The Back Story: (if you came for a solution, skip ahead to the solution section)
My wife and I bought a house last year with an old built-in Sub Zero 550. It’s a pretty neat unit, nothing quite like it on the market as far as counter-depth (something like 25 inches from wall to handle!) unless you want to replace it with another built-in unit and drop 8-10 kilobucks on it. But we started noticing some issues shortly thereafter – the drain tube had apparently developed a hole sometime in the past which the previous owner repaired with electrical tape, which didn’t prevent the water from leaking; so we’d occasionally get water dripping out of the bottom when the door was opened. I cut the broken piece and got some smaller flexible tube to attach to the drain trough and slipped the other end into the cut-off drain tube. All seemed to be fine for a while, until…
Fast forward a few months. I go into the refrigerator and notice it’s a bit warm; eggs on the top don’t seem to be quite as chilly as I’d expect them to be. I thought it might just be a fluke. But this happened a couple more times, and each time I’d bump the temperature control wheel up a notch. Soon though, the temp wheel was 10, the refrigerator thermometer I had purchased told me the top shelf was around 50°F, while certain vegetables on the bottom shelf were frozen solid. I would occasionally take the drawers out to have a look and find, to my dismay, a large block of ice forming at the bottom and the drain tube frozen solid. I surmised that perhaps the thermostat, which dangles out just above the drain trough probably thinks the temperature is near freezing in the rest of the fridge.
I started reading through forums, searching various combinations of “sub zero 550 temperature fluctuation” or “sub zero 550 drain tube frozen”, etc. and did find some useful advice, but nothing that was a silver bullet answer. I stumbled across a Sub Zero 500 Series Technical Service Manual (highly recommend reading through) and came to rest on section 3-4 – Refrigerator Air Flow:
I checked my serial number, and it looked as though by design, my evaporator fan was only intended to run when the compressor was running. But I started thinking: the problem was not that my refrigerator was not cooling properly, or that it wasn’t draining; the problem was the temperature disparity – a 20°F difference between top and bottom. What if the evaporator fan was always on, like in previous model numbers? Would that help maintain an even temperature and keep ice from building at the bottom? I had a theory, now I just needed to test it.
So what did I do? Taking a look at the schematic in the service manual, I saw that the switch on the right side of the refrigerator controls the evaporator fan. If you follow the gray wire on the diagram, it goes up to the starting relay; so it’s only on when the compressor is on. But my theory was to keep the fan on all the time, so I started looking for an easy way to make a connection:
and here’s the schematic view:
The easiest way I could find was to run a connection from the light switch directly next to it; the black wire on the light switch is the one I went from, which is the hot line. I didn’t make the most professional connection (a flying wire twisted around the switch post and shoved into the light switch’s hot line), but it was more just a test to see if it would work.
And work it did! Almost immediately, the temperature delta between top and bottom shelves closed to nothing; the temperatures never got above 42-44°F or so; I was able to dial my temp setting back from 10 to 6 and the temperatures are staying consistent; and the best part – within 4 days or so, the ice on the bottom melted on its own and the drain tube defrosted and is draining properly!
If you are experiencing temperature fluctuations and ice buildup in the bottom of your refrigerator, I would recommend giving this a try. If you are comfortable working with electricity (and you have taken the necessary precautions – looking at the wiring diagram for your specific serial number (I found 7 different configurations of the 550 in the service manual), double-checking which wires are hot and neutral even if the colors appear correct, unplugging the refrigerator before working, etc.), give it a try at your own risk! If not, ask a handy friend or an electrician, and reward him with an ice-cold beer from your evenly chilled refrigerator!
Good luck and I hope this helps someone!