Turned on the fan after replacing the 3 amp fuse the first time. Seemed to run fine. Turned on fan after replacing the fuse the second time. Also seemed to run fine. Saw this thread but it is closed:

3 Amp Fuse Keeps Blowing On Control Board

I was confused at what it could be. We have two of the same units, the other has been reliable for the 13 years we have been in the house. This one keeps having trouble, including a replacement compressor under warranty. I suspect it has part to do with the MERV 13 filter, so I replaced it with a MERV 4. But the other unit is running on MERV 13 just fine. Checked the blower capacitor, and it has a 8.89 uF out of 10 uF (rating). Blew the board with an air blower/compressed air. Have not checked the contactor or contactor wiring as in the thread.

Any advice would be appreciated. Thank you.

(I am a CECS drop out, but electrical issues should be well within my grasp. I studied it for 3 years before dropping out.)

 

In the other thread, it notes to test the wiring to the contactor, by measuring resistance to ground. When I am up in the attic, where is a good ground? Thanks!!!!

 

Welcome to the forums.

The air handler is completely grounded.

You need to know what mode is causing the fuse to blow. You turned the fan on and the fuse was fine. When did the fuse blow.... when you turned the A/C on ? When you turned the heat on ?

Is this a heat pump with backup electric or a gas furnace with split A/C ?

 

Hi Pete,

Thank you for the warm welcome! (Definitely warm the other night when it went out just before bed!)

It is when AC is on that the fuse blows. The first time was after turning it on AC auto fan a few seconds, with temp two degrees below the display AC will start, and it did for that few seconds. The second time was after AC had run to cool the room some I believe. I was in the restroom, closed the door, and it was blowing cold air, and then all of a sudden it stopped. Then it wouldn't work anymore, fuse blown.

No heat pump that I'm aware of, but I might not recognize what that is, to be honest. Looks like it's "gas furnace with split AC". (Not 100% sure to be honest.)

Thank you again for a response.

Jesse

 

If there's a gas furnace... there's a good chance you have a standard split A/C system.

You could have one of two problems....
1) a chafed wire somewhere from the furnace all the way up to the outside condensor. A typical location would be outside as the wire is not usually well protected.

2) a contactor in the condensor with a coil going bad.

 

Thank you again Pete for the fast response.

Last night it ran twice as far as I know. Seems like it will be a hotter day today, so maybe it will have more work to do and possibly blow again. I will follow up if I get the chance to measure the resistance on the long wires leading out of the house. On quick inspection on the outside I see no apparent issues/possible shorts. We have had rats/mice in the attic spaces though. (Set some traps and brought them to the mountains......)

 

AC ran at least twice yesterday, during the day. I don't think the fuse blew. Just to make a correction the new filter is FPR 4, not MERV 4. Thermostat is set to 78. Sometimes at night just before bed I'd set it to 77 for my son's comfort, but I have been scared to blow the fuse so I've left it at 78. This is the upstairs thermostat. The downstairs one is set at 77. (As soon as it is cooler outside I open the windows, either in the evening, if I'm still awake after my son falls asleep, or when I wake in the middle of the night.) The amount of dust on the circuit board and surrounding areas was pretty minimal, at least compared to the blanket of dust I once saw on a computer board that I helped my son's teacher clean out that was freezing. Intermittent problems are such a pain in electronics. Sigh....

 

When I got up this morning, I decided to try Fan On. Nothing. Sounds like fuse is blown again. Is there a way to test the contactor directly? I have the panel on the AC unit outside open. Thank you!!!

 

So I was at the condenser yesterday. I tried to test the contactor and verified 1 nF across the two internal capacitors from what I could see in the wiring diagram. I then decided to twist the wire nut connections tighter like I remembered doing last time when I replaced the run/start capacitor. This morning I woke up to a Eureka moment: What if I damaged the connection inside the two wire nuts they used to complete the wiring to the contactor. I opened up the red one and it looked nice, the stranded wires looked nicely intertwined with the solid wire. I opened up the green one and woah, not good, the two wires separated immediately and it did not look like it made good contact. So I stripped a little more of the solid to make an even amount of bare copper. I straightened out the mangled mess of stranded wire to a decent amount, put them together and twisted on the wire nut. I then unscrewed it to verify a much nicer wrapping of the stranded wire around the solid.

Maybe I've found the problem!...... (Unfortunately two replacement contactors are on their way to my local Home Depot already.)

Time to gamble one more 3 Amp fuse I guess....

 

Blew the 3 amp almost instantly on the test. Gonna wait until the contactors come in before I try again....

 

You can disconnect the two LOW voltage wires on the contactor to do testing.
The power to the condensor should be off during testing as the 24vac contactor coil power comes from inside.

 

Thanks Pete! (For some reason there is a lag in seeing your response this time!)

So I got the contactors from Home Depot yesterday afternoon. I determined they looked nothing like the original, but the specs were as spot on as can be comparing the labels. (I got two just in case one was defective.) I disconnected everything from the original contactor, measured the resistance across the "inductor coil" (sorry, is that the correct term for it?), and got 7.3 Ohm. I measured the resistance across the two new ones, and they were at 12.0 Ohms. V=IR, or so I always thought. (At least on DC circuits). This is significant difference. (Bingo, the old one is drawing too much current and blowing the fuse.) I put the new one in, replaced the 3 amp fuse with my second to last spare, set the temp to 77 as the display shows 79, turned on the AC, and in approx 2 seconds blew the fuse...

Back to the attic for me I guess. Double sigh!

 

Locating an intermittent low voltage short can be a frustrating job.

First check for an obvious rubbed wire to the pressure switch in the outdoor unit, then check for sun rotted , or weed eater damaged wires between the house and condenser outside. These options are better than pulling new wire.

If they check out fine...

I would remove power to the indoor unit, isolate the low voltage wires to the outdoor unit at the furnace and outside, then measure the Ohm reading between each wire to the other wires and each wire to ground (refrigerant copper pipe).

 

Thank you Houston204!

I definitely have a short in the horribly long wiring. Since I already measured the resistance across the "inductor" at 12.0 Ohms, I went up to the attic and isolated/disconnected the low voltage wires. I measured the resistance across the two, (should be 12.0 Ohms or higher). No such luck, it is 1.4 Ohms. I left those wires loose/open. I go outside to the condenser, (thank you Pete for letting me know what it is called), and disconnected the wires from the contactor. I measured the resistance and it was at 0.4 Ohms. I have a short, but the outside wiring looks clean. Maybe the animal I heard scrambling around the lower attic got it, based on the resistance readings the short is closer to the condenser....

The space around the attic furnace has a nice board for me to walk on, but there are a lot of areas in the attic spaces that just have the "skeleton" beams to walk on. I am not accustomed to walking in those areas. I am thinking of trying to use a fish tape and run the wire to a vent I can see, try to grab it from there and across the top of the 1st floor roof to the roof top next to the AC unit and then drop it down. Do you have any advice to what gauge wire to use/what to buy? It will be an even longer, maybe quite a bit longer, run doing it this way, but I may not have to walk in those areas where I may accidentally step in between and punch a hole in the ceiling! Is there a limitation on the length? I noticed the wiring inside the condenser is much thicker than the really long wiring from the furnace all the way there...

I must make a correction: There is no capacitance across any of the contacts that I thought I found before. Must have been because the contactor had some connections left/I did not disconnect everything from it.

Thank you for all the help! I am so happy I can work on this by myself. One time I called for service and the guy who came charged $300 just to replace the capacitor in the condenser unit. (I felt pretty dumb for not opening up the panel and checking myself first before calling.)

 

You're getting there.

It wouldn't be the first time I've found a mysterious short in the wiring.
I've found splices in the wall that were sometimes the cause.

I'm guessing you only have two conductors to the outside condensor.
If you had more than two you could switch to a different pair.

 

If you only have 2 conductors and they are shorted to each other, but not shorted to ground, there is a work around with some systems.

Use the 2 low voltage wires for the Y signal and ground the common wire from the contactor.

I would pull new wire if possible but when this isn't an option I have grounded common to complete the circuit ( if common is also grounded at the furnace).

Those darn " squirrels" love to chew on low voltage wires.

 

Thank you both for the tips!!!

I was discussing the scat (poop) that I had found in both 1st and 2nd floor attics with an exterminator (here at my parents' house. My wife, son, and I live with them). He told me to look carefully down those long channels created by the wavy tile roof, and said it is impossible to prevent animals such as mice/rats from getting in. Just to set traps continually and send them their way. (I haven't been keeping up with the traps, and I guess this is what I get.)

There are actually three wires in the "line". I was going to ask that question at some point, the third is a green one, and it is just left open/dangling. I was wondering where I should connect it. I wouldn't be surprised, though, if all three are shorted together. With the little resistance I am measuring it seems like a complete short. (That little animal must have been really hungry!) Uh oh, that reminds me, I didn't take note exactly where I pulled the two wires from. I have a 50/50 chance of putting them back in the right place if so! Argh!!!

Maybe I should try practicing walking on those rafters, or whatever it is called. (Unfortunately I'm somewhat on a budget these days to call someone to come out and remedy the situation, but if I end up having to I will. It hit 100+ here just (yesterday now).

That ground work around sounds very interesting. I'm going to think about it a little... Is that safe? Aren't you sending AC current through the ground? (Not very good understanding of AC I'm afraid...)

It's very late here now I'm afraid. I have a little insomnia, but I'm very tired now. Thanks again. I'll post what I sort out later...

 

You can very quickly check to see if the green wire is shorted to either of the other 2 wires. Strip one end of the green wire (whichever end is easier to get to), and using your ohmmeter, check to see if you have any resistance between the green wire and either of the other 2 wires. If the ohmmeter shows an open circuit (infinite resistance), then you could use the green wire in place of which ever wire is currently connected to the Y terminal.

If you need to try Houston's idea, it's very safe. There is only 24VAC between the Y wire and ground, and relatively low current at that. It's not dangerous.

 

Thank you Bob14525! Yes, I don't know what is wrong with me. I should do that first! (I've started searching for wire already.) I don't work in an electrical related field. I'm pretty much an at home dad. I will go do that first right now.... (Maybe whatever it was decided copper wasn't quite palatable and may have just possibly spared the green.....) As my son taught me, BRB....

 

Well, what do you know. Green was NOT shorted. I already spent a little time looking for wire....

​​​​​​Time to put the humane traps again, with the secret peanut butter.....

Didn't dare try it. Set the thermostat at 78 and hoping it won't blow my last 3 amp fuse. I had 5 replacements, and it was already 97 F just before noon!

Thank you! Thank you all!!!

(Had to rush to take my mom for a CAT scan but thought I'd try to update....)

 

Glad to hear that you're up and running again. Thanks for letting us know.

 

It's been said that critters don't like green wires.

 

Thank you again guys!

It reached 105 F today. I don't usually like working in the hot attic in the afternoon but I knew if i didn't at least try with what I had/without running wire, we would be miserable tonight! (I came home and the fuse had blown, thermostat said 80 but no air blowing.)

I went to pick up my son from school, and when we got back to the house I asked him if I could leave him there for 30 mins by himself, (he's 12), called Pep Boys to find that they did not have the 3 A five pack in stock per their computer. I decided I would go there and look on the shelf as online it said it is available in store and that is where I got the 5 for $3.xx. The hanger was empty for the 3 amp. So I looked through their packages of variety and found the cheapest was $9.99 for 96 fuses, 12 of which are 3 amp. It's more than a dollar each, but while the 5 pack was less than a dollar each, but small price to pay on a day like today.

I brought it home, put two in my pocket, and went up to the attic. This time I only covered the panel that activates the safety switch. Opened it up and measured the resistance with the wires all connected. It was shorted. I had connected the red and white to the same Y, which had a yellow wire there originally. On the condenser end I had twist tied the red and white onto the same green connector. The "new" green wire went to COM and the other side of the contactor. Then I remember your advice about grounding the COM. I took the red and white, and swapped them with the green, green to Y, and red/white to COM. (I hope I got the two correct, I might have them reversed.) I measured the resistance and 13 Ohms. I put the panel back on and the blower sucks the panel onto the air handler, I had forgotten and left the AC set to Auto, and it was warm enough to immediately activate.

Now I have to run and take my son to Religious Ed....

Thank you! Thank you! Thank you!!!

 

Yes...the green needs to be used as the Y wire since you already know that the red and white wires are shorted.

 

I hope that the rodent doesn't continue to chew on your wires.

 

No wonder I dropped out of computer engineering, it takes me so long to figure out a problem...

I've thought about the rodent problem some more:

I told my son I'm scared to go up to the 1st floor attic, because the thing that chewed the wires may just take a bite out of me! The traps I set are little cages that catch the virmen live, and although there is a little handle up top, there's a slight risk the mice/rats I've caught could scratch me, so I use some gloves. (I haven't gone up yet, been busy, need to move the ladder (heavy), and honestly a little, alright, maybe more than just a little, scared.)

But after swapping the wires as you guys suggested, I realized that the white and red are probably already shorted to ground. This is the only way it would still short, as I had both red and white connected to the same terminal. This would also suggest somewhere there is either a metal surface that is grounded, (rubbing a sharp corner), or literally ground/dirt (possibly yard work, animal, weed wacker, etc as suggested), where the break in the wires occurred. A neighbor/lady/friend down the street has fallen in love with bunny rabbits, and now has 3. She let them roam around and found that one has a fondness/dumbness of chewing through electric wires, in particular once under their electric reclining seats it apparently chewed through the wires and the seat no longer worked! The wires were completely severed, and I gladly jumpered the wires for her so that it now works. (She no longer lets them, in particular that one, roam under there.) I was told by an electrician that came to the house once that sometimes the rats would chew the AC (live) wires, but that would end, assumingly as they are shocked. These aren't live unless the compressor is running, so I would guess the wires would be severed if it was some sort of animal.

I am up in the middle of the night. (It is a quarter past 3 but I got up around 2:20). Since I wake at night, sometimes when it has been a hot day, and I can't open any windows before bed, I get up and open at least the 2nd floor windows. For safety reasons nobody wants the 1st floor windows open without someone around. Finally some cool air is coming in to relieve the AC from it's duties....

Thank you again for all the help everyone!!!

(I hope to stick around, and pass on some experiences I've had to others. Is there a car section around? I have an affinity to working on car related stuff. I'll try to search, as I'm new around here....)

Take care, and God Bless!!!!

 

Seems like my calculation on per fuse price was off as well! Oops.

 

"It's been said that critters don't like green wires. "

If I ever have to run wire, I will run all green!!! (Forget the electric code!!! I'll figure out the wiring somehow....)

 

Just put a little bit of colored electric tape on the ends of your all-green wiring. Mark both ends of each green wire with red tape, or yellow tape, or white tape (depending on what colors you need). Hopefully the critters won't ever see the tape on the ends and assume that they're all just green!

 

It was just a joke. Rats are color blind.

I usually run shielded wire when rodents chew the low voltage wire.
( But this is often the only control wire that I stock in my van)

 

I tried to search up what colors a mouse/rat can see. Seems like mixed reports. Sine a rat/mouse can't talk I'm not sure we can tell. (Thanks for the joke! Definitely lightens up the situation!) It was much cooler yesterday, so much so I didn't need to close the upstairs windows the entire day. Maybe fall weather has finally set in, knock on wood....

Thank you again for all the insight and help everyone! Sounds like a great idea to label the ALL green wires in my future! Did I ever mention I'm slightly red green color blind? (for real)