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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 5648
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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3 amp fuse blows on circuit board. would last a little while

Customer Question

3 amp fuse blows on circuit board.
would last a little while when replaced.
now dead short
Submitted: 8 months ago.
Category: HVAC
Expert:  airheatman replied 8 months ago.

Below are some of the most likely causes for a short and blown fuse.

1.Short in thermostat

2.Chewed or knicked control wire between the units

3.Relay or component in air handler

4.Wire touching hot gas line in outdoor unit

5.Short in switch or contactor in outdoor unit.

Finding a short can be difficult. Easiest way to find is by process of elimination.If you can get several fuses, we can try different things until we no longer blow fuse to locate the short.

1.Disconnect all thermostat and outdoor unit control wires from the terminals on indoor unit control board.Turn thermostat to call cool and see if fuse blows.If it blows, the control board in unit is shorted and will need to be replaced.If not move to step 2.

2.Reconnect all wires previously disconnect at control board. Go to outdoor unit. Disconnect the 2 control wires going to Y and C where they first enter the unit. Turn thermostat to call for ac. If fuse still blows we have a short in those wires between indoor and outdoor units.Could be chewed or skinned, possibly nicked by weed eater, etc. allowing wires to touch and create short.If it does not blow,we know short is inside of the outdoor unit.Move to next step.

3. Reconnect those 2 wires .Then locate the contactor relay in outdoor unit. Remove the small control wires on the contactor coil.Turn on AC.If fuse does not blow, the contactor coil is shorted and contactor will need to be replaced.If fuse still blows we know short is between where wires enter unit and the contactor.Move to next step.

4.Reconnect the contactor wires.Follow the control wires from where they enter unit and see what component may be in that circuit .(Pressure switches, time delay, defrost control board, etc).Follow same procedure of disconnecting wires to each of those controls and see if we find one that seems to cause fuse to blow when in circuit.Also look for any place the small control wires may be touch a hot gas copper line in unit that may have melted sheathing on wire and be allowing copper to ground to pipe.

Let me know what you find and we wlll go from here.