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fyrmnk

AC Sensor Issue?

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Hi all,

Our AC has started the following intermittent issue:

When temp is reached, the condenser shuts off as it should, but the indoor fan continues to run and says "Cool On" like it is still running and having a call for AC. The unit then has to be manually shut off and back on at the thermostat. Then the normal lockout seems to be working properly flashing "Cool On" will until time threshold is met, then it will operate normally.

Again, only intermittent, but trying to chase down what contact or sensor could cause this. I've seen posts where others replaced their thermostat with no change, so didn't want to start with that if that's not it.

Thanks, hope all are well! Kevin

Edited by fyrmnk

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That's normal operation of most older ACs when the 'fan' switch is in the 'on' position.

Check the thermostat occasionally and see if the thermometer reading on it is within 2 degrees of the temperature setting.

'Cool On' means that the thermostat is in cooling mode. It doesn't mean that the outdoor section will be running.  The thermostat decides that.

Edited by Marc

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2 hours ago, Marc said:

That's normal operation of most older ACs when the 'fan' switch is in the 'on' position.

Check the thermostat occasionally and see if the thermometer reading on it is within 2 degrees of the temperature setting.

'Cool On' means that the thermostat is in cooling mode. It doesn't mean that the outdoor section will be running.  The thermostat decides that.

Thanks Marc, but that's not the case here. "Cool On" normally does not stay on on mine when the condenser isn't running and it's in Auto. When it does this, it can get to any temp in the house while set on 70 and it won't kick on to cool it down (highest while I was gone was 78). Even got the thermostat to think it was 85 inside today while set on 70 and it wouldn't kick the compressor back on. 

Have to shut it off, then turn back on to get it running again. 

Thanks again.

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Just to clarify, when this occurs, outdoor unit shuts down when temp is reached as it should, however indoor fan does not (in Auto), "Cool On" stays on on thermostat and outdoor unit won't start again when temp inside rises unless turning system off at thermostat and turning back on, then will operate normally (i.e. outdoor and indoor shut off and "Cool On" shuts off when temp reached) until the next mess up. 

Thanks, Kevin

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1 hour ago, fyrmnk said:

Just to clarify, when this occurs, outdoor unit shuts down when temp is reached as it should, however indoor fan does not (in Auto), "Cool On" stays on on thermostat and outdoor unit won't start again when temp inside rises unless turning system off at thermostat and turning back on, then will operate normally (i.e. outdoor and indoor shut off and "Cool On" shuts off when temp reached) until the next mess up. 

Thanks, Kevin

Is this a variable speed compressor?

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Disregard.

I can't see how anything other than the thermostat or a control card in the indoor section could do this, not with that combination of symptoms.  I'm assuming you didn't remove power from the system.

Edited by Marc

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On 7/23/2020 at 10:06 AM, Marc said:

Disregard.

I can't see how anything other than the thermostat or a control card in the indoor section could do this, not with that combination of symptoms.  I'm assuming you didn't remove power from the system.

I did not remove power. So far hasn't messed up again. On another board someone said it was likely high/low pressure switches.

Found another guy that had near the exact same thing and he said his was the contactor having corrosion and carbon buildup from arcing while engaging and caused intermittent contact issues and replacing that fixed his.

Either of these sound possible causes to you? At least the contactor is easy and cheap. May even start with cleaning connections and applying dielectric grease if that is a possibility. 

Thanks again. Kevin

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3 hours ago, fyrmnk said:

I did not remove power. So far hasn't messed up again. On another board someone said it was likely high/low pressure switches.

The high/low  refrigerant pressure cut-offs would not reset when the thermostat was turned off. They reset when pressures return to normal, unless there's a control card in the outdoor section, then you'd have to get in there and reset it.  Older outdoor sections, much older, had a red push button, near the refrigerant line connections, to reset the high pressure cut-off but I haven't seen those in decades.

Found another guy that had near the exact same thing and he said his was the contactor having corrosion and carbon buildup from arcing while engaging and caused intermittent contact issues and replacing that fixed his.

He's referring to the compressor contactor.  Again, cycling the thermostat on/off wouldn't change that.

Either of these sound possible causes to you? At least the contactor is easy and cheap. May even start with cleaning connections and applying dielectric grease if that is a possibility. 

I wouldn't.  Get a spare if it lets you sleep better.  They're about $15, $20 wholesale here. The dielectric grease will flame up. When they're pitted enough to fail, the contact tip that's brazed to the coil mechanism will be nearly gone.  Nothing left worth saving.

To be completely frank, The symptoms reported don't add up. Something was less than accurate or missing.

If you want to try avoid a service call:

  • check the coils on the outdoor section.  Make sure they're clean.
  • Examine the compressor contactor.  Make sure they're not pitted too badly, They will be pitted at least a little.
  • Make sure the coils on the indoor section are not freezing up.
  • Keep a new thermostat handy.

Thanks again. Kevin

 

Edited by Marc

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23 hours ago, Marc said:

 

Thanks Marc. Coils have already been cleaned. The contactor mechanism itself is enclosed, I was referring to the wire attachment terminals since they are not and do have some corrosion. I have another thermostat from when I replaced it several years ago I keep as a backup. So far it's been normal the past several days so waiting out this heat wave a couple more days then doing some more checking. Thanks again.

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