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I am trying to determine why my blower motor will run when the thermostat calls for AC and Heat but it will not run when I switch the thermostat it to fan only. I recently replaced the old Honeywell thermostat with an Ecobee. I have the same issue with both thermostats. My control board is an ST9106b 1068.

I have 6 wires coming from the blower to the board so I assume that means I have a 5 speed motor. Please correct me if I am wrong. Here is a summary of the 6 wires coming from the blower motor along with my guess as to what each one controls:

  • White – connected to one of the neutral taps on the board.
  • Black – (High speed) Connected to both the AC and Heat taps. It seems the previous owner wanted to run the max fan speed in both modes.
  • Blue – (Medium High speed?) Capped, not connected to anything.
  • Yellow – (Medium speed?) Connected to M1.
  • Orange – (Medium Low speed?) Connected to M2.
  • Red – (Low speed) Connected to the continuous power tap on the board, then run to a toggle switch on the side of the unit which appears to have been installed by the previous owner. This allowed the previous owner to manually operate the blower and I believe he left it on all the time.

Here is a summary of what I have checked:

  • When neither the AC or Heat is running I have 24V between the R and C terminals, nothing between G and C terminals. The constant tap on the board (connected to the red wire on the blower) has 120V. The blower will operate if I throw the toggle switch.
  • When the AC or Heat runs there is 24V between the G and C terminals. Either the AC or Heat tap (both connected to the black wire on the blower) has 120V. Everything operates correctly.
  • When the fan only mode is selected there is 24V between the G and C. The constant tap has 120V and I get nothing from either the AC or Heat tap. The blower will only come on if I operate the toggle switch.
  • I have tried swapping the orange and yellow wires on to the AC tap to confirm that those speeds function on the blower motor.

Shouldn’t the board be calling for power from one of the taps, probably AC, when the fan only mode is selected? It seems like the thermostat is working correctly since I have power from G to C in this case.

Something else that I find strange is that the black, blue, yellow, orange, and red wires all have voltage when the motor is running regardless of whether or not they are all connected to the board. For instance, if the black is connected to the AC tap and the motor is on my voltage pen says that the other 4 speeds are hot as well. I don’t have a good understanding of how motors are wired so this may be normal.

wiring diagram Here is the wiring diagram.

circuit board looking down Here is the board.

blower whip view Here are the blower connections. Note that the blue wire coming off of the CONT connection on the board is spliced to a brown wire which goes to the toggle, then a red wire comes off of the toggle and runs back to the red wire to the blower. Not ideal mixing and matching wire colors. I didn't do it!

The thermostat is an Ecobee Smart Thermostat with Voice Control. The furnace is an Armstrong Air Ultra V Tech 91 Model C2E36C-1A.

Here is the wiring on the original thermostat. I had the same issue with this one installed. (https://hosting.photobucket.com/images/ff454/wbwufpack/20210531_080821.jpg?width=450&height=278&crop=fill) (https://hosting.photobucket.com/images/ff454/wbwufpack/20210531_081141.jpg?width=450&height=278&crop=fill)

Here are a couple of photos showing the furnace model. (https://hosting.photobucket.com/images/ff454/wbwufpack/20210611_062342.jpg?width=285&height=175&crop=fill) (https://hosting.photobucket.com/images/ff454/wbwufpack/20210611_062358.jpg?width=285&height=175&crop=fill) (https://hosting.photobucket.com/images/ff454/wbwufpack/20210614_054048.jpg?width=285&height=175&crop=fill)

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    Could you clarify what is meant by "constant tap"? – NoSparksPlease Jun 10 at 14:08
  • Hey, pretty thorough post, but a little more information might be helpful for your specific case. You say you think you have a five speed motor, but then you've got a capped wire and so on and so on.... So, how about adding pictures of the wiring on both ends (connections at control board and at thermostat), also give us the make/model of the furnace. While you're at it, give us the model of the t-stat. A bit of tangential advice: Since you are new, take the Tour if you haven't already. – The Ghost of Jon Jun 10 at 14:51
  • Thanks for taking the time to respond guys! NoSparks - There is a connection on the board labeled "CONT". Based on my testing it supplies a constant source of power when the AC and Heat are not active. The Ghost of Jon - I just updated the original post with pictures and the thermostat model. – Brian Jun 10 at 15:29
  • You might find these sort of videos helpful in learning how to begin troubleshooting furnace issues: How to read a furnace wiring diagram and how to read an AC wiring diagram. Side point: you can see your wiring diagram doesn't add up to your blower motor whip; you may be looking at a replacement board, this is just to emphasize the importance of identifying your furnace model rather than the board model. – The Ghost of Jon Jun 10 at 15:54
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    I want to say again, add a picture focusing on the thermostat wiring behind the thermostat and a picture focusing on the thermostat terminals at the furnace circuit board. I can't be sure without seeing the thermostat wiring connections at both ends, but its sounding like you don't have the wiring for independent fan control. [after submitting this comment I see Ed Beal is getting at this point, his response is the correct answer to your problem]. – The Ghost of Jon Jun 10 at 16:06
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What no green wire, although the colors are only recommended ones a green is usually for fan only, you have a call for heat and a call for cooling, you could use the capped wire if your board has a fan only (not all do)

In the case where the control board doesn't have a fan only I will wire one in at the air handler.

If the air handler uses 24v for the fan contactor (fancy word for a power relay) I will wire a switch to the 24v and then to the contactor coil making sure to tap the hot side not the common or that will short the transformer and blow a fuse on the board or trip the circuit breaker and worst case fry the transformer / control board.

When the switch is turned on the fan will run, if there is a call for heat or cool the furnace / compressor will start like normal and the fan never stops unless turned off at the switch then it will cycle normally.

If the contactor is 120v I tap the 120v that supplies the furnace through a switch and again to the hot side of the contactor.

Years ago 2 wire thermostats were more common and this is how I installed fan switches. the switch only had to be rated for the voltage of the contactors. Contactors only draw a very small amount 5-10 watts is common so most any switch will usually work with 24v . I normally used a standard light switch in a box and labeled it fan on / thermostat.

Since both thermostats had the same problem looking back to the control board for a cold solder joint may find the defect.

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  • Hey Ed, I just added pictures of the thermostat and circuit board connections. I get what you are saying, but I don't think that is my issue since I have a green (G) wire. The G wire is what calls for the fan right? So if I can measure 24V between the G and C when the fan is set to "On" shouldn't that trigger the blower somewhere on the board? Thank you for taking the time to respond. – Brian Jun 10 at 16:27
  • Yes I missed the green in your descriptions, if you measure 24v when in fan mode and nothing when fan mode off it should work if landed in the correct place. I will try to look everything over tonight but working on a big project and have not had much time to respond. – Ed Beal Jun 10 at 22:36
  • Thanks Ed. That is what has me stumped as well. It seems like everything is operating correctly right up to the point where the blower isn't coming on! – Brian Jun 11 at 12:37
  • I was looking at the board I see the automotive style 5 amp fuse only one so there probably is not a second fuse but tracing the connection from the green terminal to the on board relay possibly a cold solder joint could be the problem this is a more common issue on / for low voltage. I would expect the signal to drive the same relay that turns on the air handler for heat or cold so a bad solder joint could be the problem. – Ed Beal Jun 11 at 13:47
  • Okay so now we are close to exceeding my very limited electrical skills. How do I go about tracing the board to check for a bad solder joint? Do I remove all connections and remove the board? – Brian Jun 11 at 16:18
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Here is the documentation for the logic board.

According to it, a "G" signal from the thermostat causes the blower to run at "heat speed". The logic board has on/off delays that are ignored when responding to "G" so it should be quite simple: When the Ecobee wants fan, you get fan.

You have the heat and cool signals from the logic board jumped together and feeding one black wire. Maybe that's because you have a simple single-speed blower motor? And yet, this document describes the M1 and M2 terminals as "Motor Parking". It does not describe what that means or what the logic board does with those but you have the orange and yellow wires connected to them!

We may need to have more info about where exactly (in the blower) those black, orange, and yellow wires go, and if there is more info about the blower itself that would help.

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