AC Condenser Fan Power Question


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Old 07-29-17, 10:56 AM
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AC Condenser Fan Power Question

I have a Trane XL16i outdoor condenser unit with a variable speed fan motor.

Recently a tech identified the fan module/motor being bad. Quoted $1,300 to repair. I was able to get the motor assembly for $500.
Compressor runs, fan doesn't. This unit doesn't have a capacitor for the fan.

Before I put it on, I was testing the power lines again.

There are two power lines, one each from each "phase" coming in.
Then there are two power lines from the air handler, one being the high pressure cutoff switch (yellow,) the other being the low pressure cutoff switch (blue.) Only the yellow wire had power with compressor off. The yellow wire and two main power wires had power with the compressor running. So the blue wire never has power.

I went in to the air handler, and there is power off the board for the yellow wire, but now power off the board for the blue wire. Assuming I have the right wires.

Funny thing is, with the compressor running, I only found power at the high press cutoff wire. None of the other three power wires had voltage.

Should the low and high pressure wires be energized for the fan to run?

The fan would run at times, for just a second or two then it would stop.
 

Last edited by renoir; 07-29-17 at 11:14 AM.
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Old 07-29-17, 11:22 AM
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Not sure what you are doing there..................

The high and low pressure switches will shut down the entire unit on fault..... not just the fan.

If you didn't have the proper incoming 240vac.... the fan and the compressor would both be not working.

If you didn't have the proper 24vac from the air handler.... the fan and the compressor would both be not working.
 
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Old 07-29-17, 11:37 AM
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Pjmax, should there be 24 vac coming out of both the low and high pressure cutoff lines for the fan to work?

Does power enable in both cases or does it disable?

High Press Cutoff wire = 24 vac
Low Press Cutoff wire = 0 vac


Also, there's a yellow wire coming out of the time delay switch which goes into what looks like an automotive ballast resister. There's a blue wire that comes out the other end which has no power. The side of this white cermamic item says 220 OHM. When testing it, it had 22 OHMs.
 
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Old 07-29-17, 11:48 AM
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I don't have the exact wiring diagram in front of me for that unit but pressure switches for the most part are normally closed.

I don't know if both should have 24vac on them. They don't have too.
 
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Old 07-29-17, 11:48 AM
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Okay, clarification. The blue wire comes in and goes to the compressor. Then, there is a blue wire coming from the compressor which connects to the 'compressor motor contactor (?) on the left side. From here, it goes to an epoxy filled square box (which has other connectors that do have power) and it goes to both the white ceramic looking device next to the time delay as well as to the fan motor.
 
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Old 07-29-17, 11:51 AM
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And closed means that they'd be passing power through?

I see, you answered this one already. Thanks!
 
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Old 07-29-17, 11:55 AM
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High voltage is always present to the motor and a low voltage signal to the 2 wires (that you think are pressure switch wires) controls operation of the fan.
 
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Old 07-29-17, 12:02 PM
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Perhaps, when the low pressure switch is 'activated,' it sends 24vac to the air handler which in turn kills the run signal to the compressor.

Would I be crazy to jump 24vac to the blue wire at the air handler with the compressor running, to see if it stops?
 
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Old 07-29-17, 12:06 PM
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Houston204, you are amazing. That is a very hard diagram to come by on the net. I am referring to it.

The HPCO black wire to the yellow wire to the fan.
The LPCO blue wire to the fan.

These are the two wires I'm referring to and assuming are the High and Low pressure 24vac signals that go to the fan motor.

At the fan motor connector, only the yellow one has power.
 
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Old 07-29-17, 12:06 PM
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So the high pressure cutoff does not control the compressor directly.
What is MS ? Looks like a generic symbol for relay coil.
 
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Old 07-29-17, 12:07 PM
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At the fan motor connector, only the yellow one has power.
Correct. The blue is Common.
Jumping ANY 24vac wire to blue would be creating a dead short !!!
 
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Old 07-29-17, 12:11 PM
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Here's the version I was able to find yesterday.

Name:  XL16i fan diagram.jpg
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Size:  47.3 KB


The connector for the fan has a

Green - Ground
Yellow - 24vac
Blue - should be 24vac? Currently no power
Brown - 120vac - When compressor is running
Black - 120vac - When compressor is running
 
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Old 07-29-17, 01:01 PM
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Green - Ground
Yellow - 24vac - think of it as positive 24v
Blue - should be 24vac? Currently no power - think of it as negative 24v.
Brown - 120vac - When compressor is running
Black - 120vac - When compressor is running

With 24vac measured from yellow to blue means the fan should be running.

Does that make more sense ?
 
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Old 07-29-17, 02:56 PM
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Alright, plugged the new motor in without installing it. Just held it.
It started and ran fine.
Plugged old motor in just the same, it started, then within seconds stopped.

Installed new motor, have AC again.

$89 - diagnostic fee
free - unneccessary worrying
$500 - new motor

Not impressed with Trane. The old motor module is epoxy filled. But, I bet the back side of the control board which faces up, isn't. My guess, based on the rust screws, water leaked in from the back (don't know how, but there's plenty of evidence for it) and ruined the module from the back side.

Thanks very much for your replies and time.
 
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Old 07-29-17, 04:24 PM
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Good you're back in business there.
 
 

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