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Good advice noted. I had thought this was a general question, but it is
now clear that defrost implementations do vary somewhat.

Yes, indeed, it was a Trane heat pump.
But no info with it. I rarely see info with units.

Honestly, I should have figured it out from the schematic.
I jumped to some bad conclusions based on my limited knowledge.
Going forth, I will be better armed.

Thanks again to everyone for the help.

Just a tip for future questions.

It would have helped a lot if you had mentioned the specific brand and model in the first post, as not all units are the same.

By combining the hints available in post #1 and post #11, the unit in question is most likely an AS/Trane heat pump.
The air temperature sensor is the one with the black wires, and wide plug, the coil temperature sensor is the one with the yellow wires, and narrow plug.
The unit "should" have a package of information in it with with 3 things; the Service Facts, a parts list sheet, and troubleshooting information for the defrost control(including temp/resistance charts for the sensors).
The extra brown wire hanging off the board is only for use with some mechanical thermostats that have a "T" terminal. It is for heat anticipator compensation for the aux/EM heat, based on the outdoor temperature.

I need an ipad or cheap wifi laptop to which I can download
manuals and then view them. The HVAC tool chest grows and grows.

Do this as soon as you can afford it, you won't regret it.

Long story short is that this particular unit did indeed have, in perfect working
order, its two sensors used to determine defrost intact and wired. The loose,
stripped wire that I found was an option for the thermostat to use to optimize
aux heat use.

In short, I can forget about it. Thank you all for your participation.

I really did learn a lot about this even though it is in
reality just a tiny slice of the pie.

But I guess that is the way of HVAC. There is really a heck of a lot more
to this than even HVAC folks assume when they first get into it. I think
I've learned that one can't just "divine" (Discover by guesswork or intuition)
something, that manuals are really, really useful and sometimes necessary for
some problems. I need manuals. But I threw away my last printer because
ink was so expensive. I need an ipad or cheap wifi laptop to which I can download
manuals and then view them. The HVAC tool chest grows and grows.

Originally Posted by hvacrmedic

The OAT sensor in most cases is provided to lock out the defrost cycle at around 45°F or above.

It serves to lock out defrost. Help determine when to defrost. And on Yorks, it also is used to lock out the aux heaters when its bove X degrees out side.

The OP may have found a unit that a thermostat's OD temp sensor was just mounted/thrown into the condenser to be used to lock out the aux heaters.

Then you cut out the coil sensor. As Scooby would say, rotsa ruck!

The demand defrost control measures heat pump outdoor ambient temperature with a sen- sor located outside the outdoor coil. A second sensor located on the outdoor coil is used to measure the coil temperature. The difference between the ambient and the colder coil tem- perature is the difference or delta-T measure- ment. This delta-T measurement is representa- tive of the operating state and relative capacity of the heat pump system. By measuring the change in delta-T, we can determine the need for defrost. The coil sensor also serves to sense outdoor coil temperature for termination of the defrost cycle.

http://www.google.com/url?sa=t&rct=j...SIyRGA&cad=rja

Originally Posted by rundawg

Is this a heat pump? Agree with BaldLoonie.

Demand Defrost Controls use an Outdoor Ambient Sensor to determine when to initiate defrost.

They will also flash a code for an open sensor.

Do you have a model number?

Side note: to answer your "One more thing question"..... Started in the trade back in my teens, took a few years off while in the military, now back in the trade, and am now pushing 60. I'm too old to do the math.

No wonder you have so many books!!!!!!!

Hvacrmedic- good to see you back on and posting again!

Leaving notes at the office isn't good.... Trane has a great library of literature (so does mr rundawg) of which you should search and read up on it.

I have been in the field for 12 years now going from resi, to commercial ac, to commercial refer, and now to controls.

I don't know about my brother's case, but mine definitely had another sensor, circular, and although
I didn't look at it on the schematic, it was clipped to the bottom tube of the outdoor coil, if I remember
right. I assumed that was the defrost sensor? I didn't disconnect anything. The wire I found was already
disconnected. There was simply no sensor in sight to which I could reconnect it Mine indeed was a Trane.
Model number in notebook at work. Sorry.

The OAT sensor in most cases is provided to lock out the defrost cycle at around 45°F or above.

Originally Posted by georgelass

He currently has an outside unit control board with an OAT (outside air temp) fault code. We at least have determined that the thing is a thermistor, not off/on.


I thought I remembered, from reading long ago, such a sensor being for the indoor control, to let it know when heat strips might want to be added, but I have a very bad memory and I can't find any info easily. That's why I'm here.

And that purpose seems fairly innocuous to be giving an outside control board a crippling fault code. On the other hand, HVAC units never have mere warning codes, huh?

Is this a heat pump? Agree with BaldLoonie.

Demand Defrost Controls use an Outdoor Ambient Sensor to determine when to initiate defrost.

They will also flash a code for an open sensor.

Do you have a model number?

Side note: to answer your "One more thing question"..... Started in the trade back in my teens, took a few years off while in the military, now back in the trade, and am now pushing 60. I'm too old to do the math.

Rheem or Trane most likely and you disconnected a sensor for the defrost system. So now it either won't defrost and freeze up or defrost very frequently since its sensor has been removed.

One more thing.
My brother, in his defense, he works in the ghetto, for a
propery management company, mostly rentals.
He's lucky the heat pumps he sees
have any copper, never mind OAT sensors.

Outside Air Temperature Sensor Purpose?

I'm sorry. Twice.
Once for already posting about four questions this week.
Second for not being able to find this using the search
tool. (I LOVE this forum--couldn't live without it--but I'm
thinking the search tool is not the best.)

This question will help TWO people. My brother has
been in HVAC for 12 years, learned on the job. I've
been in HVAC since 2008, (started tech school then,
but given the deep recession, the time spent has not
been quality time, much more DOWN time).
He currently has an outside unit control board with an
OAT (outside air temp) fault code. We at least have
determined that the thing is a thermistor, not off/on.
I have less of a problem. I noticed, on a dead
compressor service call, a dangling wire --stripped
but not connected, ran back to outside unit's control board,
and wiring diagram says there might have been at one time
an outside air temperature sensor. My unit is running fine,
without the OAT. And at the homeowner's request, I have
disabled the heat strips at the thermostat. I can have a
relaxing weekend. No 2 am insomnia wondering if I've done
something wrong.

I thought I remembered, from reading long ago, such a
sensor being for the indoor control, to let it know when
heat strips might want to be added, but I have a very
bad memory and I can't find any info easily.
That's why I'm here.
And that purpose seems fairly innocuous to be giving
an outside control board a crippling fault code. On the
other hand, HVAC units never have mere warning codes, huh?
That leaves me to conclude that my memory is wrong, that
the sensor has some more important purpose.

One more thing. My brother and I were joking around today
about our time in HVAC and our knowledge levels. (All in good
fun--I ask his opinion ALL of the time.) So for you folks kindly
being helpful and responding, could you also throw in how long
you have been in the trade?

Thank you.