I post this in the hope that others may be able to better deal with a problem that seems to be somewhat common on Expeditions and F-series. The issue is the HVAC blower shutting off spontaneously. The cause is related to the solid-state blower speed controller that replaced the blower resistor bank in about 2010. The part is still called "blower resistor" just to add confusion. When my 2013 Expedition was new, I experienced one or two episodes in which the truck was parked in the hot sun with the AC set to "Auto". When the truck was started, the system came on with the fan set to max. The fan would begin to blow, and immediately turn off. Fan would not come back on until the key was turned off and the driver's door opened to release the accessory relay. Then when the truck was restarted I would quickly set the fan to a lower speed and the problem went away for then. I did some research and found a TSB about the blower motor harness. I have lost the TSB number, but it instructed the tech to cut the 4-way connector out of the harness and solder the wires, except that the ground wire was to be soldered to a new heavy-gauge eye that was bolted to the body rather than sharing a common ground wire as per factory wiring. I did all that.
Fast forward to summer 2018. Suddenly the same system exhibits new symptoms. Now the blower will run only on 1 or 2 bars (speed display has 6 bars) but if turned up faster the blower shuts off and will come back on as before if the accessory relay is powered down. But it also became apparent that the blower would cycle with no control changes, about 15 seconds on and 2 minutes off, then repeat. Testing showed power from the blower motor to the controller, but no continuity through the controller to ground during the failure. Control input from the head unit to the blower speed control was increasing in steps of about 1/2V per bar and holding steady at the moment of failure. So I installed a NAPA BR792 blower speed control. Same deal. Tried a second BR792. Same deal. Took the truck to Ford. It got a new speed control and new blower motor, both same pt. # and appearance as old parts. My modified harness was apparently good enough, it's still there.
So the problem seems to be high current draw in the motor causing failure of the controller. I failed to measure blower current during diagnosis, but it looks like replacing both parts is the fix. I hope this saves someone some time and money. My '90 F-150 is still on it's original blower motor and resistor...