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Turning on my ac or fan causes the 3 amp fuse on my furnace control board to trip.

Customer Question
Hello. Turning on my ac...
Hello. Turning on my ac or fan causes the 3 amp fuse on my furnace control board to trip. After i thought i diagnosed a short between the furnace and outdoor ac unit I replaced the wire inbetween them. In the process i also replaced the ac contactor and capacitor. Afterwards i tried the fan and ac and both functions caused the 3 amp fuse to blow again. While troubleshooting some more i caused a spark to occur between the red wire going to the contactor and the contactor itself. Now my furnace led light won't glow or blink at all, indicating no power to it. How do i get power back to the furnace and solve the original problem?
Submitted: 2 years ago.Category: HVAC
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Answered in 20 minutes by:
5/5/2015
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,167
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified
Welcome! We keep going until you have all of the information you need.[b/]. Extensive training & coaching also available.
If you can send me a well focused picture of the wiring diagram glued to the back side of the furnace blower access panel, I can use that to coach you.
Use the 'attach files' link at the lower left of your dialog box to attach the picture.
We can go from there.
.
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Customer reply replied 2 years ago
By "red wire" i mean the tstat wire going from furnace to ac. I had mistakenly left the power on to ac unit before trying to reattach the red and blue wires to the contactor. They were unplugged to begin with because i was checking if the furnace fan would run when those two wires were unattached to contactor.
Customer reply replied 2 years ago
Let me work on getting photos
Customer reply replied 2 years ago
Actually i won't be able to send pics until after 8pm since I'm at work. Will somebody be able to help me at that time?
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
I will be in from 8 pm to 10 pm west coast time, then all of the following day etc.
What works in this sort of trouble shooting is not random trial and error but a few very specific tests, one leading to the next... then we locate the problem.
If we start in the middle and go both directions that can take a long time.
.
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Customer reply replied 2 years ago
Ok I'll be available at 8pm PT. Is it ok to send photos before then?
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
yes, any time works.
.wiring diagram photo's need to be well lit and well focused.
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Customer reply replied 2 years ago
Hi are you there?
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
Hello again, thanks for the great wiring diagram.
Disconnect the power to the outside unit.
Disconnect the red thermostat wire from the contactor, and use a bit of tape to mark where you had it attached. Then send me two different photos if it taken from different angles so that I can see both sides of the contactor.
Tell me in excuciating detail how you carefully marked and replaced the run capacitor wires, include a photo of that.
It is 6:11 in the morning here. I am working on your question.
Meantime do not replace any more parts, it is very very easy to connect them incorrectly and cause all sorts of damage to the electronics... especially when installing a new contactor.
Do not try anything at all unless I instruct you to do it please.
------------
Next turn on power to the furnace or air handler inside the house.
Then follow the instructions on this diagram
http://screencast.com/t/tEKYcy72X
We can go from there.
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Customer reply replied 2 years ago
Thanks for the instructions. I will follow them and get back to you, but not until after 8pm PT.
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
That will work, if we are in similar time zones... if not, I will respond when I get up in the morning etc.
.
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Customer reply replied 2 years ago
Sounds good. I'm in central time FYI. I'll get back to you around 8pm pacific time today and I'll be available all day tomorrow. I appreciate you working with me on this over an extended period of time.
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
That works.
.
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Customer reply replied 2 years ago
Per the first part of your instructions, I disconnected power to outside unit, disconnected the red thermostat wire from the contactor, and marked the spot where it had been attached with black electrical tape. Included are two photos.As for the run capacitor, I removed the wires from the old one, swapped out the old capacitor for the new one, and reattached the wires the same way that they had been attached previously. Included is a photo of the connections. (Two yellow wires attached opposite each other to the 4-prong knob, one blue wire to the three-prong knob, and one brown wire to the one-prong knob.)Per the second part of your intructions, I turned the furnace on. The reading at L1 is 123.6 volts. The reading at L2 is 0 volts. The 3 amp fuse at FU1 is intact. I cannot find the fuse at FU2. Suggestions on where this might be located?
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
Hello again.
http://screencast.com/t/ge8djGpC4jL4
-----------
http://screencast.com/t/AV4DohcOq
tell me where what color wire is on each of the terminals, Comm, herm and fan and exactly where it goes.
one of the Comm wires should go to the contactor, Herm should go to the compressor and fan should go to the fan.
-------------
The read I need at the contactor L1 and L2 is **between** L1 and L2...one test probe on each of them.
Same with T1 and T2.... you need a multi meter for that, not a single probe type tester.
The indications so far is that one of the lines is dead.
We can go from there... I am off to bed in 30 minutes, if I miss you I will pick your response up in the morning.
.
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Customer reply replied 2 years ago
I can find L1 but not the L2 you indicated in the diagram. is it not on the control board? And where are T1 and T2?I'll get back to you shortly with specifics about the capacitor wire placement.
Customer reply replied 2 years ago
Looking at the capacitor, each terminal is labeled, same as the old one. Two yellow wires to the C terminal, one blue wire to Herm, and a brown to Fan. Where the wires lead to is unchanged from last summer when ac worked fine.
Customer reply replied 2 years ago
Alright i think i found L1 and L2, T1 and T2 at the contactor. With power on to outside unit, Reading between L1 and L2 is 0, reading between T1 and T2 is 0. i traced the red wire back to its source at the servicemans box and it read 0 as well. Voltage sniffer at that spot blinked red though. Is this indicative of the problem or did I troubleshoot incorrectly?
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
Thanks,
You are in luck, either Line 1 or line 2 has a blown fuse at the disconnect... it there is no fuse at the disconnect just a breaker try resetting the breaker, turn it OFF... then **PUSH it off hard to reset it.
then turn it back on, if that fails do the same at main breaker panels where the breaker marked AC is located.
If that fails you probably have a bad breaker... a person who is safe around high voltage power can use his volt meter to diagnose that.
A voltage sniffer is not the right tool for this job, because of feed backs we get in AC circuits, (but not DC circuits as with cars etc).
use a multi meter.
.
We are on the honor system here. Please remember to rate my service before you leave today. If you need anything else, just let me know. You can continue asking follow-up questions at no additional charge..
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Customer reply replied 2 years ago
Thanks.So if resetting the breaker at the disconnect box and main breaker panel has not worked, should i replace the two fuses at the disconnect box?Also, the red wire to the right side of the contactor has been disconnected since you gave me the instruction to do so. Am I ok to reattach it at any time?
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
Do not reconnect the red wire, I think that is what blew the fuses
http://screencast.com/t/VnaIrtuM
You can remove the two fuses at the disconnect box and use the ohm function on your meter to check them for continuity... if you replace them make sure there are **dual element** type fuses, not NON buss fuses.
.
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Customer reply replied 2 years ago
In that last picture the sentence is cut off. Can you say again?
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
Hello again
Generally all it takes to read the whole thing is to scroll your screen up.
http://screencast.com/t/zASm2uKVm
Let me know how that works.
.
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Customer reply replied 2 years ago
The last part reads "and would also" then cuts off
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
If you are on a cell phone or ipad there are often problems, you need to be on a computer.
http://screencast.com/t/djwqQGZR
try that
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Customer reply replied 2 years ago
It's still not working, even when I look on my computer. I think the text box is not big enough, causing some of the text to be cut off.
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Phil
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,167
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Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial

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