HVAC panel fuse keeps blowing

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  #1  
Old 10-14-16, 09:12 AM
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HVAC panel fuse keeps blowing

I have read the previous thread on this topic that was closed.

This is a Lennox c33-30b-2f-3

The fuse blows when calling for cool, works fine with heat.

I DC the low voltage wiring at the AC unit outside and inside. I get 1.6 ohms or so between the wires. I visually inspected the wires and cross checked to ground. I connected the unused green wire instead of the red one and still the same blown fuse with the wires DC at the compressor.

The fuse doesn't blow if I DC the wires at the panel. As soon as I put them on, DC outside, it blows immediately on start up.

I am perplexed. Will that 1.6 ohms that I'm getting do this or is just any semblance of resistance at all causing it? Is it the panel?

Any help is appreciated. Thank you
 
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  #2  
Old 10-14-16, 09:25 AM
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Sounds like you have a short in the cable. With the wires disconnected inside and outside you should have an open circuit (infinite ohms) between the wires and between the wires and ground.

If you've inspected them over their entire length and don't see any obvious damage, it may be faster and easier to replace the wires.
 
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Old 10-14-16, 09:33 AM
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If you disconnect the two wires outside at the condensor and the fuse doesn't blow.... then the problem is inside the condensor.

It sounds like you have a shorted contactor coil in the condensor unit.

The condensor unit with contactor should measure 10-15 ohms approx.
 
  #4  
Old 10-14-16, 09:42 AM
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Thank you for the replies.

Both wires disconnected outside.

I put one wire at a time individually on the y terminal and each wire blew the fuse. One at a time. I'm out of fuses. Went through 10 so far
 
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Old 10-14-16, 09:47 AM
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if the wires WERE NOT connected to the condensor outside than they are dead shorted to ground somewhere.

Follow Pauls advice.


Instead of using up fuses.... set your meter to ohms. Connect one probe to a ground. Check the wires with the other probe. The meter should not show any continuity.
 
  #6  
Old 10-14-16, 10:02 AM
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I get 1.6 ohms between them I also took a plain piece of that same wire and I get 1.6 ohms between them
 
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Old 10-14-16, 10:42 AM
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Apparently you aren't sure what to do.

You have two wires that connect the furnace to the condensor. One wire goes to Y and one goes to C.

If they are disconnected from Y and C and they are disconnected from the condensor..... you should have two wires that CANNOT show any continuity between them.

In other words.... if the wire colors were red and white.... putting one meter lead on white and the other on red MUST not show any continuity.
 
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Old 10-14-16, 10:59 AM
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I understand what to do I know that the Y and Carmen are connected via the two wires what I'm saying is that no matter what I do the fuse blows even if I just put one wire on the Y at a time I'm going to get a new complete length of wire and before I run it I am going to hook it up
 
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Old 10-14-16, 01:32 PM
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Update. I ran a new low voltage wire to the ac. Fuse no longer blows but now the unit won't start in either ac or heat. Two slow blinking lights.

The relays aren't tripping. I tested it after I ran the new wire and at least it started (induction fan). I then reconnected the low voltage outside and came bs k in and nothing.

I disconnected the ac wires inside and still nothing.

Board?
 
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Old 10-14-16, 01:47 PM
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I can't find service info on your furnace. If you remove the blower door there should be a chart that tells you what the flashing LED code is.
 
  #11  
Old 10-14-16, 01:49 PM
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Two slow flashing lights is normal operation.

At this point the induction fan was starting just a little while ago. Now nothing.

What else could it be but the board?
 
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Old 10-14-16, 01:51 PM
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It could be a defective 24vac transformer. I was trying to find info to determine if a separate transformer was used.
 
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Old 10-14-16, 01:54 PM
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Yes there is a seperate transformer. It's humming.

I read a thread somewhere about that possibility as well.

Is it as simple as putting a Dvom on the outputs of the transformer to test it?
 
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Old 10-14-16, 02:00 PM
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Yes... check it for +/- 24 VAC.

The primary is probably ok which is why it's humming but the 24v secondary could be burned open.
 
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Old 10-14-16, 02:12 PM
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Ok. But if that's the case why is the board even getting power. Lights flashing ?
 
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Old 10-14-16, 02:20 PM
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I'm getting 28v on the secondary. It looks pretty stable just watching it for a bit. It's mounted on the side of the bracket that holds the panel so I think it just kind of vibrates.
 
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Old 10-14-16, 02:46 PM
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Update. I connected w and r at the Tstat. Heat started. I put in another Tstat. Heat comes on.

Cool still no response.
 
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Old 10-14-16, 03:05 PM
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Ok sir handler comes on both heat and cool.

Outside nothing. 87vac going to outside via new wire. Is that right?
 
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Old 10-14-16, 03:09 PM
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The short on the Y line probably damaged the thermostat. You can connect the Y to R to test the outside condensor.

It's probably .87 ....... not 87 vac.
If it were the correct voltage it would be close to the same as your system voltage around 24v.
 
  #20  
Old 10-21-16, 04:00 PM
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To close this thread, it was the contactor switch.

There was definitely continuity between the conductors on the control wire though as well and I think the cold side of the thermostat got fried also.

Could the short in the contactor created the issue in the wire? Seems odd to me.

I replaced the contactor, the control wire, except for the last 8 feet through the wall and I had a nest thermostat to begin with but had been using some older units in troubleshooting so the nest is back on and all is well.

Thank you for the help
 
  #21  
Old 10-21-16, 04:09 PM
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A short would not cause the low voltage cable to go bad.
It looks like the contactor was the source of your problem from the beginning.
 
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