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Discussion Starter · #1 ·
I have a Trane XL 1400 heat pump with the model number TWY048B100A1. In winter the system frequently blows out cold air in the house; when it is really cold out, it seems to blow out cold air for 15 minutes every 30 to 45 minutes. I looked at the system closely and found that the emergency heat electric heating coils are broken and the fan on the outdoor compressor unit runs a defrost cycle about every 30 minutes for 15 minutes. During this defrost cycle, the compressor unit fan runs.
I found that there is a defrost control board with the model number cnt02941.
I also found a manual that explains that the outdoor fan should not run during the defrost cycle here: http://docslide.us/documents/heat-pump-defrost-control-book34-4101-07.html
Since the fan is running during the defrost cycle, it seems that the outdoor coils are not able to heat up to the required 50°F and the board uses a time override feature that prevents the system from being locked into a defrost cycle.
The manual states that if the fan runs during the defrost cycle, to replace the defrost control board in multiple places.
I replaced the board and ran a forced defrost cycle. During this test, the fan continued to run during the defrost cycle, making it seem that it is not the board. In other words, the new board does the same thing as the previous board.
Can anyone confirm that the fan should not run during the defrost cycle?
 

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When in defrost mode make sure that the board is sending 24vac to the air handler to turn on the aux heat.

Make sure the outdoor fan is wired through the board as it should be. Maybe someone did something stupid and bypassed.
 

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Discussion Starter · #4 ·
Thank you for your reply. Yes, when I jumper the test_common pin to the FRC_DFT pin on the control board about a second later the compressor unit's reversing valve solenoid activates and it makes the normal loud noise where the refrigerant switches direction. The large diameter pipe going into the house that is normally warm (around 120°F), then drops to about 30°F - 40°F and the small diameter pipe becomes warm to melt any ice off the outdoor coils. When I tested it today, it was about 50°F outside so the system stayed in defrost mode for about 30 seconds then switched back to the normal heating mode where the large diameter pipe going into the house returned to 120°F.

Do you have any ideas why both the old and new defrost control board are not turning off the compressor fan?
 

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Discussion Starter · #5 ·
Thanks for the suggestions!

I will check for 24V between X2 and B to see if it calls for aux heat.

I am not quite sure how to check that the outdoor fan is wired to the board correctly. I know there is only one YL wire connected to the PWM terminal on the board that leads to the fan motor ECM. There is a LOW_FAN test pin that should slow the fan down when it is connected, which it does not do on either board. However, doesn't the PWM terminal tell the fan motor when to turn on and at what speed, so if it wasn't connected correctly then the fan wouldn't turn on at all?
I will look to see whether this yellow PWM wire is connected correctly to the fan motor.
 

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Check to make sure the Yellow wire from motor terminal 1 to PWM isn't shorting against anything.
 

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Following with been, If the yellow wire to the fan motor is shorting to ground, its possible to have low speed fan all the time. It would also cause more defrost cycles the normal. If you disconnect the yellow wire from the Defrost control board, the fan should turn off. If it stays on, then look very carefully for shorts, or the ECM itself may have an issue.


 

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Discussion Starter · #9 ·
Thanks for all your replies and suggestions! I looked to see where the yellow PWM wire goes and found that it goes to a separate 6 connector plug that isn't plugged into anything.
The fan motor has a model number of D934 and it has 4 wires, brown/white and brown for a capacitor and white and black for power. This a single speed (non ECM) motor. The black wire goes to a double throw relay that also controls the compressor, so whenever the compressor is on, the fan is also on. I'm sorry for not looking at the motor type sooner. I had assumed that it was a ECM motor based on what the defrost control board manual calls for.
I have the idea to add a normally closed relay inline with the fan motor and use the X2 and B wires from the defrost control board to open the relay when it calls for aux heat. I found a relay with the model number 90-380 that looks like it will work with the 220v 1.7 amp motor.
Will this idea work?
 

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One of the outdoor fan motor wires should run through a relay on the defrost control board. Turning on and off with the compressor is not correct.
I'd wire it to OEM specs before I tried to change things around. Might dig yourself a deeper hole.
 

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Discussion Starter · #11 ·
There are only two small relays (K3 and K1 on page 27) on the defrost control board and they do not have terminals like my other heat pump defrost control board does that go to the fan motor. On the other working heat pump (with a CNT02938 board), it has a K4 relay that has a wire going to the fan motor to turn it off during defrost. On the board with the continuously running fan, it doesn't have a relay for the fan to plug into; only a PWM terminal. I think this is why the fan was wired to the same relay as the compressor.
My guess is that at one time, the ECM motor was replaced with a single speed motor and the defrost control board wasn't changed to work with the single speed motor. This is why the PWM wire that should turn off the fan is just leading to a disconnected connector.
Are you suggesting I buy a new defrost control board that has a relay for a single speed motor?
 

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I have the idea to add a normally closed relay inline with the fan motor and use the X2 and B wires from the defrost control board to open the relay when it calls for aux heat. I found a relay with the model number 90-380 that looks like it will work with the 220v 1.7 amp motor.
Will this idea work?
That would work so long as you understand the fan will turn off any time there is an Aux heat call, and not just from the condenser (defrost board) either, but also from the thermostat too.
 

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I actually looked through the manual and Trane literature on the fan control. It barely meets the definition of pwm. It's just Trane being more complicated then they have to be. It was a simple 2 speed on/off fan that was way too expensive.

A separate relay, (24v coil, min 220v, 2A contacts) will work just fine. Wire it to break the fan leg(s).

For the coil:
1-Disconnect the fan control, it's now useless. (if it's still there)
2-connect a wire from y to t1 on the defrost board.
3-connect a wire from t2 on the defrost board to the coil of the relay.
4-connect a wire from the other coil pin to c (b for this unit)

Cheers!
 

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Discussion Starter · #15 · (Edited)
I am fairly concerned with Bob Sander's comment and didn't think about that situation. If I understand him correctly, if the temperature in the house drops about two degrees below the set point, then the thermostat will call for aux heat and if the relay is added, will turn off the fan. This would cause the outdoor coils to freeze.
So, based on this, I think the best option would be to get a defrost control board that works with the fan motor that I have.

Looking at the different defrost control board in the manual, it looks like the CNT03716 De-Featured Control - Scroll 1-Speed PSC O.D. Fan Motor defrost control board would work, however, I am not sure what the "To L2 (MS-2)" wire goes to. Is this just the power wire going to the fan motor?
Also I am not quite sure what kind of compressor (scroll or reciprocating) I have with a TWY048B100A1 unit, but it looks like it is scroll from searching for the model number online.
Do you think this replacement board would work?
 

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Discussion Starter · #16 ·
For the coil:
1-Disconnect the fan control, it's now useless. (if it's still there)
2-connect a wire from y to t1 on the defrost board.
3-connect a wire from t2 on the defrost board to the coil of the relay.
4-connect a wire from the other coil pin to c (b for this unit)
1 - The yellow PWM fan control wire was already disconnected. It was just going to a disconnected 6 pin connector.
2 - y as in the Y yellow wire going to the thermostat?
3 - My defrost control board (CNT02941) doesn't have a T2 terminal?
4 - I can connect the other coil pin to pin b, the blue wire from the defrost control board. Should I disconnect the blue wire from the indoor thermostat at the compressor unit so the coils won't freeze if the thermostat calls for aux heat?
 

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Discussion Starter · #18 ·
I took Bob's suggestion to order a White Rodgers universal defrost control board and I just installed it yesterday. Now the heat pump works great and does exactly what it is supposed to do! The defrost cycle isn't lasting for 15 minutes anymore!
So, Bob, thank you for the suggestion and everyone else for your help in helping me diagnose the problem.
 
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