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  1. #1
    Join Date
    Sep 2004
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    Hey, I was at a house today and unit is goodman gmp100-5 and during heat mode everything works up until flame comes on then once blower starts clicking sound turning off gas valve and blower keeps blowing flame starts again and flame turns off again. Narrowed it down to flame sensor, what do you guys think?

  2. #2
    Join Date
    Mar 2002
    Concord, CA
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    Call a licensed and insured professional.

  3. #3
    Join Date
    Mar 2003
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    flame sensors are 5 bucks, get one and put it on.
    when gas valve is energized control board waits a few seconds and if burner is not detected gas valve is deenergized. control board uses flame sensor to detect burner has lit.

  4. #4
    Join Date
    Sep 2004
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    Thread Starter
    Thanks, thats what I thought it was. But here in town parts stores close very early and Sunday not open.

  5. #5
    Join Date
    Sep 2004
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    There will always be a hero trying to impress someone

  6. #6
    Join Date
    Aug 2001
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    then clean it

  7. #7
    Join Date
    Jul 2004
    West TN
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    Check flame sensor microamps
    Check flame sensor wire AC voltage to ground

    (this is a reference you build over many units... there is no 'set' value on this one... I've seen Johnston Controls ignition control modules read 12 volts and have a relatively low operating microamp reading at about .5mA... then again I've seen read 138volts and have a relatively higher operating microamp reading at about the way these are both 24volt controls.

    Before I move on.. for the others that wonder where I'm coming from...

    First... and formost... build a reference sheet!!!
    At each PM you go to... Take the microamp reading and the voltage to GROUND between the flame sense wire and GROUND.

    You'll need unit model, serial, board model, input volage
    to board (need to know if it has 19volts or 28 volts), AC voltage to ground, and microamp readings.

    ALSO... take a magic marker and write down in the unit the readings... I confess I normally just write down volts to ground and microamps

    Lets say you go to unit A and read
    88volts to ground 5ma

    you go back next year...
    22 volts applied to board, 88volts to ground, 3ma
    You clean sensor... still 3ma
    Whats changed??
    Bad board you say.. not necessarily...
    Whats the 'whole' path for the board to sense??
    From board, through wire, to sensor, TO GROUND, then back to the board.
    Alot of boards have a ground wire going to the cabinet.
    Or even a ground screw that goes into the corner of the board. Also.. alot of these new inshot burners get rusty on the ends... this increases the electrical resistance of the circuit and the board don't sense like it should.

    Lets year you go to the same unit...
    22 volts applied to board, 0 volts to ground, 0ma
    Hopefully you would see that its the board now and not the flame sensor...
    Sense the board is not sending a signal to the flame sensor in the first place (ala zero volts to ground) how can it sense it???)

    After a while, you will see a distinct pattern.
    Low voltage to ground will equate to lower mA readings
    Higher voltage to ground will equate to higher mA readings

    Give you one more pop quiz to see if you get it right....
    19 volts applied, 70 volts to ground, 2mA... furnace showing irratic behavior.

    hmmmmmm whatcha think???
    could it be that somebody hoooked up the transformer to the wrong voltage tap??

    You can use the 'voltage to ground' reference on the prob you have at hand....

    Inducer comes on, pressure switch closes
    Hot surface ignitor comes on, gas valve opens
    quick before it shuts down... measure volts to ground and microamps. It might take a couple tries to get both readings since you have to set up the meter in two different ways.

    Are the voltages and mA's good? (this is were the reference sheet comes in handy)
    Run through a cycle and monitor mA at the moment it shuts down.
    Run through a cycle and monitor volts to ground at the moment it shuts down.
    If you maintained a volts to ground and lost mA... then I'd go with the direction of flame sensor.
    If you lost volts to ground then I'd go backwards and go toward the board.
    Try it again and check volts applied to board... do you lose voltage at the moment the blower comes on?
    Did any of your safeties open during the moment the blower was called to come on?

    Last but not least... the remote possibity that the furnace is not grounded properly. (our gas dept has been replacing gas lines with the plastic stuff and replacing gas meters... if the furnace was not properly grounded and ermmm using the gas line as a ground... then the board will do scewy things like turn the gas valve off when the blower is called for. I've also diagnosed a gas leak in the ground just before the meter while on a complaint.
    While I was fixing the complaint, the gas dept came out and dug down to an electrode that attached to the gas line.
    It had completely disintegrated. The gas line was painted/coated and did not get a good ground like it should since the ground electrode was gone. Thats where the leak was too.)

    whew I sure do end up writing long posts
    I need to learn how to keep things short and simple
    nobodies gonna read this hehe

    For what its worth... when I go into detail on a prob such as this... its not just to help the person out with the prob... its to help everyone out.
    Who knows.... I may know stuff you don't know and you may know stuff I dont' know....
    I'm getting tired and started to talk jibberish now.. so I'll leave it at that

    [Edited by wormy on 03-13-2005 at 01:18 AM]

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