Hi all! I've been reading the posts regarding fuses blowing in the secondary voltage circuits on Bryant furnace/blowers. I blew the 3 amp fuse in this circuit two weeks ago on my gas Bryant furnace. Repairman diagnosed the problem using the flashing led light (2 fast, 4 slow). He replaced the fuse and the problem was solved... for about a day. Then the fuse blew again overnight. I replaced it myself the second time but now it has been blowing every few days or so and this morning it blew and now every time I replace it, the new one blows immediately after the front panel is replaced and the system is turned on. I've checked the wiring that I can access (mainly the red and blue wires to and from the transformer) and everything appears to be normal. I have a new thermostat on order and will try that when it is delivered but could the transformer be bad? Do transformers ever have problems? The system is 8.5 years old and have never had any problems. I am a bit curious that it never had a problem, then it blew a fuse... then 3 days later again, then every few days and now every time. A definite progression/accelerating trend. I am a bit of an electrical noob. I can replace fuses and thermostates and check wiring but beyond that I am lost. Any help would be greatly appreciated!! Thanks in advance!!

Now that the intermittant is not an intermittant anymore, you have a couple of choices:

1. inspect wiring
2. Disconnect everything at the t-stat. Put furnace together.
If it works - not likely tstat. If it doesn't likely wiring.
3. Disconnect at furnace (determine if wiring)

There is a post about a light bulb trick that can allow you to trace a short. You basically replace the secondary fuse with two automotive light bulbs in series that draw slightly less than (40 W/24) amps. Disconnect wires until the light goes out.

Awesome! Thanks for the fast response! Will try those suggestions. I've had someone suggest that I put a slow blow (delayed) fuse in just in case it's a momentary amperage surge but I don' really like that option. I figure that the 3 amp fuse is there to protect stuff and allowing more than 3 amps to go through for even a few seconds longer probably isn't too prudent. I'll try your suggestions and report back... Thanks again!

Here is the thread.

Hi,
When my wife found your posting, I was surprised to see how similar our situations are. My gas furnace, Bryant model 373LAV, a horizontal unit in the attic started with a blown fuse 3 nights ago. I first thought it was a bad thermostat, so I switched a new thermostat replacing the builder stalled Honeywell T8000 with a digital one. Nothing improved. Then I found the blown fuse and switched it out. Heater worked for a few hours after that, but didn't last through the night. I then replaced the fuse again and just monitored the operation. It seems that after the flame goes on for 20-30 seconds, the fan kicks on, and that's when the gas shuts off and the whole cycle starts again. I suspected low air flow, so I inspected the filter and it was dirty. But after replacing the filter and another fuse, the heater worked for a few more hours, then it burns out the fuse again.
I too was tempted to put in a 5 amp fuse, but am hesitant for the same reasons that it might cause more damage to the unit. I suspected wiring touching each other at the thermostat end when I replaced it. I went back and inspected the wires carefully to make sure they are not in contact, and I actually put the old thermostat back.

I think my next step is to call the tech support for Bryant and see how far I can get. If you get some resolution to your problem, can you email me and let me know? Or post your solution? I will certainly do the same. Thanks.

I can't find my post, but the idea is to get two 12 V light bulbs wired in series that draw about 40 VA/24 Amps. I just forget the lamp numbers. I can find them I guess.

Put this where your fuse goes. When the system shorts the ight lights. It's a current dependent resistor.

Then the next step is to wiggle wires to see if the light lights.

I'm not sure what your thermostat wires are or if it's a heat pump.

Basically shorts at the thermostat end except for the common wire might be rare.

Short going to the outside unit is definitely a possibility.

Give me your thermostat wires that are used, I'm particularly interested in the use of the common wire.

I'm also interested as to how long it takes to happen and what modes it happens in.

Does it happen in fan only, heat only or heat and cool?

We can go from there.

Remove all t-stat wires from low volt terminal board and use jumper wire to see if short occurs while cycling unit with jumper.

Hi guys, thank you very much for your input. I couldn't really find any obvious wire problem, and frankly just don't have enough time/energy to climb into the attic (where the furnace is) to try the various thing. I called a heating/cooling tech, he came out today and informed me that it was the transformer. He said something about the fuse was to protect the secondary circuitry and it's the primary circuitry that was shorting out the fuse. He will come back on Monday to replace the transformer. I just hope it won't cost me an arm and a leg. And so, thank you all very much for your input.

If he tells you he blew another transformer and wants to charge you for two of the watch out. LOL It has happened before.

Sorry, need to use jumper with inline 3 amp fuse when doing the low volt test, or a mini 3 amp breaker., thanks 1000.

If true, a primary to secondary intermittant is a hard one to find unless he got lucky or was able to use a megger on the transformer which would have found it easily.

Did the new transformer take care of the problem? I have the same symptoms!

HI, so did the transformer fix your problem?

I'm still replacing fuses once a week. Was waiting to see if it helped "KISS" before I replaced the transformer.
Sorry -
Jeff

A low voltage short can come from anywhere. Its common for low voltage wire to rub into the refrigerant lines on the A/C and short out and be intermittent as the system vibrates. Is your short after you run the heat or the A/C?

Seems random, don't usually notice it until the house gets cold and I go looking for the problem. I'll check for some abraded wires though and see if I can find anything. Thanks

J

So it sounds like you notice it when your trying to use your heat? If so you may have a gas valve going bad. Not knowing What type of system you have makes it hard to answer. Not to discourage you from giving me more info but its like you trying to guess what number I'm thinking of from 1 to 1000

No problem. I'll cycle it a couple times and see if I can force the issue and find out when it occurs. I also have a scope I can look at the transformer output with and compare to one of my other zones that are working to see if the output voltage is clean and on target. Appreciate your help.

If you have a zone system it's a whole new ball game of what to start checking for the short. Any damper, limit or thermostat in heat or cooling mode could have the short.

Did replacing the transformer fix the problem? "rclee71"

I have been having very similar problems to what I have been reading about Bryant furnaces and the 3amp fuse blown. I have lived in my house for almost 5 years and the house is 11 years old. I have replaced the 3amp fuse probably a dozen times and I have recently replaced the t-stat. The furnace has always worked for an extended period before the fuse blows again. Just yesterday I replaced the blown fuse only to have it blow twice within minutes of turning on. My furnace guy came by this time determined to find the cause. He checked all of the wires leading into the t-stat and found continuity when there should not have been. He traced the wires from the circuit board as they come out of the unit and head up the side towards the t-stat. Just about two feet up the side of the sheet metal the installer put an installation screw accidentally just enough into the t-stat wires that is was causing an intermittent short that has progressively gotten worse as the furnace vibrates causing the pointed end of the screw to work its way further into the t-stat wires. We spliced the t-stat wires to remove the damaged section. My furnace is working properly now and I believe this will no longer be an issue. Hope this helps someone else.

I have this code but have no idea where to find the secondary voltage fuse. My Bryant air conditioner was functioning fine until I changed the batteries in the thermostat. At that time the air quit working. At first I could jump run the blower in the on position but not in the auto position and the outside unit wouldn't come on at all.
Then things got worse. Now nothing will work. I have a 2 rapid/4 slow code and would like to replace this fuse if I know where to look for it.
Please help!
Thank you... Tim

I know this is old (3 years old) but wanted to see how it went when the tech replaced the transformer in your Bryant. I am having the same issue and am thinking I need to replace my transformer.

I know this is old (3 years old) but wanted to see how it went when the tech replaced the transformer in your Bryant. I am having the same issue and am thinking I need to replace my transformer.

What is looks like is I have a short somewhere in the low voltage circuitry that is blowing my 3A fuse on my control board.

HERE IS THE ACTIONS THAT OCCUR
1. Fan turns on
2. About 8 seconds later the pilot lights
3. Burns for 60 seconds
4. Then fuse blows, and blower turns on blowing cold air through out the 5. House (because there is no more flame)

Unfortunately I cannot find the short. Any idea's

What burnes for 60 s?

Pilot or Burner?

It would seem it could be the main gas valve.

The pilot lights, then I hear clicking over and over again like a grill lighting, but the burners never turn on. At 60 seconds the blower starts at the same time the 3A fuse blows.

I have been debating if the flame sensor is dirty, or bad, or if the gas valve is not working. I have a new control board and wiring harness. Verified the voltage before and after the transformer and its 110 in and 24 our to SEC1 and SEC2.

Not sure how to test to see if the flame sensor is working(I can't even find it on the old Bryant Plus 90) But thought is either the flame sensor is not telling the control board there is a pilot, or the control board knows there is a pilot and is telling the gas valve to open, but the gas valve is not opening.

I'm confused somewhat.

Verify at least that the fan works in FAN mode and that it works in the AC mode. I suspect it will.

In Heat mode, the furnace controls the fan. In AC mode, the thermostat controls the fan usually,

It's also possible that the main gas valve is bad and don't know if you can disconnect and see if the fuse doesn't blow.

Unfortunately, I'm not sure what kind of system we are dealing with, In some cases, the "thermocouple" gets heated and it turns on the gas valve.

i.e. A purely mechanical failsafe. ON, then thermocouple gets heated and the gas valve turns on due to the expansion of the junk inside the tube.

So,not sure if you have an older system or an electronic flame detector. That uses flame rectification and you have to be able to measure a very small current.

Easy to tell, if the flame sensor has wires and goes to the control board it's electronic and it can be cleaned with emory paper.

A mechanical flame sensor, "thermocouple" will generally be a small copper tube.

Pictures, wiring diagram could help.

The 24 VAC fuse blowing when it does, I would think indicates a probem in the 24 VAC system and I "don't think" it's the thermostat or wiring to/from the stat.

One other possibility is the 24 VAC phasing if the sensor is electronic. If you accidentally reversed the wires to the 24 VAC secondary or Primary, the flame sensor may not work.

The fan works in AC mode, and on just fan mode. The fan also works in heat mode.

"It's also possible that the main gas valve is bad and don't know if you can disconnect and see if the fuse doesn't blow."
So from this, if I get what you are saying, I should be able to disconnect the wires that go from the control board to the gas valve, and if after 60 seconds (when the blower turns on and the burners should be lit) the fuse does not blow, then it would mean the short is in the gas valve?

The unit is an old Bryant plus 90, upflow furnace. About 15-20 years old. But I don't see the flame sensor, but the burners and pilot are in a burner box which is hard to get to without tearing the burner box apart.

Tonight I will post some pictures of the burner box, gas valve, control board and the wiring diagram from the Control board and inside of the side panel.

Here is a link for the YouTube video that I recorded with the full cycle and pictures of all the components
Full cycle until furnace 3a fuse blows - YouTube (http://youtu.be/djoiEkWYaV0)

http://youtu.be/djoiEkWYaV0

Do you have an actual model number from the nameplate?

I think the first part of the model points to the carrier base 398AAW0 and the second part 360600 has something to do with Bryants naming.



The full model on the sticker is 398AAW0360600

The product on the sticker is 398AAW0360600AEBA


So my guess that Bryant added 360600 and AEBA probable is some sort of a version information