When I bought my truck, my vents in the cab had resorted to default or defrost only.
To fix this, Ford calls for replacement of the electronic vacuum pump.
To put others' minds at rest, I put together a simple little write up on how to change the vacuum pump that fails on just about every Powerstroke Diesel out there.
QTY 1 of a I got this from Wabash Ford Truck parts
Use the multimeter and test Fuse #27, 15A in the Central Junction Box located at knee height under the steering wheel.
If the fuse is in working condition, and is not blown, remove the plug shown in the following picture.
With the key on, use the volt meter to test for voltage. You should see a minimum of 12vdc+.
If the meter reads at least 12vdc+, test the vacuum pump for resistance as shown.
Turn the vehicle off at this time.
(Good pump on the left, bad pump on the right)
A faulty electronic vacuum pump will read zero ohms. A working vacuum will read around 10.5 ohm.
If the pump and the pump circuit pass, refer to your nearest Authorizes Ford Service Center for help.
If the pump fails the ohm test, continue on with removal.
Locate the electronic vacuum pump with orange plug, as shown in the picture.
Remove the orange plug by squeezing in the tab(s).
Set the orange plug and harness off to the side to aide in the removal of the electronic vacuum pump.
Remove the small 1/8" rubber hose from the vacuum reservoir.
Remove the two 10mm head bolts from the fender that attach the vacuum pump to the vehicle.
Remove the pump from the vehicle.
Clean and inspect for reuse
You most likely won't be doing any inspection of the failed pump, with the exception of comparing the old pump with the new.
Transfer over the 1/8" rubber hose vacuum reservoir connection to the new pump.
With your clean rag(s), remove dirt, bug, and debris that have accumulated under the pump and bracket.
Inspect the electrical connection for corrosion, exposed wiring, and broken connector tabs. Clean with electrical contact cleaner.
Lubricate the two 10mm head bolts with anti seize. Set them aside with in your reach.
Install the new pump into the vehicle.
Install the 2 10mm head bolts through the pump bracket, and thread into the fender.
Tighten the bolts. The bolts shouldn't require more than 5-6 ft lbs.
Connect the 1/8" rubber hose to the vacuum reservoir nipple.
Connect the orange and black plug to the vacuum pump.
Turn the vehicle on and verify that the vents in cab are operational to all positions.
An in depth look at the failure
I decided to take a look at why my pump failed. Turns out the motors in these pumps are the culprit.
That big gold / yellow looking cylinder is the motor. My pump smelled something to burnt plastic inside, even though the armature of the motor still turned.
In this motor you can see the pump diaphragm to the top right, and the micro switch on the bottom left.