Trying to be as thorough and concise as I can, zero experience with hvac.

I have a year 2002 Trane xv80 that is giving me the three blink code, which indicates a pressure switch issue. I disconnected both the low speed and high speed switches, blew into them, and they seem to be making the right kind of clicking noise as I blow or suck. Kind of a cluck-cluck noise.

When I power up the furnace, the main blower starts to circulate are thru the ducts. However the inducer motor is not kicking in after 30 or 45 seconds which by memory is when it should kick in, followed by the igniter. My theory is if the inducer will not spin, then the pressure switches sense inadequate draft and they will then not allow the igniter to ignite, which in turns means no gas will flow and burn, and thus no heat.

I have installed a new flame sensor only because I cleaned it last year and it seemed to solve the problem. However, it may have been a coincidence. But, at approx $12 it was worth a shot. I think the igniter is OK, it ignited a few times over the past few days as I have been troubleshooting, butno longer will ignite.

I unplugged the inducer motor - three wires in the plug, a fourth green is external and goes to a grounding terminal. I put the multimeter on the inducer plug and have continuity. I then inserted the probes into the plug coming from the circuit board and checked voltage after starting the furnace. Nothing registered after a couple minutes of watching, but I may have been doing it incorrectly

Looking for advice. Is there a way to tell if the inducer motor works? I can insert a screwdriver and the fan blade spins. I am not opposed to removing it altogether. Or is there a way to test the circuit board to determine if it is sending voltage to the inducer and if yes, how much should it send and where should I place the probes?

Or would disconnecting the clear plastic tubes from the pressure switches fool the board into thinking all and allow the inducer to go? I am not opposed to ordering parts, just want to know what to replace.

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    The blower should not start immediately, it should only start after ignition and a 45 second start delay. Are you sure you've got your motors right? – Tester101 Nov 17 '15 at 15:04
  • Sorry for confusion, here's what's happening. The big blower - they unit that sucks in air through the filter and blows air into the ducts fires up as soon as I make a call for heat if I dial the Trane xv80 to "heat" and "fan". Nothing from the smaller inducer motor (brand FASCO) – user1276141 Nov 18 '15 at 2:01

The normal sequence of operation is:

  1. Thermostat calls for heat.
  2. Control module runs self check, and verifies 1st stage pressure switch is OPEN, and limit switches are CLOSED.
  3. Draft blower is energized.
  4. 1st stage pressure switch closes.
  5. Igniter warm up begins (17 seconds).
  6. Gas valve opens.
  7. Flame sensor confirms ignition within 4 seconds.
  8. Delay to FAN ON begins (45 seconds).
  9. FAN ON delay elapses, and blower motor is energized.

It sounds like you're having a problem at step 3. Where either the control module is not energizing the draft motor, or the draft motor is dead.

I'm not familiar with your specific unit, so I'm not exactly sure which speed the control module runs the draft motor at. I would guess that 1st stage uses low speed, but I could be wrong.

To verify the motor is being energized, follow this procedure.

WARNING: You'll be working on live equipment. If you're not comfortable with this, please contact a local licensed professional.

  1. Turn power off to the unit, using either the breaker, or the serviceman's switch.
  2. Remove the wire harness from the draft motor.
  3. Restore power to the unit, and trigger a call for heat.
  4. After a delay, measure the voltage between the white IND-N pin (pin 3), and the red IND-LO pin (pin 2) on the inducer motor wire harness.
  5. Measure the voltage between the white IND-N pin (pin 3), and the black IND-HI pin (pin 1) on the inducer motor wire harness.

You should measure line voltage (~120 volts) between the white wire, and one; or both, of the colored wires. If you don't, the problem is with one of the other limits or the control module. If you do, the problem is likely with the motor.

You mentioned that the blower motor is starting, as soon as the thermostat is calling for heat. If this is the case, that could point to an open limit switch. As the blower should not come on until after ignition, during normal operation.

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  • Yes, I think my problem is at step three as well. The draft blower (FASCO inducer) is not energizing. Yet it spins fine if I flick it with a screwdriver, and on the odd occasion it did work, it spins without problem. I will investigate with a multimeter and let you know what I find. – user1276141 Nov 18 '15 at 2:08
  • I tried the multimeter on the plug of the FASCO inducer. I got the following after a call for heat (thermostat set at 80, actual house temp 59, I am cycling the breaker to create the heat call). Leads in white-red: 0 Leads in white-black, four calls, 12, 3, .3, 7.6 - volts Leads in red-black: 1, .1. .3 I'm not getting ~120 volts at any time. I can hear the "click" meaning thermostat is trying to get the inducer to spin up, but nothing. Please let me know what else to check. – user1276141 Nov 18 '15 at 2:49
  • Is there a model number of the control board? Is there a model number on the inducer motor? Is there a wiring diagram printed on the motor? If the board is bad, there's not much to do aside from replacing it. But they're usually expensive, so you want to be sure that's the problem. – Tester101 Nov 18 '15 at 3:38
  • @user1276141 You've followed the reset procedure? – Tester101 Nov 18 '15 at 3:46
  • There should be a 12 pin wire harness plugged into the control board. Turn off power to the unit. Unplug the harness from the board, and check continuity between pins 6 and 11 (should be the only two yellow wires). That will tell you if the limits are closed, which they should be. – Tester101 Nov 18 '15 at 3:53

Hope this helps whomever has a similar prob. Unplug the harness that connects the Fasco inducer and put the meter probes into the white and black (high speed is black) female ports. Do a call for heat and if no power then it's most likely the board failing to energize the HIGH speed of the Fasco. But check your three temp switches first for continuity (normally closed) and check both pressure switches for no continuity (normally open). If your switches are as they should be then it's probably the board. I purchased an aftermarket board 21v51u-843 for half of the cost of the White Rodgers board. New board has thermo terminals integrated, old one does not. Label with tape all of your wires first with initials such as EAC, HUM, XFMR, etc. then remove all wires. Match the dip switch settings on new board, follow manual - note dip switches are in a new location and in a reverse order! The manual will show an example of the switches. Then plug into new board according to labels and attach thermostat wires. New board is a diff size thus I mounted it with wire thru the board's holes and made sure it was secure. Furnace works fine now. As always turn off all power before doing any work on the board and take several photos of the old board's wiring and dip switches to use as a reference.

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