Help with 3A fuse blowing on cooling
#1
05-21-15, 09:50 AM
Help with 3A fuse blowing on cooling
I recently came back from a trip, and the thermostat was still working. On turning the AC on, the wall-powered thermostat decided to die.
Upon investigating in the attic, I noticed the 3A fuse had shorted. I put a new one in, but this one burned as well, as soon as the cooling kicked in.
Following this, I disconnected the yellow and blue (Y and C) wires that lead to the outside unit from the control board and tested the system. On activation of cooling from the thermostat, the fan turned on, and everything worked fine.
I plugged Y and C that lead to the outside back into the control board, and unplugged the yellow and blue (Y & C) from the contactor coil, outside. I let the system run, and the fuse burned.
I go back inside to investigate, unplug Y & C (now Y & C are connected to nothing) and do a beep test between Y&C - no beep.
I connect outside Y&C back to control board, and now altogether, Y&C from thermostat and Y&C from outside meet with their respective colors. Thermostat is off, but when last powered, was set to cooling. I beep test the Y&C terminals directly on the board, and it produces a beep.
Unplugged everything, directly beep tested board Y&C terminals - nothing.
Did a ton of other beep tests:
tY (thermostat Y) to oY (outside Y) - nothing
tY to oC - nothing
tY to bY (board Y) - nothing
tY to bC - nothing
tC (thermostat C) to oY - nothing
tC to oC - nothing
tC to bY - nothing
tC to bC - nothing
oY to bY - nothing
oY to bC - beep (why?)
oC to bY - nothing
oC to bC - nothing
Any ideas what might be wrong, or what the next steps I should check are? I don't think there is a short in the condenser coil... and I really hope it isn't a board problem.
Upon investigating in the attic, I noticed the 3A fuse had shorted. I put a new one in, but this one burned as well, as soon as the cooling kicked in.
Following this, I disconnected the yellow and blue (Y and C) wires that lead to the outside unit from the control board and tested the system. On activation of cooling from the thermostat, the fan turned on, and everything worked fine.
I plugged Y and C that lead to the outside back into the control board, and unplugged the yellow and blue (Y & C) from the contactor coil, outside. I let the system run, and the fuse burned.
I go back inside to investigate, unplug Y & C (now Y & C are connected to nothing) and do a beep test between Y&C - no beep.
I connect outside Y&C back to control board, and now altogether, Y&C from thermostat and Y&C from outside meet with their respective colors. Thermostat is off, but when last powered, was set to cooling. I beep test the Y&C terminals directly on the board, and it produces a beep.
Unplugged everything, directly beep tested board Y&C terminals - nothing.
Did a ton of other beep tests:
tY (thermostat Y) to oY (outside Y) - nothing
tY to oC - nothing
tY to bY (board Y) - nothing
tY to bC - nothing
tC (thermostat C) to oY - nothing
tC to oC - nothing
tC to bY - nothing
tC to bC - nothing
oY to bY - nothing
oY to bC - beep (why?)
oC to bY - nothing
oC to bC - nothing
Any ideas what might be wrong, or what the next steps I should check are? I don't think there is a short in the condenser coil... and I really hope it isn't a board problem.
#2
05-21-15, 11:32 AM
Member
I had a little trouble following everything that you did. However, my understanding is that if you disconnect the Y & C wires going to the outside unit from the control board, the fuse doesn't blow. However, if you connect the Y & C wires going to the outside unit to the control board, even though the contactor (what the Y & C wires are connected to inside the outside unit) is disconnected, the fuse blows.
If the above is correct, then I would look for a break in the insulation on the Y wire, probably where it goes into the outside unit. The Y wire is probably shorting to the case of the outside unit, which is grounded. The C terminal is also grounded, so if the Y wire touches the case, it's effectively a short circuit.
If the above is correct, then I would look for a break in the insulation on the Y wire, probably where it goes into the outside unit. The Y wire is probably shorting to the case of the outside unit, which is grounded. The C terminal is also grounded, so if the Y wire touches the case, it's effectively a short circuit.
#3
05-21-15, 03:11 PM
Thanks Bob, you're absolutely right, Y has a connection to ground somewhere...
I made sure the Y&C at the furnace is not touching anything grounded. The wires stretch over a sea of attic insulation, and I don't see anything that it would touch when it descends into the wall and outside.
However, at the outside unit, I also removed the Y&C as far away from the casing, but there is still a ground connection.
Is there anywhere else I should check? I'm not quite prepared to rip open my walls to get at the cables.
I made sure the Y&C at the furnace is not touching anything grounded. The wires stretch over a sea of attic insulation, and I don't see anything that it would touch when it descends into the wall and outside.
However, at the outside unit, I also removed the Y&C as far away from the casing, but there is still a ground connection.
Is there anywhere else I should check? I'm not quite prepared to rip open my walls to get at the cables.
#4
05-21-15, 05:17 PM
Member
I think you've identified the problem, now you just need to find the easiest way to solve it. As I see it, you have two options. First is to run a new "Y" wire from the furnace control panel to the outside unit. This is the best solution, but also the most difficult. The second option is to "troubleshoot" the current wiring to try to isolate where the wire is grounded. If it's not too difficult to splice the Y wire back together (long enough and accessible), try cutting the Y wire somewhere between the house and the outside unit. Disconnect the Y wire from the control board and check resistance to ground. If it reads open, the wiring inside the house is good, and the problem is beyond where you cut the wire. If this is the case, carefully examine the Y wire near the outside unit. My guess is that you'll find a cut in the insulation around where the wire comes into the outside unit.
Report back what you find.
Report back what you find.
#5
05-21-15, 07:43 PM
I accidentally didn't shut off power to the compressor and brushed the outside Y with something and saw sparks - oops! But to my surprise, Y no longer had a connection to ground.
I left Y & C disconnected from the contactor coil outside, but reconnected them to the furnace control board, and the fans turned on with no incident.
I connected Y & C to the contactor coil, and turned the system on again. Encouraged, I returned power to the compressor, and let the entire system run, and hear everything working.
Then a minute later, the fuse shorted... and the connection Y had to ground came back.
:/ ultimately I still don't know where the short is, it just miraculously went away briefly after a short. I'm tempted to replicate the accident.. but that's probably not a good idea
There didn't seem to be any breaks in the insulation outside, but I'll look again when it's bright out. Hopefully I've just been blind and the problem is sitting right in front of me, but you never know.
I left Y & C disconnected from the contactor coil outside, but reconnected them to the furnace control board, and the fans turned on with no incident.
I connected Y & C to the contactor coil, and turned the system on again. Encouraged, I returned power to the compressor, and let the entire system run, and hear everything working.
Then a minute later, the fuse shorted... and the connection Y had to ground came back.
:/ ultimately I still don't know where the short is, it just miraculously went away briefly after a short. I'm tempted to replicate the accident.. but that's probably not a good idea
There didn't seem to be any breaks in the insulation outside, but I'll look again when it's bright out. Hopefully I've just been blind and the problem is sitting right in front of me, but you never know.
Last edited by unkani; 05-21-15 at 09:05 PM.
#6
05-21-15, 08:02 PM
Group Moderator
It's a little hard to follow your troubleshooting.
A short to ground could open after a short circuit. No.... you don't want to repeat that.
A short to ground could open after a short circuit. No.... you don't want to repeat that.
#7
05-21-15, 09:08 PM
Sorry about that... This is all new to me, so I'm wording things poorly.
I can't seem to edit my original post anymore, but I tried to better word everything else I've said.
I can't seem to edit my original post anymore, but I tried to better word everything else I've said.
#8
05-22-15, 06:43 PM
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#9
05-22-15, 09:01 PM
Thank you all for the help.
I decided to give up on the Y since I was unable to figure out where the short was occurring.
Fortunately, my system included the other unused wires (white, green, red) that also ran the length between the attic furnace and the outside unit, so I simply used the green wire instead, as if it were the Y, since it didn't have the same short problem.
I don't know why I didn't think of that earlier. The entire system is up and running with no shorts
I decided to give up on the Y since I was unable to figure out where the short was occurring.
Fortunately, my system included the other unused wires (white, green, red) that also ran the length between the attic furnace and the outside unit, so I simply used the green wire instead, as if it were the Y, since it didn't have the same short problem.
I don't know why I didn't think of that earlier. The entire system is up and running with no shorts
#10
05-22-15, 09:54 PM
Group Moderator
