It is a 3A fuse on the control board, it blows 2 times when turning on the AC

Does this only occur when the a/c is turned on? If so, check the low-voltage wire to the outdoor unit for damage. If OK, the contactor coil is likely shorted. Be sure to turn off power supply to outdoor unit before venturing into the control compartment.

Hi KC,

Thank you very much for your quick response. I am a homeowner not a electrician, so my knowledge is limited to the AC field, your posting and the picture will help me a lot. My A/C unit is a Carrier 58ZAV, I turned on the fan only works, then I turned on the A/C. it failed. Found the LED is blinking, the manual listed the code as 24, and I need to check the secondary voltage wiring. I don't understand what is that, so Google lead me to this site, and found a lot of useful information, thanks a lot.

Now, if I shut off the power at the break box, the outside condenser unit will have no power? So that I can check the Contactor and the wires going into and out of it?
If the wire is damaged how do I rewire them? I think it is going into the house with a conduit.
How do I know if the contactor is bad? Can you advise?

Again, thank you very much for your time to share good advise with me.

If the control wire is enclosed in conduit then damage is unlikely. I have seen cases where an exposed wire was chewed by an animal. Turning off the power to the outdoor unit can be accomplished either at the main panel or the outdoor disconnect. Check the contactor coil (wire winding near the base) - if it appears burned or badly discolored, it has likely shorted. Try this: turn off line voltage power to the outdoor unit, then disconnect the low-voltage wires from the coil and position them so they aren't touching anything or each other. Wrap with tape if necessary. Now try setting the thermostat to turn on the a/c. If the fuse doesn't blow, the contactor is bad. If it blows, the wiring is shorted.

Hi KC,

Thank you so much for your updated instruction. I will do that tomorrow, cause it is raining here today. I will keep you posted about my findings when I finish the inspection. I hope it is just the bad wire, so that I can bandage it and the unit will up and running thereafter. Great help!

Too add to KC13's post, if the secondary is shorted after test he mentions, then those 2 wires need to be disconnected at the Air Handler to help further locate.
In the Photo above of the Contactor, If the outside disconnect is on, you should have 240 volts into 2 termonals, then out to Compressor and Outside Fan. When the 2 side terminals receive 24 volts A/C from thermostat, contactor will energize.

Hi Stratmando,

Thank you for your advice too. You guys are great helper for someone like me. I will work on that problem and keep you everyone posted.

Also Problem can also be between the Thermostat and Air Handler wiring, not just between the Compressor Unit and Air Handler.

Guys,

I changed the low voltage wire since it is kind of old due exposed in air. But the fuse blown again. Meaning the problem is still there. May be I have to change the contactor? Could that possible the thermostat shorted? Thank you for your help!

If you remove the small low voltage control wires on outside contactor, and you turn unit to cool and it doesn't blow the fuse, contactor is bad as mentioned, the contactor gets 24 volts ac from thermostat and air handler transformer.