Originally Posted by jcain
flushing a tranny that late in the game is risky as you might clean out all the grime that's actually holding it together!
Baloney! Old wives' tale. The tranny engineers at Ford don't agree with you. Properly draining and flushing an automagic tranny will never cause a problem that wasn't about to happen anyway.
However, properly draining and flushing means using only MERCON V ATF. Never flush with anything except nice clean ATF. Those harsh flushing chemicals are the cause of most tranny problems that come up soon after a flush and thereby contribute to the old wives' tale.
I'm not up with the details on the tranny in my F-150, but I know a lot about the 4R100 tranny in my old F-250 diesel. The details of how to flush the 4R100 are at
The details of how to flush the tranny in the F-150 will differ by how much ATF is required and the location of the tranny cooler return line, but otherwise they would be very similar to the procedures to change the ATF in the heavier-duty 4R100 tranny. I've used these procedures to change the ATF in an older F-150 that had a 4R70W tranny with no problems. But the 6-speed tranny in the 2011-up maybe be slightly different.
The following link is a how-to on this website, but it applies to a 4R70w tranny too. The video shows a Magnefine in-line tranny filter in the rubber part of the return line near the front axle, but your truck may not have that filter.