Finding Low Voltage Shorts Fast! HVAC Troubleshooting!
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- Опубліковано 2 жов 2024
- In this HVAC Training Video, I show How to Quickly Find Low Voltage Shorts with a Multimeter! Once you know where to look and how to divide the wiring, troubleshooting the problem can be done fast! Supervision is needed by a licensed HVACR Tech while performing tasks as Experience and Apprenticeship garners Wisdom and Safety.
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Let me suggest a method for finding low voltage shorts without removing all those wires. First I'll be obvious, the reason the fuse is blowing is because something is shorted and therefore has very low resistance or zero resistance drawing lots of current and causing the fuse to blow. The temporary solution is to add series resistance (inline with the fuse or replacing it) to the circuit with a known level of resistance drawing less current than the rated fuse. The simple answer is to insert a 24 volt bulb in a socket with two wires or a commercial short finder (all these things are is a 24 volt bulb packaged nicely with wires). The current draw will be 2.5/3.0 amps which isn't enough to blow the fuse. The bulb will light because you have 24 volts on one side and the short giving you a ground on the other side. Grab your multimeter and wrap the current jaws around each low voltage wire on the control board until you find the wire/circuit drawing 2.5/3.0. That is the wire/circuit where your short is. Now track that wire to its termination point which could be a component. Disconnect the component and if the light goes out you found your shorted device. If it doesn't track that wire along its length to find where the wire is touching another wire or the frame of the furnace. In any case you cut your troubleshooting of the short down to five or ten minutes. You can use the same procedure on a cars, but use a 12 volt bulb. I've used this process for years to track down short on cars, furnaces and even circuit boards. I've found the current jaws on a multimeter the most underutilized item in a tool bag. Hope you find this useful.
Yes clamp meter is a great tool, makes those parasite draw much easier to find, great tip!
What would my amps be of the other low voltage wires? It never dawned on me to use my claw to read low wiring amperage?
I didn’t think a multi meter could pick up amps that low such as 2-3amps
Great point! Resistance testing often produces flawed results for a host of reasons.
You can use your clamp meter to measure very small AC currents. For example. If you clamp your meter on a single conductor carrying 1 amp, your meter will of course read 1 amp but if the same conductor passes through the jaws twice, now you will read 2 amps and 10 times, your 1 amp will now read 10 amps on the meter. It's not very handy to wrap a wire around the jaws 10 times and then insert that in series in the circuit you want to measure but it works. And a tip on finding a short circuit. You don't want to go crashing in on a short because in the process you may clear it and then it won't be there anymore to track down, then in a couple of weeks have the short return and the fuse blow again. I think a person is fortunate to have a dead short rather than one that is intermittent. Sometimes I also use the light in series with the load to track down shorts. I bought your refrigeration book last year and it really helped me understand the subcool and superheat I've heard about and didn't understand. It's always nice to learn something new.
Dude I know you made this due to feedback. You're awesome!
Hey, thanks!
Excellent Video, very good visual and explanation.
This is so complicated 😞
Don’t get discouraged! Keep watching these videos. Learn a little more each time.
Aww, and I thought you were gonna tell us what the magnet was for. Oh, well, loved the isolation procedures! Thanks.
It’s to close the door switch so power can run with blower door off
Probably to hold the door safety switch closed.
There's an easier way. Take all your thermostat wires off and then touch each wire one at a time to the red hot wire.and if you get a noticeable spark on say orange wire then you know it is on that wire circuit, do all wires this way except the common wire. This is the easy way to bypass all that nonsense.
Low voltage Shorts are time consuming and can get frustrating . Your "process of elimination" is well explained . I find this video very helpful to deal with this issue. Thank you very much for sharing this video.👍😊
I always use continuity to find low-voltage shorts.. Please explain why ohms is better then using continuity on a short to ground or common?
To keep from making one million trips wire a T-stat to carry w/ you for the amount of wiring you need. You can use 2H/1C T-Stats for furnaces if you wire the W to your Emergency Heat in your diagnoses T-Stat. If it’s a Heat Pump set then just continue as the wiring should be applied. Attic trips are a killer, don’t forget your fuse pop switch so you don’t burn through fuses.
Explain in more detail please!
This is a great explanation for the process of elimination. I would also recommend to especially younger technicians to get a Lil Popper fuse reset tool or something similar. They come in 3 amp and 5 amp sizes. They are helpful during the troubleshooting process so that you aren’t just burning through fuses.
I was using one the other day on a twinned system and it kept blowing the fuse on the second air handler and not my popper… I was so confused
@@anthonys6187 I have seen that happen before too. Now that everyone is switching over to cool mode, make sure the contactor is not shorted. If they are in cooling mode and you are blowing through fuses and transformers, that is likely your problem with these seasonal changes.
Other things
Did you try and isolate the 2nd twin from the mother system to see if it was the one popping the fuse? I naturally want to look at the outside thermostat wire to see if an animal has been chewing on it or a melted wire on the outside condenser. Sometimes the low or high pressure wires touch the hot copper line.
Greatest video so helpful
Glad you think so!
This video helped me diagnose a short to ground from our humidifier, which was causing our ecobee to only call for heat for less than a second, and the 3 amp fuse on the furnace board to blow when contacting Rc and W1. Turns out one of the wires poking through a sharp hole in the cold air return got nicked, who knew?!
Thank you so much, you've saved me a bunch of trouble and money.
For boards I often find thermal imaging shows the blown component ( seek thermal pro for smartphone ) you can also usually spot the hot flash of the groundcshort asthe fuse trips... excellent video as always
Beautiful, this trade is becoming more easier watching you sir. Thank you!
So nice of you
Great video. How do you fix an issue where the problem Is not a direct short where the fuse doesn’t blow right away.
I
Hello sir. Do you know what would cause the outside ac unit fuse to blow. I replaced the capacitor already. Thanks
As always thank you for sharing your thoughts and knowledge
So nice of you
it is the best channel for HAVC systems ... welldone man
Not sure how old this video is but it was tremendously helpful to let me today. While my problem turned out to be something unique (an unused wire from the contractor coil outside had lost the electrical tape that was shielding it and it was arcing on the cabinet) . using your guide, I was able to determine everything it was not which eventually led me outside to look at the contractor coil. Thank you!!!!
Glad it helped!
My fuse does NOT blow when I turn fan on. When I turn AC or heat on it blows instantly. Any tips?
That short could be any where.....!
Great video I love the set up the app it's a great learning
Thanks for watching
Your vids are top notch and have helped me become a better tech for my customers and business partners! Thanks a million sir.
Thats awesome to hear, thanks!
Do you have a 24v trouble shooting card made up? Easy to keep the the truck.
Wanted to give a big thanks. Had the dreaded 3amp fuse issue on a 4yr old carrier split system. Out of all the videos I watched yours by far was the easiest to follow and quickest to resolution. Thanks for sharing your knowledge.
Very knowledgeable and straightforward thank you.
OR lol you could just pull the stat and ohm out C to all other leads and Boom walk straight in the direction of the problem
I love and watch a video just about everyday..
Want to find a short even faster than my suggested method using current jaws? First, use the technique explained in my previous post by adding serial resistance with a 24 volt bulb in place of the fuse or in series with it. This will prevent the circuit from blowing the fuse while allowing it to remain active drawing somewhere around 2.5 amps. Next drag out a Thermal Imaging Camera and look over the wires and control board (remember all wires whether from the thermostat or outside units terminate at the control board). The shorted circuit will be obvious. Thermal imaging cameras can be had pretty cheap these days and worth adding to your tool kit. Hope this suggestion helps in your troubleshooting.
Wowe I’ve got the tools in my van. I’m purposely going to cause a short and test it. Good advice
Film a UA-cam video showing us in field footage.
good morning Craig! all of your hvac articles, videos and books are very clear and well explained. I got your mini split hvac books from Amazon and I am still learning it so far very helpful. so. common and ground are same because they are touching each other. when hot wire touched to the ground it will just blow the fuse but if hot wire touch the common directly than it will blow the transformer with fuse. but common and ground are same on the other hand. why short with common directly will kill transformer not just the blow the fuse?
It will still blow the fuse but may also blow the transformer because both hot and common are connected to the transformer. Hot to ground will still blow the fuse but is less likely to blow the transformer unless the path of least resistance is back to the common on the transformer. regardless, we do our troubleshooting with blowing fuses or breakers and by using electrical resistance measurements, thanks!
First principles.
The whole problem with the ENTIRE hvac industry is service and parts for profit.
Of course you will get shorts,opens,semiconductor fails, bond wire, substrate die junction, when they do single board designs without conformal coatings
whilst no protection from dust, moisture, heat, vibration. I have had to rebuild entire control cabinets of NEW 600 ton chillers from
CARRIER because of this BS. Yes this voids warranty and service guaranties but I will never need them. They don't even use real ball, pin or roller bearings in motors in the smaller common garbage.
Thank you for this video. It is 2024 Feb., and I hope that ppl or you still will see this reply. I am trying to learn about heat pumps. and was wondering if you or anyone could explain about the "common and the ground" on the 24v circuitry. i just don't get it, how there are the same as you say here. HOW IS THE COMMON, the ground? Or how is the ground common?
I recently dealt with an HVAC company who someone else called to fix something. He blew up THREE transformers before he told me that there was a dead short in the thermostat wires.... Which I already knew and already told him.
Why didn't I just do it myself? I was working there temporarily and i didnt make the call and i was too busy to deal with it. Ultimately, I used an unused wire to the stat as a substitute.
Thank you for the informative video. I'm just wondering if you can upload a video on how to troubleshoot the water source heat pump. Thanks
Do you have to disconnect the wires on the board in order to get proper ohm readings? I noticed it had a measureable resistance when it was connected to board and not OL even though nothing was calling and the power was off. Can you explain it? I'm just curious as to why it wouldn't read OL, i would assume the wires aren't touching on the board until a call is made
How do I fix the fuse blowing if the fan is turned on? We have already replaced the fan motor the thermostat and checked the green wire for continuity along with red. I can’t find the problem.
So, what does it mean if the voltage between the red wire and the green wire is 18 volts, but it's 27 volts on all the other wires? That's the problem I'm having, and although I could be wrong, I suspect it has something to do with why my fan doesn't always come on.
I had one today. The led was in standby mode. As I was testing I crossed W with R
Blow the fuse. But didn’t blow the fuse when
The thermostat called for heat . The furnace didn’t come on . As I was going through the troubleshooting I found it . The high limit switch was shorted to the frame .
Gas furnace??? Is this the same for small commercial buildings???
I have installed transformers and the metal frame of the transformer is connected to. Ground and as I understand the bare copper wire is connected to the ground frame terminal and back to the service disconnect ground terminal which then goes to the ground rod to earth ground.
I swapped my 5 amp for a 10 and not I just hear a faint hum furnace won’t turn on at all now , my 5 amp fuses would blow every 5 seconds
This is great, helps narrow down the search to 1 run. Thank you.
Glad it helped!
Sweet video ! I just ran into this issue. When I turn my thermostat to either AC or the heat pump, it blows the 5amp fuse on the board inside the furnace unit. If I use just electric heat, then no issues.
I isolated the short to the yellow wire by unhooking it from the AC unit outside. I turned my thermostat back to cool and the fan on the furnace came on with no issues and the fuse didn’t blow again. I decided to use the spare wire from the outside unit and used the black/white wire. Once that was hooked up again, the fuse blew as soon as I requested cool. What I also noticed on the board where the wires hook up, is that I have 2 wires on the same connection. (Yellow and white, but now black/white and white). It’s an older unit so I don’t have wires at my thermostat to test. I did trace the thermostat wires from the furnace unit and put together the new black/white wire with the white wire. Now the fuse no longer blows when I switch it to cool, but now my outside unit is not switching on. Any ideas ? 🥲
The only thing I don't understand is how attaching the thermostat faceplate in off mode changes anything or determines if the thermostat is good or not.
Young Maria Walker Eric Walker Elizabeth
Once again, great video and explanation.
Thanks for watching
awesome craig! thank you so much for sharing! alway's lovin your video's!.
i changed the thermostat and now nothing works except the blower... even without the thermostat connected to the board it stays on.
i have not blown a fuse or lost power in the main board.
The gas valve is a smart valve and power for the inducer motor is coming from main board to the gas valve but no power is going to the inducer from the gas valve, does that mean the gas valve connector/board is no good?
i checked the limit and rolls outs, all showing continuity with power coming and going.
The connector on the gas valve for the inducer motor is 4 wires.. black and white IN (which has power ) and black and white out ( to inducer having NO power) ...is this a direct connection ? or is there something else that needs to be satisfied before power is sent to inducer?i trying to figure out if there is anything i didn't check
Isolate wires, one at a time put wires back which ever one blows fuse is your prob.
What if the thermostat not lit due to power outage? Tripped the circuit breaker
You are a Great! teacher for that reason i purchased your books.
Wow, thank you!
Williams Sharon Garcia Michael Gonzalez Matthew
I have a question on the harness which was the first wire you stuck your lead too ???
Y keeps blowing the fuse even with new cables. What should i do?
I’ve done refrigeration for 30 years, I don’t get much practice on home units. Very informative! Thanks for sharing!!
Glad it was helpful!
Craig i appreciate all your time and effort u put into your videos thanks for helping all of us that are starting out u have teached me alot and thanks to u i have hope to one day have my own company ur very inspiring to all of us i want to let u know that god bless your family always thanks for helping us all out man your an awesome human i admire u
he's been a great help for us guys starting out for sure!
I have a unit that pops 3amp fuse in heating, only 4 hrs later.!
Thank you. This video taught me something today. Where can I send you a check 😅😂. The hats seem nice I’ll take better look when I get home great job explaining
Great video, I found a rare problem when troubleshooting a circuit board fuse problem. The contactor coil in an outdoor unit was the culprit, I measured the resistance of the coil and to ground both readings were normal. When 24 volts was applied the fuse blew instantly, the only symptom I found after removing the contactor was that the contactor had a burnt insulation smell. Normally when a contactors coil is energized with 24 VAC a magnetic field is produced opposing electrical current, because the coil insulation was damaged several turns of wire in the contactor shorted together. As a result the coil did not produce a magnetic field allowing excessive current to flow through the coil destroying a 3 AMP fuse.
Was that the permanent fix?
@@Swoliosis1 That was the permanent fix, the original AC contactor was damaged due to faulty wiring when the owner changed his furnace circuit board. After properly installing a new circuit board and replacing the AC contactor the problem was solved.
@@practicemakesbetter7132 We recently had some very bad storms in my area... a day later, my system blew the 3 amp. Replaced it, blew again instantly as soon as the contactor energized.
Checked all over for shorts... replaced fuse, disconnected contactor... fuse did not blow at FAN ON or COOL ON. Replaced contactor, system now works. Contactor tested at ~16 ohm.. not shorted to ground. Don't know?
@@Swoliosis1 Good job finding the bad contactor fault! Normally when 24 volts AC is applied to the coil of a contactor, the coil produces a magnetic field that impedes the flow of Alternating Current thru its coil. Because the coil windings were damaged, there was no magnetic field created and there was very little impedance to the flow of current thru the coil; that is why the 3 Amp fuse blew when 24 volts was applied to the coil.
@@practicemakesbetter7132 About 72 hours out and we're still running...
The DIY videos, web sites, etc... don't really talk about parts still being bad, even though they measure well (by typical methods). Maybe at the moment the contactor engages and/or under load it exhibits the symptoms and only then. If so, may need a lab setup to prove it... either way, old contactor in the trash.. ordered a new one for $10...
Thank you for the help you've given me so far. I determined that I have a low voltage short coming from my AC compressor (read 0.5 ohms instead of 15-20 ohms). Do you have a video for troubleshooting at the compressor that I can watch?
Thanks.
Wow I wish my unit was this simple. I have what looks like another thermostat inside of my furnace and a black box with a red timer knob on it and also another black box looped to it by 2 wires with 3 wires connected. I am so frustrated with this. Your video is great but my unit is just a lot more complicated. By the way I'm blowing fuses when unit is calling for cooling. Doesn't happen on heat and not happening when calling for fan.
This video is very helpful but I am still having issues. I have a split system, AC and heat. I tested the system similar to what you did. I have two sets of thermostat wires going into my FAU. The ones from the Thermostat does not trip my fuse. When I plug in the thermostat wires from the AC unit outside It trips the fuse. I even tried switching the wires and still trips so its not a break in the line or grounding issue. Any ideas on what it could be? I am replacing the AC capacitor as a last resort of something cheap before calling an HVAC.
How are you sir, i want ask you a question, can l replace resettable transformer to a normal transformer, instead burning the transformer l can just reset the switch??? Thank you
I have seen in another application of the secondary of the transformer that the common wire connected to ground would have to have a diode from anode to cathode and cathode connected to ground that way the back current cannot go back because the diodes reverse bios will stop any back current flow!
I have a 35 yr old air handler, what to do if it has no circuit board and a 3 amp fuse? I bypassed the thermosthat and now my AC won’t turn on. I guess I short circuited the unit.
So I have a honeywell smart wifi thermostat and a carrier gas furnace. Whenever I run the heat and the furnace does it's heat cycle, soon as it goes into 90 second cool down where the fan keeps goin but the furnace isnt calling for heat my thermostat cuts off and then restarts once it's done. Any thoughts?
Where'd you get that magnet for the door switch? Approximate O.D. diameter please. Thx much!
Do this exact thing with a basic gas garage heater. I unplugged everything from the board and it still blows the fuse. I ordered a new board after running out of options.
Hello, If a compressor was recently replaced in January and not cooling due to low freon, and constantly running and trying to cool the house but the temp keeps rising, could this constant load of the compressor trying to cool with low freon for hours cause a disconnect to short out?
Thank you for sharing your knowledge 👍
So nice of you
thanks for sharing. Questions: does anyone know what it means if you do get a resistance reading, for example: common to green, instead of OL?
Easy way to do this quickly without making trips to the thermostat is install your popper fuse and unhook your wires. Leave your thermostat calling and hook them up one by one until it trips. Of course use your meter to verify your suspicions, but that’s a quick down and dirty way to find the problem.
I sure was hoping to see how you use that microwave donut magnet that was hanging by the door switch to override that door switch...LOL. 🙂
Watching this again
I had a house on Friday where there with a 6 zone pneumatic zone control. After about 2 hrs of tracing a maze of wires, there were no marrettes on the humidifier, and they were touching the duct work. The ac low voltage wire was also bad inside a finished ceiling, and the R wire entering the zone control was broken and touching the case of the box. It was a mess.
Great video thanks for sharing
Thanks for watching!
On the 3A fuse protection for the transformer, which is best, fuse on the primary or secondary? Thanks for your fine videos.
Another quick way to find it is to unhook the tstat and cu wires. If fuse blows problem is inside furnace. Hook them back up one at a time with the call on that's blowing fuse. When the fuse blows you've found the bad wire. Most times it's outside in the weather.
I dont have a control board. I have a blend air box. Would that affect how I test for this.
Ran into a problem where the breaker kept tripping every time the furnace came on ended up being the igniter wires melted together.
When you pull the fuse out and test red and common do you take the other wires off or you leave them connected
I wish you put videos on vrf /vrv
Easy to follow detective work!
Very!
If you have a resettable 3 amp breaker is it possible to insert that into the fuse spacing and use it to diag the system? Just to prevent burning multiple fuses?
very common too...dog bite on the control wire into condenser (to contactor coil) causes a short.
What happens when the transformer gets hot then dies but not blowing the fuse?
Great video. Saved me a bunch of cash. Thank you!!!
I dont understand why when is reading OL thats not the problem. Is an open line.
Ol is the normal reading between common and hot wire(low voltage). Essentially it’s the same as touching your 2 low voltage contactor wires together creating a short
My furnace works fine I was trying to practice
Doing this on the wiring harness and it wasn’t reading any ohms just O/L between every wire what did I do wrong ?
Testing the resistance of the board last are you still looking for a low value resistance of .5?
You just simplified low voltage shorts
Awesome videos and I just placed an order for your book Refrigerant Charging and Service Procedures for Air Conditioning Spiral-bound - May 24, 2019
by Craig Migliaccio (Author)from Amazon. thank you
Awesome, please let me know what you think of it after reading!
Fantastic explanation! This helped me isolate my yellow wire which was shorting -- it was wrapped over the edge of a housing where the access door would rub on it every time the door was opened, slowly fraying the wire until it finally made contact with the housing creating a short.
What setting is your multimeter turned to?????
Great troubleshooting tips Craig. Thank you.
Thank you for the support!!
Hey I have a question, when you check for resistance values you disconnect all wire at the board except common right, but do you also disconnect the wires at the thermostat ? Or you leave those connected?
Pull the face off the t stat but leave the wires connected. This way you eliminate the t stat face relays out of the equation
Hi Craig, my HVAC unit was not started and after seeing this video, I have checked the fuse and it was blown out. Then I replaced the fuse. The unit works fine in fan mode and heat mode. when i turned on the cooling in thermostat the fuse blew out again. Please let me know what could be the possible reasons and what else I can check. Please advice
Contactor is a good starting point imo. But your meter is your best friend when troubleshooting electrical issues
Like how you show exactly step by step on finding a low voltage short circuit. This issue is always on my job I’m the ac tech on my job we have heat pump. The units are over 10 years old and the wiring in the condensing unit is exposed to heat and cold weather mostly I find shorts on line from evaporator to condenser. Also through all the years of being used 24/7 the thermostat do fail most of the time.
Very good tutorial on troubleshooting the low voltage side of the furnace. Thank you !
Fantastic explanation on every part of the hvac house. Thanks