6HP26 Fluid Change at Dealership
I created this post as a heads up to all have the 6HP26 transmission. To my knowledge, the 2005-2008 Navigators have this 6 speed transmission, but not sure if/which Expeditions have it.
I recently purchased a 2005 Lincoln Navigator and it has 94,000 miles on it. Knowing that preventative maintenance is important and knowing that the vehicle had a special transmission that was also put in other cars like certain Jaguars, I decided to take it to my local Lincoln dealership for the service since I figured they see them all the time and know how to do the job properly. I was wrong.
I dropped it off and picked it up later in the evening after they had done the work, they had also flashed an update to the PCM while it was there. Driving it around, I noticed that it would almost seem to slip followed by a moderately hard shift going into second gear from a stop. I also noticed that when on the highway, when going to pass someone it would very harshly downshift into a lower gear. I knew something wasn't right, but thought maybe it's the transmission "relearning" my driving habits. I called the dealership and they said to keep an eye on it and if it doesn't go away then bring it in. Well, it didn't after 3 days and I decided I better bring it in the next day.
In the meantime, I did some research. The 6HP26 transmission MUST take Motorcraft Mercon SP fluid. There is even a service bulletin from 2004 that was there to let dealers know that it is only to take that fluid and not be mixed with ANYTHING else. I looked at my printout from the service, the transmission fluid listed was "Mercon". I called them up, they said to bring it in. The guy who took the car in said that they use a "universal" ATF that works with everything. I had a copy of the service bulletin in hand, showed it to them and they thoroughly flushed the transmission and the lines/cooler twice to get all of the regular Mercon out of the system. Sure enough, the transmission immediately functioned properly again without slipping or harshly shifting gears.
I had tried to find stuff on the internet about incorrect fluid being put into the 6HP26 transmission but couldn't find much more that 1 post somewhere that said it's very bad to put anything other than Mercon SP into the transmission.
My warning to everyone with this transmission: If you are going to have any shop flush your system, make 100% sure that they are using ONLY MERCON "SP" ATF. There are no substitutes.
Hopefully this will be helpful!