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I've read the posts with the opposite issue, which have been helpful (you guys are amazing!) but still no solution. Problem is that the blower fan does not come on when system is on "Cool" and fan is on 'auto' or when system is on "Off" and fan is on "on". Interestingly, we discovered fan comes on when system is on "Heat" and fan is on "auto".

Things I've checked and tried:

  • Replaced thermostat first; no issues w/ install
  • Blower motor capacitor was weak but have now replaced
  • Tested motor by testing common wire to each of the speed wires (High, Med, Low), all showed about 7-10 ohms
  • Relay seems to check out, and the switch does go on/off when we turn the fan "on" or "auto" on the thermostat. So I think the thermostat is working fine, but fan still doesn't come on.
  • Since the issue seems to be on the cooling/fan side, I was convinced that we had an issue with one of the green or yellow wires connecting the thermostat to the relay or the relay to the fan/main limit, etc. So I tested continuity on the connecting wires by disconnecting for example, the orange between whatever you call the terminals where the thermostat connects and the bottom of the relay. I tested continuity on 5 different yellow or orange connectors, and all looked fine (i.e. 0 ohms).
  • Longshot, and earlier in the process, I tested the Door Interrupt Switch and it was fine.

I must not be understanding some part of the wiring that affects cooling and the fan, but not the heat. Could the Fan and Limit Controller be the culprit? I am guessing not, because it seems that this relates to heating (a shutoff if plenum gets too hot, right?) but would not affect cooling or just running the fan. I am so frustrated... could use your help. thank you so much.

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If you want anything close to an accurate answer. You're going to have to tell us a bit more about the wiring between the thermostat and the furnace, or include some photos of the wiring. And include make, and model numbers for all the equipment involved.

Most furnaces manage the blower themselves during a heat call, i.e. the thermostat simply energizes the W wire. Some also manage it during a cool call, but not all do. So when calling for cooling, the thermostat usually energizes both Y for cooling, and G to turn on the blower.

Since you say the blower doesn't come on for cool calls, or when the fan switch is moved to the ON position. That points to a problem with the "fan circuit". Between the thermostat and the furnace, there should be a wire (usually green) connected to the G terminals of both devices. When this wire is energized, it tells the furnace to start the blower. Some furnaces have an on/off delay, so the blower may not come on immediately. On some units, this delay is adjustable.

Since you haven't told us the make and model of any of the equipment, there's no way to be specific about what happens in your furnace. Some furnace control boards have terminals for HEAT, COOL, and FAN. While others may only have HEAT, and/or FAN. The motor speed wires are connected to these terminals, to allow the furnace to run the blower in different speeds depending on the situation.

The first thing I would do, is test for voltage between the grounded ("neutral") and the COOL terminal on the board, during a cool call. If you measure about 120 volts (line voltage), that means the board is energizing the terminal to turn on the blower. This means there's a problem between the board and the motor, or with the motor itself. If the terminal is not energized, then you'll have to start moving "back" through the circuit.

If the furnace is signalling for the blower to turn on, but the blower is not starting. You could try connecting a different speed wire to the terminal. Since the blower works for heat, you might try putting the wire that's on the HEAT terminal on the COOL terminal. If the blower fires up, you know it's a problem with the motor. WARNING: Don't run the system this way for long, you're just checking to see if the blower starts.

If the furnace is not signalling for the blower to turn on (no power to COOL terminal), then you'll want to start by making sure the thermostat call is reaching the furnace. Disconnect the wire connected to the Y terminal, and set the thermostat so it's calling for cool. Measure voltage between C (or the common side of the transformer), and the wire that was connected to Y. You should get about 24 volts AC. If you don't there's a problem with the thermostat, or the wire between the thermostat and furnace. If you do, then the problem likely lies within the control board.

Without more details about the system, it's impossible to offer any more specific advice. It's possible that there's an open limit, which is preventing the blower from turning on. However, without a schematic, there's no way to know if there are any limits that might be involved.

  • Thank you for responding. I wasn't sure how to post a picture, so I can do that. Will also try these diagnostics you suggested and report back. Again, my sincere thanks! – eliza1 Jul 29 '15 at 12:16
  • @eliza1 Here's a link that should explain how to upload images. – Tester101 Jul 29 '15 at 14:22
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I have seen where the furnace itself manages the fan when heat mode is called, but not when cool mode is called. Not sure why that is, but it makes sense of what you are seeing. If the W terminal is energized, it turns on the heat and the fan at the furnace. If the Y terminal is energized, it turns on the a/c outside unit, but not the blower fan. Typical thermostats will energize the G terminal when in cool mode, or when the fan is turned to manual "always on" mode.

It sounds to me like the G terminal is not being energized. It could be at your thermostat, or at the furnace. The G wire could be broken, open, disconnected somewhere.

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