I had a thermostat get hot and smell, replaced thermostat. Still not working found fuse on control board blown replaced fuse. Lights on control board indicate no problems. Replaced 18/5 cable from control board to thermostat. Still not working. Replaced cable from control board to condenser. Was able to manually start compressor and fan on condensing unit. But still not turning on. Did jumper tests from R thermostat lead and fan and R to A/C wire on control board, both caused 3amp fuse to blow on control board. Replaced 24v transformer was getting high voltage. Still not working. I don't know what to try next, I would think a control board but the heat works and the lights say it is operating fine. This is a Lenox brand system.

  • Does the furnace have a multi/variable speed fan? If you flip the fan switch on the thermostat to ON instead of AUTO, what happens? – Tester101 Jun 25 '14 at 10:15
  • Including the make and model of the furnace and condensing unit might be useful. A schematic for the furnace would also be helpful. – Tester101 Jun 25 '14 at 10:18
  • Has this setup ever worked with the A/C? Did you change anything, or did it just stop working? – Tester101 Jun 25 '14 at 10:21
  • No single speed fan tried auto and on positions. Yes this setup has worked in the past. Only change is thermostat after failure. Furnace model is Lenox G43UF-36B-070-07. Condenser unit model is Lenox 13ACD-030-230-01. – Paul Jun 25 '14 at 15:00
  • If you flip the switch on the thermostat to ON instead of AUTO, does the fan run? – Tester101 Jun 25 '14 at 16:17

If the system run normally when the thermostat is calling for heat, and when the fan control is switched to the ON position. Then the first thing I'd look at, is the contactor in the condensing unit.

WARNING: If you're not comfortable opening the condensing unit, and/or working with electrical devices. PLEASE contact a licensed and insured HVAC technician.

  1. Start by turning off the breakers for both the furnace, and condensing units.
  2. Next pull the serviceman disconnect at the condensing unit, and place the pull out in your pocket, or place a lock (that only you have a key to) on the disconnect switch.
  3. Open up the condensing unit to expose the electrical components.
  4. Locate and discharge any capacitors.
  5. Locate the contactor.

    enter image description here

  6. Label and remove the two low voltage wires connected to the contactor coil.
  7. Using a multimeter set to measure resistance, measure the resistance across the coil.

    Contactor coil leads labeled

Typically the coil should measure between 9-12 ohms, though this may vary by manufacturer. You may want to contact the condensing unit dealer, or the contactor manufacturer for model specific values.

High resistance

If the value is higher than 12 ohms, it's likely that the coil will not be able to pull the contactor closed. Which means the condensing unit will never turn on.

Infinite resistance

If there's infinite resistance across the coil, the coil is completely broken. Again, this will never pull the contactor closed.

Low resistance

If a low resistance is measured, it's likely there's a short in the coil. This will typically result in burnt out transformers, and/or blown fuses.

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If the system runs normally when the thermostat is calling for heat, but not when the fan control is switched to the ON position. There's a short somewhere between the thermostat and the G terminal.

Looking at the documentation for the unit, there seems to be a relay on the board that controls the fan speed selection. The heat speed is connected to the normally open contact, while the cool speed is connected to the normally closed contact. When the thermostat calls for heat, the relay is pulled, and the heat fan speed is used. If the thermostat is not calling for heat, the fan will run at the cool speed. Though the documentation states that the fan will run at heat speed unless in cooling mode, it doesn't appear that way in the wiring diagram.

  • Check the connections of the Y and G wires at the thermostat, and the control board.
  • Run a continuity test on the Y and G wires, to insure there are no shorts.
  • Make sure there's no jumper between Y and G at the thermostat or control board.

The furnace doesn't do much when the thermostat calls for fan or cool, other than turning on the blower. So there shouldn't be many places for a short to be.

The other thing you could try, is to run the blower at a different speed when in cooling mode. You'll not want to run the system this way for long, since it may not push enough air through the system and could cause the evaporator to freeze. This will tell you if the high speed tap on the blower motor is bad, though I'm not sure why a fault on the line voltage side would cause the low voltage fuse to pop.

  1. Remove the brown tap wire from the PARK terminal on the control board.
  2. Move the black tap wire from the COOL-H terminal, to the PARK terminal.
  3. Connect the brown tap wire to the COOL-H terminal.

This should run the blower at medium-high speed, when the system calls for cool. If the system works in this configuration, it means the high speed tap on the motor is shot and the motor likely needs to be replaced. Again, I'm not sure why this would blow a fuse on the low voltage side, so I'm not convinced this is the problem. It's just another thing to try.

Without actually getting my hands on the system and poking around, there's not much else that can be done from afar. Aside from saying, go through the entire system and check all switches and relays for shorts.

For reference, this is the wiring diagram I'm working from (which may or may not be correct, but it appears to be the documentation for your unit based on the model number you gave).

enter image description here
Click for larger view

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