Hi everybody, I was visiting my sister in South Carolina earlier this week and heard her Heat Pump buzzing while not in operation. The fixxer upper in me said, that doesn't seem right. A little googling and quick trial and error pointed me to the reversing valve coil. With the 220v pulled, the buzzing was still there and when switching the thermostat to off, it went away. Her unit is a 12-14 year old Nordyne FT3BC-036K and I located the model number for the reversing valve coil assembly from and online manual. I came up with PN 668770. I also read it could be the transformer and frantically searched locally for a 110v to 24v transformer and could only come up with one for a doorbell. I did actually get ready to install the new transformer but the one in the fan/heater enclosure was actually a 220 to 24v transformer fed from the board. So no ability to replace that while I was there.
I did some further googling and somehow came up with a cross reference (in an Ebay auction) of the 668770 Coil to PN669343 and was able to find that on eBay and the seller said he could hurry up and ship it out to me so I could repair it before I left her house to return to Chicago. Once it showed up, it was a good .5" -.75" longer (and actually the PN was 6693430) than the one that was on the unit (same diameter hole for the shaft though). I did some more googling to see if there would be any problem using a longer Coil on the valve and my results were inconclusive. The coil that arrived was a "Ranco L30-12 Used on V1-V12 50/60hz 6/4W N-3072 A02". The only differences I could find besides the length is that it didn't have a metal frame around it like the original. I really couldn't read anything on the old coil since all the letters were worn off and I can't even find the Nordyne 668767 anywhere on the internet to even research or buy!
I tested the new coil while energized and placed it on the stem and could hear the valve actuating. So I thought, I should be good.
Not wanting to put everything back together without fixing it, I put the new coil on, and bought a little bit longer screw and used a couple washers and a lock washer so I could make it snug without bottoming the screw out in the stem.
AC ran fine the rest of the day, the next day and a third day. On the fourth day, I got a call from my sister saying the Thermostat was blank. At first I thought it had to be a huge coincidence and told her to replace the battery. She said there wasn't a battery and she was right. It was a Robertshaw 9420 and it ran solely off the 24v transformer. She doesn't have a Multimeter to check if she is getting 24v across C and R but my guess is that she isn't and that is why the thermostat is blank.
Sorry for all the lead up, but I thought more info was better in this situation. I did wiggle some of the wires connections on the AC control board because the thing looked like it had pretty significant aging and I wanted to make sure there was good electrical connection between all the connectors just in case that was part of the reason for the buzzing. If I was there, I could troubleshoot it and get it back going I'm sure of it. But from 1000 miles away, I am trying to help my sister from paying a lot of money because I wanted to get rid of a buzzing noise!
Did the slightly larger coil burn out the transformer or did replacing the coil finally push the transformer to die? Or is this all a coincidence and the transformer just decided to die 3 days after this repair?
