Secondary voltage fuse is open

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Old 05-13-11, 01:28 PM
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Secondary voltage fuse is open

Have a 5 year old Carrier furnace and a/c, programmable ritetemp thermostat. Attempted to start up the a/c this year and there was no response. Checked the diagnostic on the furnace which was showing an error code of 24 (secondary voltage fuse is open).

Fuse was blown so I replaced it. Fan working, heat working, but the moment I flip the tstat to "cool" the fuse blows again.

Wiring looks good both inside and outside, nothing exposed or chewed on. Checked at the control board, the a/c unit, as well as the tstat but all looked fine.

Thought about replacing the tstat but figured I'd look around first. Any suggestions as to what else I could check?

Thanks in advance!
 
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Old 05-13-11, 02:42 PM
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There are two wires (one cable) between the furnace and the outside unit. They are connected to the Y and C terminals in the furnace. Disconnect these (leave all others connected) and try the system in the cooling position. The furnace blower should start but the outside unit will do nothing.

If this checks out and doesn't blow the fuse your next step is to disconnect the wiring at the outside unit and reconnect in the furnace. Make sure the two wires at the outside unit are not touching each other or the cabinet and then try the system again. If the fuse blows the interconnecting cable between the furnace and the outside unit is bad. If the fuse holds then the problem is most likely the power "contactor" in the outside unit.
 
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Old 05-13-11, 04:12 PM
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Thank you, I appreciate the quick response.

I disconnected the Y and C terminals as you suggested, the fuse held and the blower came on. Reconnected those wires and then went to the unit outside but I'm having some difficulty getting at the wires. Almost looks as if the main a/c cable wasn't long enough when it was installed so it barely makes it to the box. Without any slack I can't get the box open to get at the wire coming from the furnace. Started raining as well which didn't help so I'm going to try again later.
 
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Old 05-14-11, 05:27 AM
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remove the electrical service panel on the a/c after killing the high voltage at the breaker or disconnect by the a/c.
 
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