ruud gas furnace board wont power up

Paul_69

i had a ruud gas furnace where power light wont come on at the mother board. i replaced stepdown transformer as stated in guide. worked 1 day and went off again. i was getting 120 volts out of board going to primary side of transformer.but i put another transformer on and still wouldnt make 24 volts going out to feed board. could there be a bad board or nuetrul problem in board? power good into primary of board also. i am almost thinking maybe there is a electrical problem going to furnace because after awhile it just started working properly again out of nowhere. i even put the hot into the power wire from the switch directly to the hot side of the transformer and left nuetral hooked up to the board and recorded 120 volts and wouldnt power up and after shutting off power playing with this thing for a long time it started up again! i dont know what to d. should i replace the board or have the circuit looked at by a electrician first where this is so intermittent.it ran many cycles no trouble but i know its not solved. i am atech and i cant put my hands on this and i dont want to replace the board unless i am sure. any help please it is a ruud model ugra-09ezajs  serial eh5d707f390119502

furnacefigher15

How

How are you testing for 120v?



Between what 2 points?

Paul_69

board

between xfmr terminal and nuetrul on board.i even stuck the wire in the power side also. but whats strange is after shutting off power enough times i lit up board and cycled fine

Paul_69

board

one time i even took a jumper cord plugged into a socket in basement and hooked it up to board and it started right up. then i put wires back on from furnace circuit it then started now on this circuit i shut it off again and turned it on again and it didnt work .and then one more time it went on again. i dont have board and its cold so i left it on for now.it is intermittent and i dont know what to do because i cant prove out whats wrong

Tim McElwain

Are you using NEMA

rated Class II standard DC 20-92 transformer and if so what is its VA rating?



What is the make of the board and its model number?

Tim McElwain

Lets walk through

this:

Ruud gas furnace board wont power up

I had a Ruud gas furnace where power light wont come on at the mother board. i replaced step-down transformer as stated in guide. worked 1 day and went off again. i was getting 120 volts out of board going to primary side of transformer.but i put another transformer on and still wouldn't make 24 volts going out to feed board.





 I ASSUME THE FEED FROM THE SECONDARY OF THE TRANSFORMER IS GOING TO X AND C ON THE BOARD, X IS 24 VOLT HOT "C" IS COMMON IS IT CONNECTED THAT WAY?





 could there be a bad board or neutral problem in board? power good into primary of board also. i am almost thinking maybe there is a electrical problem going to furnace because after awhile it just started working properly again out of nowhere. i even put the hot into the power wire from the switch directly to the hot side of the transformer and left neutral hooked up to the board and recorded 120 volts and wouldn't power up and after shutting off power playing with this thing for a long time it started up again!





AT ANY TIME THIS PROBLEM WAS OCCURRING DID THE 120 DISAPPEAR FROM THE HOT NEUTRAL FEED ON THE BOARD?



GO THROUGH ALL OF THE WIRING ON THE FURNACE AND MAKE SURE THE SERVICE SWITCH IS WORKING ALL THE TIME AND IS NOT WORKING INTERMITTENTLY.





 I don't know what to do, should I replace the board or have the circuit looked at by a electrician first where this is so intermittent.it ran many cycles no trouble but I know its not solved. I am a tech and I cant put my hands on this and I don't want to replace the board unless I am sure. any help please it is a Ruud model UGRA-09ezajs  serial eh5d707f390119502





IS ANY OTHER EQUIPMENT IN THE HOUSE HAVING PROBLEMS WITH 120 VOLTS? IF YOU FEEL THIS IS AN EXTERNAL VOLTAGE PROBLEM HAVE AN ELECTRICIAN LOOK AT IT. THAT WAY YOU ELIMINATE THE POSSIBILITY OF THE HOUSE VOLTAGE BEING SUSPECT.

furnacefigher15

Paul

"one time i even took a jumper cord plugged into a socket in basement and hooked it up to board and it started right up. then i put wires back on from furnace circuit it then started now on this circuit i shut it off again and turned it on again and it didnt work .and then one more time it went on again. i dont have board and its cold so i left it on for now.it is intermittent and i dont know what to do because i cant prove out whats wrong"



This sounds like a bad service switch on the furnace.



If the voltage is there with no load (between L1 and neutral), but not once there is a load (transformer) Then I would check voltage from one side of the service switch to the other while the trouble is occurring. If there is any voltage reading across the service switch, the switch is bad. The blower door switch can also give the same trouble.



From the symptoms, I think the switch has gone bad, and is acting as a resistor, and consuming the voltage before it can get to the furnace.



The other possibility is what Tim was eluding to with other power problems in the house.