this was also similar to when my PTU was bad. It also feels like I have more EV acceleration, measured by sitting at a dead stop and releasing the brake. Finally, with the first few days of driving with the fuse removed, I've noticed a slight increase in MPG, which is also something I've noticed when my PTU failed.

What I found most interesting is the front seat heaters are on the same fuse as the 4x4 system. A couple of years ago I was going to remove the front seat heater fuse because when my kids climb in and out of the car while playing they often hit the seat heater switch, which obviously isn't needed in the summer (I always place my laptop bag in the passenger seat on my way to work and every-so-often I would notice the bottom of my bag is hot and then notice the passenger seat heater is on). At that time, I didn't remove the fuse because the 4x4 was on the same fuse.

I wonder if the Ford Engineers did this on purpose. Both the 4x4 and the seat heaters are not needed in the summer. The fuse is one of the most easily accessable being on the end of the bottom row. Removing the fuse doesn't trigger the MIL to turn on. There is only a subtile reminder on the info center when first starting the engine (similar to the low fuel warning).

I just had my PTU replaced and I did notice that my MPG's dropped, obviously because the new PTU needs to be broken in, and perhaps that is the increase in MPG that I noticed (with the PTU disengaged). I know that most have stated that the rear wheels are seldom used, but I live in an area with a lot of curvy roads and a lot of hills. Whether I am using cruse control or not, my rear wheels have some power going to them 40% of the time (which must have contributed to the premature failure of my PTU). Also, while in EV mode, I have at least 10% power going to the rear wheels almost all of the time (per SGII).

What I do know is this, there is a little over a mile of gradual incline to my house with a couple of stops where the speed limit allows me to be in EV mode all the time. With the 4x4 fuse installed, I couldn't make it to my house without the combustion engine engaging. My battery charge would go from 55% down to 40% before I would reach my driveway. With the 4x4 fuse removed, I am able to make it to my garage and park with 42% charge left.

If anyone has a AWD FEH or a MMH, it would be interesting to know what your experience is with removing the 4x4 fuse. Right now, my plan is to leave it out and reinstall it for winter and remove it every summer. In my '07 MMH, the fuse is in the passenger side kick panel in the center column and it is fuse number 35 (bottom row closest to the engine, 5 amp fuse).

 

There's only one FEH owner here that I know pulled the 4WD fuse to save MPG on that was on a '08 and it only powers the 4WD module and the "wrench" icon came ON. Same goes for my '09 FE, so the '07 might be different. There are two 12V inputs for the 4WD module on '08/'09. Not sure about the '07. Is there two?

Just looked at a '07 MMH owner's manual. You pulled the wrong fuse. Should be #18 not #35 although it probably still disables the 4WD. On a '09 FE/FEH pulling that second fuse would disable the power steering!!

 

You are correct that both #18 and #35 disables the rear drivetrain and at least in the '07 MMH, it should be #35 that is pulled as pulling #18 causes the MIL to turn on.

In my product design experiences, often the constraints of the marketing product specifications dampens design engineers ideas and findings and design engineers find ways to incorporate their discoveries in a manor which conforms with the marketing product specifications.

During design of the FEH, marketing wanted a vehicle where the hybrid system was transparent to the owner and fit the regular FE drivability. The product design engineers perhaps discovered that disabling the rear wheels in a AWD vehicle would yeild greater MPG and decreased PTU wear when not needed and suggested a switch be added. Adding this switch was outside the product specifications and was denied. So the engineers incorporated the switching function in a fuse and coupled it with the seat heaters which is another system that is not needed in summer.

Then when the FEH was redesigned in the '08 model, the original design engineers were gone or on different projects and the idea was lost. The seat heaters were move to the climate controls and they moved the power steering in its vacated position.

Or it could be an amazing coincidence. At any rate, pulling #35 in my '07 MMH has yeilded great results.

 

"..perhaps discovered.."

No, it is generally well known, at least in the automotive drive train design engineering community, that having both the front and rear drives engaged, even partially engaged, continuously will result in so much driveline windup and tire scrubbing that the mechanical and tire life would be severely foreshortened.

And then the knowledge level of, or lack there-of, the general buying public must be considered.

Have a switch to turn on "AWD" mode...."..? Then you best have a serious public education program if you wish to sell it outside the "off-roader" venue. The general public is not even aware that the majority of the myriad of "AWD" designs, most especially F/awd designs, now available are not functional in the very conditions for which the vehicle was purchased. Then consider, if you will, that most of these F/awd designs are actually patently UNSAFE to life and limb when driven in those conditions.

Trust the general buying public, SUV buying public, to know when to use, and when NOT to use, an AWD "engage" switch, NEVER gonna happen.

These new F/awd systems are designed in an effort to somewhat alleviate the "patently unsafe" aspect of FWD vehicles. Ask someone with "first level" knowledge in the SUV community what's wrong with RWD or R/awd vehicles and they will undoubtedly respond with loss of control due to spin-outs or skidding.

Why doesn't the same thing apply to FWD and F/awd vehicles...?

Super sensitive TC (Traction Control), and VSC (Vehicle Stability Control) systems, that's why. And of course it helps mightily if at the time of threatened spin-outs due to excessive throttle you can automatically move a portion of the engine torque to the rear drive.

So now, rather than automatically derating (dethrottling) the engine in the lower gear ranges, you can have, and make use of, SAFELY make use of, engines in excess of 200HP in a FWD (F/awd) vehicle.

Detroit can now go back to selling what it's good at, selling HP. Ford SHO...F/AWD

EcoBoost = HP, NOT FE, actually to the detriment of FE since the EcoBoost engines must run in derated/detuned mode 99% of the time.

Just look at how, and why, the new Ford Explorer got castrated.

 

The fact alone that a FE/FEH owner thinks that they have a AWD shows one how the manufacture has BS'd the general public! The FE/FEH's that are still tagged as 4WD got it partially correct. They stopped putting the 4WD tags on the read liftgate, I guess they ran out of room!

 

I changed my PTU fluid twice in 103k miles. I just am getting my car back from the shop getting the PTU replaced.

I wish I had read about this post earlier. I am wondering if this was the culprit since it seem that the PTU needed fluid substantially before a general maintenance check.