Why doesn’t the Trane rooftop furnace ignite

Here is the basic ignition sequence for a furnace.

• Thermostat signals for heat.
• Exhaust blower creates a vacuum in the tube and causes the vacuum switch to operate.
• Pilot valve opens allowing gas to flow to pilot burner (if there is a pilot).
• Spark generator produces spark.
• Pilot lights and flame is rectified (if there is a pilot).
• Main burner valve opens.
• Pilot ignites main burner.
• Blower motor blows air across heat exchanger, and into conditioned space.
• Thermostat signals set temperature is reached.
• Gas valves close, main burner and pilot shut off.
• Exhaust blower motor shuts off.
• Blower motor shuts off.

If your exhaust motor is coming on, but the gas is not flowing. The first thing you should look at, is the vacuum switch. This switch is used to tell the furnace that the exhaust blower is on, and that the blower will clear any exhaust gases produced. If the switch does not close, the furnace will not light to prevent toxic gases from being spread through the conditioned space.

Check the tubing attached to the switch, to insure it's clear. Sometimes creepy crawlies like to make homes in things like this, so make sure there are no obstructions.

To check the switch...

• Turn off the breaker to the furnace (or the serviceman switch where applicable).
• Locate the vacuum switch (it will be a small metal bit, with a rubber tube attached to it).
• Disconnect the wires from the switch (make sure you mark them and remember where they're supposed to go).
• Set your multimeter to check continuity.
• Connect the probes of the multimeter to the wires on the switch.
• Check continuity (should be open).
• Suck on the tube (lightly).
• Check continuity (should be closed).

Professionals will likely use a different technique, but since it's not the safest practice I will not explain it here.

If the switch tests OK, you'll have to move on to the next part of the system. If the switch does not close, replace it and test the system again.

The next thing to check would be the pilot assembly, control module, and gas valves. However these checks are probably best left to a professional.

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.