PDA

View Full Version : Trane Defrost Control Board Energize "W" in cooling



jcmorris2
30-03-2007, 03:58 PM
Hello All,

Here’s a problem for which I have run out of ideas. The system is a residential 1.5 tonn split heat pump by Trane, model # 2TWB0018. The system has been in operation for a year and a half with no issues. Suddenly inadequate cooling. Closer inspection reveals the “W” circuit energized. Disconnecting an isolating circuits brought me to the defrost control board.

Easy fix right? There is an identical unit available that is operating correctly so swap the boards. Ok, did that. The donor board did the same error. The “bad” board operated correctly in the donor machine. So, fine, not the board.

Sensors bad? The paperwork and diagnostic charts were still in the units and the thermistors ohmed within normal limits. Ok maybe the pins are dirty. The board has a procedure to check to see if the sensor signal is good. It checks out.

Power? AC at 60hz measures 29v. Maybe a little high, but the other unit has the same voltage on it’s board and that one operates.

So here’s the only real clue I have. The board energizes the “W” circuit when “O” is energized and the “W” circuit is de-energized when “O” is de-energized. Also the “W” circuit is not energized when the unit is forced into the defrost test cycle.

My only thought was that there might be a partial short in the “O” circuit causing a voltage drop when current is pulled as would occur when the switching valve is energized. Of course there is no technical data provided for the board tolerances and I can’t find a number for the guys at Trane that would know.

Any ideas? Any phone numbers to Trane folks?



Thanks

goodguy
01-04-2007, 01:27 PM
jcmorris2, I don't have a lot of experience with Trane equipment but I am curious. Does 'W' energize the board through the thermostat, or is W sending 24v to evaporator to start electric heat for defrost cycle.? Is 'Y' energizing your compressor or 'W'?

jcmorris2
01-04-2007, 03:23 PM
Hello Goodguy,

"W" is the same as X2. It powers the heat sequencer that operates the heat strips. The unit will call for the heat strips on 2 occasions. First when there is greater than 3 degree difference between ambient and the set point at the thermostat. Second, when the unit is in defrost the Defrost Control Board in the outside unit will energize the strips to compensate for running the refrigeration circuit in reverse.

So to answer your question, "Y" only energizes the compressor and "W" can be energized from the thermostat or the DCB

goodguy
02-04-2007, 02:01 AM
Hey jcmorris2, sounds like a real brain teaser. :confused:
If I understand correctly, strip heaters are coming on with a call for cooling ("O" being energized). All I can suggest is if you determined the DCB to be functioning properly you must be getting a back feed to "W" through the thermostat or through circuitry within the air handler (relay, Sequencer).
Do the strip heaters stay on if you disconnect "W" wire from outdoor unit?
Is programming on the thermostat correct? There is always that guy that likes to take things apart and change settings.
Good luck to you and keep me/us posted.
Cheers:)

jcmorris2
02-04-2007, 12:31 PM
Nope, the strips are not energized if the wires from the DCB are disconnected. I replaced the thermostat to rule it out, that's when I figured out the problem was at the DCB. I ruled out the board when I swapped it out with a different unit. The only variable that I can see on that board is the "O" circuit. Yellow is energized during heat and cool as well as red and blue. It has got to be somewhere in that thermostat wire.

jcmorris2
04-04-2007, 03:29 AM
Guess what! The DCB has a capacitor on it that will keep phantom voltage on the "X2" wire. To properly test the circuit you must connect a relay and see if it has enough power to close. This unit did not, which indicated that it's functioning as it should. It appears that there was a short in the T-stat wire that was eliminated during troubleshooting. Everything works fine now.

Thanks for the help. --jeremy

analog
02-06-2007, 03:40 AM
Hello All,

Here’s a problem for which I have run out of ideas. The system is a residential 1.5 tonn split heat pump by Trane, model # 2TWB0018. The system has been in operation for a year and a half with no issues. Suddenly inadequate cooling. Closer inspection reveals the “W” circuit energized. Disconnecting an isolating circuits brought me to the defrost control board.

Easy fix right? There is an identical unit available that is operating correctly so swap the boards. Ok, did that. The donor board did the same error. The “bad” board operated correctly in the donor machine. So, fine, not the board.

Sensors bad? The paperwork and diagnostic charts were still in the units and the thermistors ohmed within normal limits. Ok maybe the pins are dirty. The board has a procedure to check to see if the sensor signal is good. It checks out.

Power? AC at 60hz measures 29v. Maybe a little high, but the other unit has the same voltage on it’s board and that one operates.

So here’s the only real clue I have. The board energizes the “W” circuit when “O” is energized and the “W” circuit is de-energized when “O” is de-energized. Also the “W” circuit is not energized when the unit is forced into the defrost test cycle.

My only thought was that there might be a partial short in the “O” circuit causing a voltage drop when current is pulled as would occur when the switching valve is energized. Of course there is no technical data provided for the board tolerances and I can’t find a number for the guys at Trane that would know.

Any ideas? Any phone numbers to Trane folks?



Thanks

You have a bad defrost thermistor brother, keep the faith