Fan runs for AC/Heat in "Auto" position but NOT "On"

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Reed34

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I have a York TG9S080B12MP11A furnace. The fan will run in "Auto" but not in "On"

I will hear a click when the Thermostat sends the "Fan On" signal, but the fan doesn't turn on.

I've tried the following:
1. Verified the wiring - everything is correct. The G wire goes from Thermostat to Control Board
2. Replaced the Thermostat. Assumed it wasn't this, since I can hear a click from the furnace when switched to On. But, I had an extra on hand, so I tried it.
3. Replaced the Control Board - I thought this was the issue. But the new Control Board has the same issue.
4. Replaced the Capacitor. (wasn't sure how that could be the issue but tried it anyway for $10)

I know the fan works mechanically. It turns on with Heat or AC. I assume it receives the "same" signal from the Control Board whether it turns on in "auto" or "on"

Also, there are no Error Codes flashing with Fan in "On" position.

Are there any other parts that could affect this operation? Thanks.
 

WorthFlorida

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Is this a new issue? Have you ever tried to run the fan in "on mode"? It sounds as if in auto mode the fan turns on when the thermostat calls for heat or cool. In heat mode, the fan will not immediately run. First the forced inducer fan must be running, the gas valve is opened and then the plemum must heat up to a set temperature or from a timer then blower motor will turn on. However, in the fan on mode it should be overridden.

As a test, place the thermostat in cool mode and set the fan to on mode. The fan should immediately start. I found the wiring diagram but it is only shows mostly terminal connections. The G wire (fan) only show the terminal connection, nothing else.

https://www.manualslib.com/manual/80393/Johnson-Controls-Gg9s-Mp.html?page=33#manual
https://data2.manualslib.com/print_page/en-0e669d33b2e28d879f6ee068d1165605.pdf
 

Reed34

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Thanks so much for responding.

I'm not sure if it is a new issue. I've never tried it prior to this discovery. I wanted to bring in some fresh air with the windows closed.

When I set it to "cool" with the fan "on" (and thermostat temp way up) -- still no response from fan. I have waited several minutes in case there was a delay.
 

WorthFlorida

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What brand and model is the thermostat. The display should not have went away. Do the following to eliminate the furnace or the thermostat as the source of the problem. Color codes for thermostats are not mandatory but generally the colors follow a industry standard. Older homes color can be all over the place. https://www.systoncable.com/thermostat-color-code
  • With a voltmeter measure between the R or Rc or just R if there is a jumper (red wires) and the C wire. There should be around 24v ac.
  • In heat mode raise the thermostat to turn on the heat, at the same time read the voltage between the G (Green wire) and C wire. When the fan turns on there should be about 24 v ac on the green wire, zero before the fan turns on.
  • With everything off and the voltmeter on the G and C terminal, the voltage should be zero, then move the fan to "ON". The voltage should measure about 24v ac. If it does not, turn off everything and
  • disconnect the R wire (red) and the G wire (green). Short the red wire to the green wire. The voltage should read about 24 volts and the fan turns on. If the fan does not and the voltage drops, the problem is not the thermostat.
 

jadnashua

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So, you hear a click IN the furnace when you set the fan switch to ON? That does sound strange if it runs when on auto when it should. It's the same relay, just activated in a different manner.

FWIW, the typical HVAC system in a home only circulates the air IN the house already...it does not bring in outside, fresh air. Now, it is possible you may have a fresh air energy recovery system, and then, yes, it might bring in fresh air, but with an energy hit, as it can't recover all of the energy from the already conditioned air as it is exhausted, and fresh air is brought in. Generally, having the fan on constantly can make the home more comfortable, especially if it's a variable speed one (higher air movement speeds will feel cold due to the wind chill factors). I have Trane equipment with their optional variable speed fan (16-speeds). WHen I turn the fan on, it only runs at speed 1, so it's quite quiet and helps keep the air from becoming stratified.

Without reading the whole manual, it implies there's a 30-second delay after turning the fan on before it actually starts...how long did you wait?
 

Reed34

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Thanks for the responses. I haven't yet done any tests with my multimeter -- I just don't trust myself to troubleshoot it correctly.

But, jadnashua -- thanks for that info. I guess I had a brain fart thinking the blower "fan only" would be pulling in fresh air. I guess the outside air is really only used for the heat ignition, right? Of course the blower fan would just be pulling in air via the Air Return -- not sure what I was thinking!!!

And yes, I waited for the delay (tried cool and heat... waiting 5+ minutes), and it never comes on WITHOUT a call for heat or a/c.

But everything else you described is accurate. I hear the click from the furnace when the "Fan On" is engaged from the thermostat. I already tried replacing the thermostat with no luck. I even replaced the limit switch (as well as the circuit board). I might just leave it as a mystery at this point, since I was really just trying to bring in fresh air without opening the windows, but I now realize that wouldn't be happening anyway.
 
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Fitter30

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Thermostat either calling for cooling or fan on put power on the g terminal to turn the blower on. Do you have a volt meter? Tape the blower door safety switch closed check power at thermostat terminal strip
R-G with blower calling 0 vac blower should be running
G-C 24 vac blower should be running
R-G 24 vac blower not running
G-C 0 vac blower not running

Heating blower on is timed ,cooling is not
 
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