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Discussion Starter #1
The second of my two Carrier heat pump units is not switching from heat to cool. I pulled the flow reverser valve actuation solenoid off to test it. It was not exerting any magnetic force on my screwdriver, but I noticed that the solenoid is getting a 24 volt signal as it should, when it is open circuit, ie, when I pull the plug off of the solenoid. Whatever is powering the solenoid has developed a high impedance. The compressor comes on, so the actuator is being controlled by its 24 volt source. My first thought was maybe pitting on the relay on the control board that powers the solenoid, but it measures only .3 ohms when the relay is closed. Any ideas for what to look for?
 

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Measure the resistance of the reverser solenoid coil. It should be around 20 ohms. You may also compare it to the one on your other unit... even switch them for a real test.

Always be careful when running a solenoid coil without the armature inserted for very long since the coil impedance is considerably lower under those conditions and it will heat up pretty fast and possibly self destruct.
 

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Discussion Starter #3
Yes, thank you. The coil is about 13 ohms resistance. It draws a strong 3 amps when energized from my bench power supply at 24 volts. Yes, I don't leave it on very long without a core, as you mentioned. Not sure why the voltage drops so much. The relay that drives it seems to be good. I will try to remove and clean all of the spade lugs. There is a transformer behind the compressor contactor, and I wonder if that could possibly be bad. It appears to be the transformer that reduces the 220V to 24V.
 

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I'd say 3 amps is too much current flow for that device. Most pilot operated solenoids wouldn't draw much over 200 mA. (@24 vac) You may have measured it while removed but still... sounds too high... possibly shorted turns. If it didn't blow the class 2 fuse, it would overheat so check for that as a clue to indicate shorted turns.

You could check it for ringing with a scope on component check if you have one.
 

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Discussion Starter #5
Oh, duh, I was feeding the solenoid with 24 volts DC. Maybe with reverse EMF, it will draw less under 24V AC. I'll test it again with AC. If it is still high, I will seek to replace the solenoid. Actually, yes, I can compare with the solenoid from the working, smaller unit. I will do that. Thanks for your help!
 

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Discussion Starter #6
ok, I have verified that the solenoid works correctly. I put it on the other unit, and it buzzes, and pulls strongly on a screwdriver. Both solenoids work on the secondary unit. The first unit is just not getting enough power to the solenoid. I pulled the control board off and measured the 5W series power resistor. It's 330 ohms as stamped. Without a wiring diagram, I don't what to do next. Under the contactor, there is what looks to be a transformer. Is this where the base 24 volts is coming from, or is it coming from inside the house somewhere? I can't find a wiring diagram on the Carrier site.
 

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If you have 24 vac between the C and O terminals when the thermostat is calling for cool, that will be normal. If that isn't getting to the reversing valve solenoid, you will normally have to replace the board.

For a test, unplug the reversing valve solenoid and feed power directly to it from the C and O terminals while the thermostat is calling for cool mode. If the valve switches, the board is defective and normally not repairable unless you can do troubleshooting down to the discrete component level and find the defective component and then be able to get a suitable replacement. The boards aren't too expensive so they usually just get replaced if the unit isn't too old and has good prospects of lasting longer.

If you can give me a model number of the Carrier heat pump, I can try to find a schematic that might be helpful.
 

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Discussion Starter #8
Here are the model and serial numbers that the installer gave me:

I don't know which unit is the larger one and which one is the smaller tonnage unit.

25HBA442A003 -31010 2007E30681
FKCEH3001F15 5007V80988
FX4CNF042000 -AAAA 4907A84693

25HBA424A003 -31010 0508E13202
KFCEH0801N08 4907V82572
FX4CNF024000 4907A85845

I do have a background as an electronic technician in a former career, so I have the tools to replace discrete components.

Thanks so much!!
 

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Unfortunately those numbers refer to the air handler, heaters and defrost thermostat but not the high side condensing unit.

If you are able to jump the 24 vac directly to the reversing valve solenoid, that will be useful information. Since the RV is generally directly fed 24 vac from the control transformer in the air handler and all other functions are working except the cool (which requires the RV) it tends to indicate that the RV itself is the problem. Since you mentioned that it had a lower voltage at its terminals, I'd think the armature is stuck and causing a low impedance (similar to inrush current) to be presented to the control voltage and pulling it beyond its capacity, yet not enough to blow the protective fuse.

Now consider this, if the compressor and the condenser fan is able to run when the thermostat is calling for cool, then the control voltage is obviously ample for the compressor contactor and fan relay to work so it should also be ample for the reversing valve solenoid. Points to a defective RV doesn't it? HOWEVER, if that voltage is fed via a triac or mechanical relay, all bets are off until you clear them as suspect. I can't tell you that since I have no schematic and would need to trace the path on the board. A triac could fail and still deliver approx. 50% effective power.

Any function of this board can be bypassed quite easily and jumping power directly to the reversing valve for testing has no downside possibilities that would damage anything (well possibly blowing the control fuse if the triac was limiting the current flow to a defective RV).
 

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You should be able to get a model number from that board. That's the best hope for getting a schematic.

Edit: It just occurred to me that you haven't checked to verify that the "O" terminal is getting a full 24 vac from the thermostat. Could be a thermostat problem and the RV is a red herring.
 

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Discussion Starter #11
Super. So, I will try to drive that solenoid from the 24VAC source directly, as you said. I might try to first put a 5W, 330 Ohm resistor in series, in case a low impedance source causes too much current to flow. That seems to be what they are doing on the control board, but I will double-check. I will get back to you with results. Thank you sir!
 

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Discussion Starter #12
So, I finally threw in the towel, and called an A/C guy. He found that a wire, presumably the one that carried the 24V source from the unit in the attic, had failed. This was a tough one, but he finally figured it out. I asked him: "How does a wire fail? Was it exposed, and I bumped it with something while I was up in the attic?" No, said he, sometimes this just happens. Somehow, it had a kink, or what have you, and somehow went resistive. Thanks for your help--hopefully this is useful information for you as well. Blessings!
 
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