pilot ok, but main burner not on, 0 voltage(AC) on MV.


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Old 02-25-10, 10:16 AM
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pilot ok, but main burner not on, 0 voltage(AC) on MV.

The furnace control unit is Robertshaw 780 715.
The pilot flame is ok. After pilot is on, I no longer hear the click, click sound which means the flame sensor is ok, otherwise the click sound will be there for a long time (Am I right here?)

I read through many threads, people usually say the flame seneor is not working.

In my case is the flame sensor ok?
Is it true that my control unit is wrong?
 
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Old 02-25-10, 01:25 PM
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The flame sensor has nothing to do with the pilot, so forget the pilot. Flame Sensor problem usually happens like this: pilot lights the burners, burners stay on a a few seconds and then burners go off and it waits a minute then retries. if thats whats happening then flame sensor is bad. clean with something or replace it. Also ive never heard a click that says flame sensor is ok but then ive never even heard of robertshaw before. Is the pilot always on or is there an electrnic ignition modle that lights pilot? does the thing click really fast and light the pilot? could be that clicking that you hear
 
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Old 02-25-10, 04:28 PM
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Pilot must be very clean, you may not know a dirty pilot. Might as well wipe off the flame rod too. The module you have is very common. And they do go bad.
 
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Old 02-25-10, 06:43 PM
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The pilot is always on,
the Robertshaw 780 715 is the electrnic ignition modle that lights pilot.

Does the thing click really fast and light the pilot? Yes
could be that clicking that you hear? Yes.
After the pilot is on, I no longer hear click anymore.

the burner is never on. I measure the MV point it is 0v. the PV point is 28V (AC)
 
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Old 02-25-10, 06:50 PM
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:::Pilot must be very clean, you may not know a dirty pilot.
How did control module know Pilot is not clean or not?
If control module sense the Pilot is on, it should begin to open main valve.
Otherwise it should keep lighting the Pilot. (but I did not hear the click sound to light the Pilot after Pilot is on) Based on this , I say the control module sense the Pilot is ok.

but the main valve's voltage is always 0. So I guess the control unit is partially wrong.
Am I right?
 
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Old 02-26-10, 08:27 AM
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I can't speak for mbk3, but I think the theory is if the pilot flame is not burning "cleanly", it does not burn as hot, which then can make the sensor not sense the heat of the pilot as well.

If say the sensor is in the pilot, only engulfed by a fringe 1/8th, lets say, and would normally work that way - it may not all the time, if the flame quality was compromised and not generating it's maximum heat. It might be the pilot flame volume, it's steadiness, and it's color (yellow or blue), that get affected.
 
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Old 02-26-10, 09:10 AM
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As I understand this problem, the pilot is lighting and the spark is shutting off. The next step in the ignition sequence should be that the 24 VAC switches on to light the main burner.

I suppose there could be a problem with a dirty pilot burner ----cleaning the pilot is ordinary maintenance that probably should be done, anyway.

But I'd be inclined to look for the reasons why the main burner isn't turning on: is there 24 VAC to the gas valve? If not, the likely problem is that the ignition control needs to be replaced.
 
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Old 02-26-10, 09:48 AM
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Thanks all.
Let me try to play with my pilot first.
 
 

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