HVAC help - 3a fuse blowing after furnace cycle

RatLabGuy

RatLabGuy

You look like a monkey and smell like one too
Joined
May 18, 2005
Location
Churchville, MD
First, the situation: Typical gas furnace w/ a seperate A/C unit. No backup heat. Cranked up for the first time this weekend. T-Stat turns it on and it goes through the expected cycle - first the main fan blows a bit (90 secs), then the purge cycle starts, main fan turns off, inducer motor spins, the igniter will heat up (I can see it), then after about 20 secs the 3a fuse pops.

One might think its the gas valve coil, but here's the thing - it does this even w/ the gas valve coil completely disconnected, and reading the wire/ground going to the gas valve during the time it is 0V AC. Like no signal to it. Igniter resistance is 48ohms. It also does it with the igniter unplugged.

So it seems it can't be the igniter or the gas valve coil (right?). I replaced the control board (ICM282B), no change in behavior. Pressure switch reads no continuity at start, few ohms when the inducer motor is running. 27v when it starts cycle, goes to 0v .

The LED inidicates normal function, no indication of lockouts or anything.

I am out of ideas... any suggestions welcome.

Potentially relevant: this summer we had a problem w/ the timer on the A/C unit, when it failed it took out several things. Then we replaced the timer, relay, main 24v transformer, thermostat, and the control board (it was an original ICM282, replaced with the new IC-driven -B version). Everything worked great afterward but this is the first time heat has been used.
 
First disconnect the blower motor from the board and try again, see if it still happens. If so, check for a short on the flame sensor next. A fuse popper is really useful in a situation like this.
Pictures would help, there's a low voltage short somewhere
 
Short on low voltage. Remove disconnect/turn off breaker. To test tstat wire, on one end take all wires and put them in same wire nut. Other end get a meter and try to ohm all the different wires between each other.
 
83oldyoda4x4 said:
Short on low voltage. Remove disconnect/turn off breaker. To test tstat wire, on one end take all wires and put them in same wire nut. Other end get a meter and try to ohm all the different wires between each other.
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Done this. All stat wires are fine, AFAIK the stat goes through cycles as it should. Its been running A/C fine, and trips heat on.
braxton357 said:
First disconnect the blower motor from the board and try again, see if it still happens. If so, check for a short on the flame sensor next. A fuse popper is really useful in a situation like this.
Pictures would help, there's a low voltage short somewhere
Click to expand...
Wil ltry this. Not following why removing motor would matter, as its full 120, but what the hell. Agree its likely low voltage.
And yea a fuse popper would have been useful, I've been through so many thus far it would have paid for itself... ordering now for next time lol.
 
RatLabGuy said:
Done this. All stat wires are fine, AFAIK the stat goes through cycles as it should. Its been running A/C fine, and trips heat on.

Wil ltry this. Not following why removing motor would matter, as its full 120, but what the hell. Agree its likely low voltage.
And yea a fuse popper would have been useful, I've been through so many thus far it would have paid for itself... ordering now for next time lol.
Click to expand...

Yeah you are right on that, I was just thinking through furnace start sequence. Not really an easy way to isolate a relay on a board. Have you tried a larger fuse? Pictures would definitely help
 
83oldyoda4x4 said:
What wire you got pulling in heat, white? Unhook that at furnace and see if it still pops.
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I'm sorry I'm not quite following what you're asking. I'm pretty sure the white stat wire is heat.

Not sure what pictures would be useful, here's the board. Stat connection in upper left.
IMG_20251015_212741598_MP.jpg


And the unit itself
IMG_20251015_212942755_MP.jpg
 
braxton357 said:
If so, check for a short on the flame sensor next.
Click to expand...
I pulled the flame sensor. Since it isn't ever firing, I can't get an amp reading on it, but reading resistance across the connector and the metal collar around teh ceramic is... interesting. Its hard to get a consisten treading at all, bounces around a lot between 4 and 19 megaohms. Its really crusty where the metal collar is around teh ceramic. For $8... I've ordered a replacement to try out tomorrow (along w/ a 3A fuse tester lol) once the Amazon man stops by
 
RatLabGuy said:
I'm sorry I'm not quite following what you're asking. I'm pretty sure the white stat wire is heat.

Not sure what pictures would be useful, here's the board. Stat connection in upper left.
Click to expand...
Pull the white wire off the board and see when calling for heat if everything else works. should run the fan etc without popping fuse. this will tell you a problem on tstat side or board/furnace side. I have had tstats themselves bad out of the box, obviously it has no problem running fan and calling ac condenser. Just working down the line.
 
83oldyoda4x4 said:
Pull the white wire off the board and see when calling for heat if everything else works. should run the fan etc without popping fuse. this will tell you a problem on tstat side or board/furnace side. I have had tstats themselves bad out of the box, obviously it has no problem running fan and calling ac condenser. Just working down the line.
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Ah you mean the incoming white wire from the stat?

Dumb question, how do I trip it to start then.

Stat has no problem activating that line to start the heat cycle.

EDIT: Earlier my son did exacty this by jumpering the R &W terminals. Kicked off the sequence, still popped - confirming its not stat related as I suspected.
 
Measure the amperage on the 24v side of transformer. Many times you will see the spike in amperage when the component that is the issue is powered. May need to use the max function on meter if you don’t see spike the first cycle. This may help you narrow down what part of circuit is causing the issue.

From what you are describing I’m leaning toward gas valve circuit since that would be the next thing in line to receive power after ignitor. But if not a dead short and the issue is only raising the amperage slightly above 3amps may take several seconds to blow fuse. So could be something to do with the ignitor relay. Check harness and molex plugs very closely.

Had one the other day condenser motor blowing fuse but everything would Ohm out fine. motor and harness. After tracing harness found a knick in the insulation of one of the wires where it was zip tied to fan bracket. It never showed on my meter but was causing a short once powered.
 
Update. Got a 3a circuit popper and replaced the flame sensor. No change, but the circuit breaker doesn't pop. Then I disconnected both ends of the molex adapter connecting the gas valve and sensors to the PCB and reconnected, and just like magic... now at least it DOES measure 27v going to the Blue and Green wires that serve the gas valve for a few seconds, then it goes off and re-sets the cycle.
Interestingly if I leave those leads connected and measure voltage, there's nothing at all. Only with control wire disconnected. Pins on gas valve read only ~3 Ohms and measuring between what should be its ground (that the green wire connects to) and another ground gives the 27v.
So I think the gas valve coil is bad.
As extra check (since I know the little PCB on that connector has a rectifier in it) I pulled that off and measured just between the two coil pins on the right here:
IMG_20251019_235756346_MP.jpg

and they also measure only 3Ohms difference. So... yeah doesn't this mean that coil is stuck closed?

Unfortunately I don't see any way to disconnect just that coil, nor can I find a part for it solo. In fact almost impossible to find the whole valve new. And if its gotta be replaced I'm guessing that means adjustment / tuning w/ a monometer or something, and I'm over my head.. .time to "call the man"...

I'm guessing that the poorly connected molex was creating juuuust enough of a bad connection/short to trip it, and that jkilled the coil...? So @Curtis_H and @83oldyoda4x4 were on the right track.
 
83oldyoda4x4 said:
I have always been told to measure to common wire not ground. Any truth to that someone?
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Yes there is some truth to it. It has to do with if the transformer is bonded to the chassis of unit or not. For the most part you can get by with using ground on a simple residential unit but best practice is to measure to common. I often see false readings to ground with larger commerical units.
 
83oldyoda4x4 said:
I have always been told to measure to common wire not ground. Any truth to that someone?
Click to expand...

Yes for sure. It also helps to have a meter with LoZ with control wires like that, they aren't twisted pair or shielded so it's fairly common to have ghost voltage on a tstat wire. A lot of times you can just measure to the nearest steel but sometimes it will fool you.
 
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