Please Help! I have a Lennox Furnace Model G60uhv(X) after years of working fine I switched the thermostat to AC and it failed to start the cooling cycle. I suspect some sort of problem with the furnace blower motor. Neither the blower or the outside AC unit come on with a call for AC.
The furnace works fine when switched to Heat. In Heat mode the blower only works in auto position however, and will not come on when switched to the on position. The furnace blower will spin by hand with no trouble. I can force the outside AC unit to come on by pressing the contactor switch and manually making the electrical connection. Using my Multi Meter I discovered The furnace is not sending a 24v to the outside contactor switch to turn the ac unit on. So the outside unit seems to be fine just not getting the signal to turn on.
I suspected a possible thermostat wiring issue and tested the cooling wire by swapping the W (heat) with the Y(cooling) wire at the thermostat and the control board with no effect. I have switched them back now.
I also replaced the thermostat and the situation remained the same.
I read a thread that the thermistor on the blower control board can fail so I pulled the blower motor and looked at the blower control board and thermistor. They visually look fine and not burnt. So I have reinstalled them without repair or changing anything. I then purchased a used furnace control board from ebay and installed that and the situation is the same. so the issue does not seem to be with the furnace board.
I then direct wired a thermostat with a short wire direct to the thermostat contacts on the control board and nothing changed.
Also, I do not have a clog switch in the evap line.
The Diagnostic lights on the main control board seem to be blinking in the normal operation pattern.
My question at this point is if the blower is having trouble in cooling mode (High speed) will the control board on the furnace prevent the outside AC from getting a 24v signal to come on? The blower seems to have about 16 wires going to it that are all black I think the number 6 wire is the cooling cycle wire but I am not sure and also not sure how to test it.