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Help! 20 year old Ruud 90 Plus condensing gas furnace with JCI G961DAJ-2401 controller keeps blowing its 24V AC 2 Amp fuse. I tightened the wires, nothing looks wrong.

I measured the current for each step:

mA       Step
84       Start. No call for heat.
260      Draft induction motor on.
770      Gas valve open, flame on
880      Blower motor on
300      Gas valve closed
84       Blower motor off

Photos: https://photos.app.goo.gl/63jWLBLmiqGFB56U8

I am measuring the current in series with the inline fuse from the 24V AC transformer on its way to the Johnson Controls controller board.

I'm stuck. Any ideas?

  • Those image links are both invalid. What are you measuring the current of? – JPhi1618 Nov 1 '19 at 15:26
  • What kind of thermostat is connected? Have you inspected the thermostat cable at the furnace controller, where the cable exits the furnace, at the thermostat, and any other place where it's accessible and subject to damage? – Greg Hill Nov 1 '19 at 15:34
  • Hello, and welcome to Home Improvement. We'll need more info (and working photos) before we can help you. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Nov 1 '19 at 18:59
  • Yes Greg, cleaned and checked the wires everything looks pristine. Thermostat is a White-Rodgers 1F85U-22NP. I did notice/feel a bit of vibration when the blower was shutting down last week, not sure if that is relevant. Does anybody know if 880 mAmp is within normal operating conditions? – MarkA2 Nov 1 '19 at 19:23
  • This photo: hamtramckhardware.com/product_info.php/products_id/1110854 seems to indicate this board should not draw more than 400mA. I was unable to find specs or schematic to confirm. Several of your measurements greatly exceed this 400mA number. Is the transformer getting hot? I'd start there... – peinal Nov 5 '19 at 2:13
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I had a similar problem but it was because I had two furnaces and a damper control on the fresh air intake. Because the two furnaces were wired with independent transformers, and the transformers were wired opposite, the transformers would basically fight each other through the damper control and pop the fuse. It would only occur when both furnaces called for heat simultaneously, which wasn't very often as we usually have only one or the other turned on at a time. Fixed by removing the damper control.

A short to ground might cause the fuse to blow, also.

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It's more likely a short than some device using too much current. I would start by inspecting all of the wires to look for breaks, burns, or worn insulation. Pay particular attention to where wires are joined together, and where wires go through a bracket or housing.

Next, get the system to run, even if just in fan mode. Then give the various wires a good wiggle. This could help narrow down our search area.

If that doesn't work, get a second fuse holder and couple of fuses, plus a larger fuse (i.e., 5 AMP). Put the larger fuse in to the existing fuse holder. Then wire the second fuse holder in to various areas and test. For example, put a fuse in line with the R wire to the thermostat. If the new fuse blows, then the problem is with the thermostat wiring.

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  • Smart. I'll try that. – MarkA2 Nov 1 '19 at 19:53

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