Goodman gas furnace

[paw paw]

I have run into a problem that has me puzzled. Maybe someone can help. This unit was running just fine before this couple went on vacation for two weeks when they returned the unit would not run .
the unit is blowing fuses, I checked control wiring at hp unit, t-stat, and connections at the furnace there's no damaged wires, there is no visual(front or back) damage to control board.....so I completely disconnected control wiring from furnace replaced fuse, turned on power and it blew fuse, so I checked voltage at transformer 26.9v. There is only 2 components in the furnace control board and the transformer. Is the control voltage to high or is the board bad?????

[nomadpeo]

maybe i misunderstood, but did you disconnect all external low voltage wires from stat and cu ? if you did and it still blew a fuse, it must be the board. your secondary voltage is fine.

[heatseeker]

just curios how can you have 24 v at the t-former if you keep on blowing fuses? How are you able to check voltage at all at the unit?

[nomadpeo]

the secondary of the transformer is the voltage source. the fuse is on the board. if it blows, the transformer secondary voltage is still present. the fuse senses overloads in the circuit and opens the path to the voltage source.

after rereading, i'm convinced the board took a hit during that period, possibly lightning in the area. after isolating the low voltage wiring, if the fuse blows with power present, the board is the most likely problem. typically the transformer secondary will be slightly higher than the 24 volt nominal rating.

[paw paw]

Thanks Guys, just needed a second opinion, this was was exactly my thoughts the board was the problem, OK now I unplugged the big plug on the control board, leaving the control wiring connected and the fuse did not blow. so does this mean there's a short in the internal wiring or maybe a some kind of limit switch maybe bad, at the furnace.

[nomadpeo]

we would typically expect a problem of this type to be the board or an external device such as external wiring or external load like the contactor coil. by isolating the external field connections, you ruled out those items. you have seemingly ruled out the cirucit board. of course, as you know, things are not always as they seem. the limit loop is a logical suspect if you were running heat, but i'm assuming you were in cool mode and heat was not running during this unmonitored period ? the 'divide and conquer' approach is sound, but still does not rule out the board. i think i would isolate power and test low voltage pins to ground with the plug isolated using an ohmmeter. maybe you do have a short to ground on one of those pins. good luck. let us know what you find.