Our Ford Flex had been having an occasional problem with the transmission slipping out of gear (engine revving, but no power), and in each case my wife was driving and was able to continue after pulling over, shutting the car off, then starting it again. No check engine light or anything helpful like that. After some research, it sounded like the likely culprit was the transmission range sensor. This sensor is attached to the shift linkage, and gives feedback to the computer to indicate which gear (or "range") is selected. Seems like a needless complication, and to make matters worse the Flex has it mounted inside the transmission and the solenoid body and valve body need to be removed before the sensor can be replaced. I could not find one single writeup or video or any pictures online, of anyone doing this themselves. So this seems to be the first post about Ford Flex transmission range sensor DIY replacement. Be forewarned, this one is not for the novice. If you don't have the factory repair manual, or an alldata subscription, do not bother attempting this. If you have plenty of shadetree wrench-turning experience, are patient, and take your time, you'll do fine. There is an easy electrical test you can do to determine whether the transmission range sensor is faulty, in fact Ford issued a TSB about it.

Here is a pic of the new sensor assembly. I couldn't find any pictures of the sensor assembly online that definitively, absolutely stated that this actual sensor assembly was for the Ford Flex. Well here it is. It looks nothing like what I expected:

The writeup that follows is NOT a complete procedure for you to follow, without the aid of proper documentation. I am going to mention a few specific things that may help you, but you NEED to have a proper procedure to follow... I am using alldata for both our Flex and my R32. To start with, the basic disassembly of components in the engine bay, to gain access to the front of the transmission:

So you can see what sort of room there is to work with. It's a bit tight, but reasonable. You will have to move the coolant hoses up and out of the way in order to remove the dipstick funnel... it needs to be rotated 90 degrees and then pulled straight out:

Even though the official procedure has you completely remove the upper transmission oil line, I disconnected the end going up to the top of the transmission, and that was sufficient for me. It did make access to a couple of the bolts a bit tricky though.

The shift cable needs to be disconnected at some point, so that you can remove the lever from the top of the transmission range sensor (part of the sensor assembly protrudes through the top of the transmission, and this is what the lever attaches to). Before the sensor assembly can be removed, the solenoid body and valve body need to be removed. The solenoid body is attached with bolts of varying lengths, so it's important to keep track of them. It's impossible to tell which bolts are holding the valve body itself together, versus those that are holding it into the transmission, so the official procedure will again be necessary for this.

You will need to pry out the retaining pin that is holding the sensor assembly in place. You won't be able to see it without a mirror, you should be able to feel the head of it with your fingers. There isn't much to pry against, just a very small boss around the hole. A screwdriver won't work, unless you bend and grind it to fit. I made my own little pry bar to fit, and it was still pretty tough going. Eventually I got it worked loose and could finally see what the critter looks like. Here are pics of the little tool I used, and the pin itself:

When re-installing the pin (the official procedure says to install a new one, but I re-used mine), you can just use a short drift punch and the broadside of a hammer to drive it back into place.

Follow the official procedure for disengaging the sensor assembly from the transmission housing and the parking pawl. This is your final warning, do NOT attempt this without referring to the official procedure. If the parking pawl lever gets pulled out a bit too far, you will have to remove your transmission from the car and disassemble it to reinstall the lever. Take time to get yourself calm and collected before this step. It's easy to get into a state where your patience has worn thin, and you're left with the parts in a position where you cannot let go of them without risk of the parking pawl rod pulling loose. I will say that it's not quite as easy as the official procedure makes it sound. You'll need to be patient and carefully work at it.

With the sensor assembly successfully removed, you should be staring at a pretty empty cavity in the front of the transmission. In this picture here, everything has been removed, and the end of the parking pawl is just visible inside the orange circle:

There were no real tricks during reassembly, again just take your time and collect yourself before attaching the parking pawl to the new sensor and getting it into place. Pay attention to the torque specs and tightening sequences for all of the bolts you need to reinstall. Reassembly took me about a quarter of the time it took to disassemble everything. Altogether, I spent a solid day on it... from about 9am to about 5:30pm. First impressions after test driving are favorable. There used to be a strange noise shifting into 4th gear, and I could never quite decide whether it sounded more like a heat shield rattle or the muted sound of gears not engaging properly. At any rate, the noise is gone now, and I expect that there will be no more problems with it slipping out of gear.