The details in this instructable are specific to the whirlpool 6ED25 series fridges, but the principle is the same for any similar side-by side fridge.
Step 1: The Culprit
To control the airflow, there is a sliding door in the air diffuser that opens and closes to allow more or less air through. In the more sophisticated models, this door is controlled by an electric motor which is controlled by some electronics. As the refrigerator cabinet warms up the controller moves the motor to open the door and let more cold air in, and when the refrigerator cabinet gets too cold, the motor closes the door to stop the cold airflow.
In my case, the problems began a couple of years ago when we moved house. I decided that it would be a great idea to give the fridge a good hosing out to get all the nasty smells out before we put new food in it. That was a great idea, but I got water in the air diffuser, which froze and caused the diffuser to jam. After unjamming it with a bread knife (not the most recommended technique), it all seemed to work reasonably well until just recently. We were preparing for a party, and having the fridge door open quite a bit on a humid day got more water into the diffuser and it jammed again. At first the refrigerator was too warm, so I did the bread knife thing again to open the diffuser door up. That worked, but now everything in the fridge was starting to freeze. Not good.
After I got the diffuser out and opened it up, I found the culprit. The sliding door was broken.
Getting to the door is a little tricky. The styrofoam casing is made up of two parts that are glued together. I carefully cut around the join to separate the two halves, revealing the sliding door and motor.
Then to separate the motor from the door I slipped a knife blade under the motor block and released the four clips holding the motor onto the door by squeezing the clips together with a pair of needle nosed pliers. After a bit of fiddling, all four clips can be released and the motor removed from the door.
As I suspected, the sliding part of the door was broken.
Of course that sliding door isn't a replacement part, you have to replace the whole diffuser ($140 here in Australia), so I decided to use my 3D printer to make a new one. Oh, and beef it up a bit as well while I'm at it!
When I was satisfied with the new part (I printed 2 of these because the first one didn't fit properly), I reassembled the door and motor mechanism then reassembled the diffuser and glued the styrofoam case back together with epoxy. I didn't use a lot of epoxy, and it is vital to make sure there is no epoxy on the sliding door.
After reinstalling the repaired diffuser in the refrigerator and allowing the fridge to cool down, I checked that the diffuser was worked correctly by observing the diffuser door while changing the set temperature - setting it to the "coldest" setting made the diffuser door open, and setting it to the "warmest" setting made the door close - success!
My refrigerator is now at 4 degrees C, and my bacon is safe (until I eat it, that is!)