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Ruud furnace won't light

I have a ruud deluxe 90 nat. gas unit built '87. The furnace turns on and the igniter glows, but won't light. The relay to the gas valve comes on about 15 to 20 sec. after igniter but I don't fell the gas valve open. I measured the voltage at valve and get .02V then 10.47 volts for a few sec then drops to .02V. ( From my vom meter I have to ground the red wire or I get a Neg. voltage reading. ??? )
I pulled the yellow wires off the circut board that come from the transformer and I don't get 24V or any voltage for that matter when 110v is on.
I don't think it's the valve because I can get it to open from the power of a 9V battery.
Is it a bad transformer or circut board problem or what?
Any ideas?

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  • 2 more comments 
  • awtinting Nov 04, 2007

    I check the two wire comming out of the transformer, Both yellow, with my meter, one probe on one yellow wire the other brobe on the other and get 0 volts.

    The circut board has both ac and low voltage on it along with relays that might look like they are removable. There is a smaller curcut board mounted on top that holds the relay for the gas valve, a white resistor and some diodes.

    I will try the ac. I did not think of that.

  • awtinting Nov 04, 2007

    The ac works and the transformer voltage jumps between .08 to .15 volts. A far cry from 24V.

    What next?

  • awtinting Nov 05, 2007

    Thanks for staying with me.

    Yes, I connected a 9v dc battery to open the silenoid valve and and the main burner opened up. Just long enough to smell gas.

    I removed the pilot gas tube and it is clear of debris, however there is no gas that comes out of the tube while the coil is at glow.

    Think it's cloged inside the valve?

    How does it know when to turn pilot on?

    FYI I did replace transformer today.

    Now what? Thanks.

  • awtinting Nov 05, 2007

    I saw the orifice and it is clrear and the tube has no kinks and is tight at all jounts.

    The heater was working last week.



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First lets try the transformer. With your meter check for 24v on the output side. You cant check to ground on this, you have to check your meter with both wires to the transformer. If this is too complicated just try turning on the A/C, if it comes on, the transformer is good. The problem may be with the time delay relay. try by-passing it and then see what happens. since its an '87 when you say circuit board I assume its a board with all the low voltage wires attached to it right? Or does it have resistors, diodes, and relays molded into it?

Posted on Nov 04, 2007

  • Rob Luna
    Rob Luna Nov 04, 2007

    If the A/C is working with no problems, that means the contactor and fan relay are getting the correct low voltage, so the transformer can de eliminated as the problem. I am having a hard time understanding your readings for the low voltage. (make sure your meter is set for the correct type of current, ac/dc). Lets start from the begining. you have a glow coil right? it gets red but the the pilot is not lighting, right? So lets start with the pilot orifice, which is a very tiny hole that sometimes gets pluged up. so make sure there is gas coming out of it. Also, just to make sure I'm understanding you right, you connected a 9v D.C. battery to the valve and the main burner opened up?

  • Rob Luna
    Rob Luna Nov 05, 2007

    When the pilot flame ignites there is a sensor of some sort that tells the main gas valve that there is a pilot flame and its O.K. to open up. Its possible that the sensor or the coil in the gas valve that opens the pilot gas is bad. Also when you checked the pilot tube at the very end there is an orifice insert @ the flame end that has a VERY tiny hole in the tip. its easy to overlook. I know I mentioned this already, but its worth double checking. Also make sure the pilot tube is good and tight. a leak can cause the flame to not light.



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