Fixing a 24-Year-Old Air Conditioner

Yesterday afternoon my wife texted me and asked me to call her. I couldn't because of a meeting and so she sent me another message saying the air conditioner in our house broke. When I could finally call her, I had her run through some troubleshooting steps to see if we couldn't isolate the problem. Ultimately nothing we tried would bring the AC back on. As this is the really hot part of the year in Utah, cooling the house is very important.

I left the office and rode back to my boat trying to figure out what the problem could be. I thought about all of the steps I had my wife try and figured that we either lost the blower motor, the capacitor for the blower motor, or the computer that controls the blower motor. Based on some issues we had a few weeks ago, I figured it was a 95% chance the computer died. Using the power of a smartphone with a camera, my wife helped me figure out the part number for the old computer and find a replacement part on the Internet. The only problem is that I couldn't guarantee the arrival before the end of the weekend when I would have to catch a flight from Utah back to the Bay Area.

This morning I had my wife call around and see if she could find a replacement part locally. She did and it cost $175 while the same part can be purchased for $75 on the Internet. When it is 95 degrees outside, $100 is a small price to pay for being able to cool the house immediately and so I had her pick up the part.

This evening I boarded a plane back to Utah and my wife picked me up at the airport. We came home and I began working on fixing the AC. One would think it would be as simple as labeling all of the dozen or so wires and plugging them back into the new computer. Nope, the new board had a completely different configuration and only a few of the wires had the same labels.

I went to my desktop computer and looked up part numbers on the Internet. My old AC computer is part number ST9120G2008 and my replacement is ST9120U1011. I searched for a bit but couldn't find instructions on how to rewire the computer but I did find one clue: white wires are neutral. I found all of the neutral connections on the computer and started there. Next I found labels on the the old computer and matched them with the new one. That left only five unknown wires. Fortunately the instruction manual came with a very nice wiring diagram. I just had to recall all of my Electrical Engineering homework from a long time ago and I matched the remaining wires.

While carpenters like to measure twice and cut once, electrical engineers like to recheck their work and so I did. Then I checked it again. Once you let the magic smoke out of electrical components, they don't work very well after. I didn't want to have to spend another $175 for a wiring mistake. Once I convinced myself everything worked, I did a quick test. The blower motor started right up. I then tested the AC and it is now working to bring my house down from an unbearable 85 degrees to a more manageable 76.

I am not an HVAC specialist and normally I would call a repairman for such a job. In fact I have done that in the middle of winter when the heater stopped working. The problem turned out to be a $4 light switch. I still had to pay $120 for the service call. I figure at that rate, I can spend a bit of my own time and try to figure it out. Furthermore buying a new part through the repairman would run me about $300. I figure I saved myself around $245 doing the work myself. I also learned a lot in the process.

If you find yourself in a similar situation but have the slightest concern that you didn't correctly diagnose the problem, then call the repairman. If you think the Internet will help guide you through the fix, then give it a shot. You might just save yourself some serious cash.