Hi,
We have a trane furnace model tud120r954k2. Here's what is going on with it. the control board shows normal call for heat. Now everything starts up fine. Every time it ignites, the flame stays on for 3-4 seconds then it completely shuts down. I have cleaned the burner tubes, the flame sensor, the ground wire by the burners, and any dust in the area. It still kept shutting down after 3-4 seconds of ignition. So I just replaced the flame sensor, and it still shuts off. My multimeter shows the flame sensor is drawing power - about 188 mv dc.

The control board never throws an error code, ever. Any Ideas?

Thanks

 

Welcome to the forums.

That sounds like a classic flame sense problem although it should set a trouble code.

When you check the flame sensing circuit you need to use a meter set to ua (microamps).
You would disconnect the flame sensor. Connect one meter lead to the flame sensor terminal and connect the other meter lead to the wire that was removed from the rod. This puts the meter in series with the flame sensor.

 

Pete thanks for the help. I checked the microamps. It's at 2 before the shutdown. Still no error codes. Board continues to flash normal call for heat. I also cut off the old spade connector to the flame sensor and put a new one on. It seems like there's a fault with the board at this point

 

2 ua is usually too low to keep the burner lit. When everything is working properly, you typically see about 5 uamps.

Measure the AC voltage that's being applied to the flame sensor and report that here. That's the ac voltage between the flame sensor wire and chassis-sheet metal ground.

Also, check to be sure that the 24 VAC voltage is staying on at the pressure switch. That voltage could be dropping out for some reason, which would shut off the ignition sequence.

 

I checked the ac voltage and it's jumping around. I checked it 4 times when the burners fired 2 times it was 3 volts. Once it was 8 volts that tapered to 2 before shutoff and the last time it was really low....about .2 volts.

 

Try measuring the voltage without a call for heat. I believe it should be around 80vac.

 

The ignition control module should provide an AC voltage to the flame sensor, usually 24 VAC, sometimes 120 VAC.

Usually that voltage should be turned on whenever the pressure switch is closed ---- whenever the thermostat is calling for heat and the inducer motor is up to speed.


When the furnace flames engulf the flame sensor, a small DC current is rectified from AC to DC by the flames and flows through the flame sensor, the flames, and the furnace sheet metal back to the ignition control ground.

If the ignition control detects that DC current flow, it leaves the burners on. If it doesn't detect that DC current, the burners are shut off after a three second or so trial for ignition.

So.

Check to be sure that the ignition control has a GOOD ground. Usually there's a wire from G to a screw on the sheet metal ---check that and be sure there is a good connection.

Check to see if you have a good AC voltage being suppled to the flame sensor. If you don't have a good voltage being supplied, you wont get a good DC current.

If you have a good ground, a clean flame sensor and still get a low reading of 2 uamps, the likely reason is low AC voltage being supplied by the ignition control.

Replace the ignition control if you've checked everything else and get a low or erratic AC voltage.

 

S.P. ... not to contradict you but the AC voltage should always be present on the flame sensor looking for a "flame sensed with no call for heat" condition.

 

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Well, my experience is that ignition modules vary in how they are designed. I've seen many such modules that don't check for a flame sensed with no call for heat.


I was aiming to keep things simple rather than to consider all possible alternatives a DIYer might encounter.