Blower will not run in fan only mode

Question

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.

Here is the wiring diagram.

Here is the board.

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)

Answer 1

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.

Answer 2

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.