3 amp fuse keep blowing when airconditioning is turned on. Any suggestions why?
Control board look fine. How do you exactly determine of there is really low voltage wires are shortened? Based on the blog, I suspect that but I have to check tomorrow.
Is there a procedure of testing it starting from outside unit or thermostat?
Please advise.

Appreciate the response.

Wazimi

Following the question, house is built in 2005 and I don't suspect any corrosion or chewed wires.

Fist place to check is the contactor in the outdoor unit. See if it is burnt.
http://www.chinacontactor.net/Products/Contactor/MBC-1N-AC-Contactor.jpg

Thank you, will check it.

Just quick for the test of the outside conductor, if I remove low vol- wires and set tstat on heat (gas furnace) and start the unit for control panel and new fuse test, will that make sense? Because it is not a heat-pump.

I checked the contactor, it looks fine and nothing seems that has burnt. Any other suggestion?

Even though the contactor doenst appear burnt, it most likely is still the culprit. Disconnect the two low voltage wires from the contactor and turn the system to cool, if fuse doesn't blow, you need a new contactor.

Thanks a lot for the responses. But my issue is still the same. I replaced the contactor with new one. Did not resolve the problem.
Still blown out fuse immediately. I disconnected the new contactor from the low voltage blue and brown wires, fired up the unit, no response to the unit to start, thent disconnected the face of the t-stat, put it back on off position, switch to cool, unit started and motor did run and blow air, but lasted only 3 minutes and fuse blew again.
Any suggestions from here?
Is my t-stat maybe defective?
Last year I replaced twice the fuse (last summer and then in winter, but lasted till now... now it keeps blowing).
What is the wiring test if I bypass the t-stat? To test if the problem is with t-stat? In that case should I disconnect the contactor from low voltage wires too?
Can it be the C-board at all?
Thanks for all your responses/

Good folks, I still awaiting any suggestions.
Thank you for your time.

Replacing the contactor was throwing money away.

Somewhere on this site, I put in the procedure for doing troubleshooting. You basically replace the fuse with two 12 V lamps that draws 30-40 VA at 12V. e.g.. About 30/12 Amps. I forget off the top of my head what bulb is a good suggestion.

Thus, you look for the bulb to light which indicates a short.

First part of the troubleshooting procedure to determine if it's G or Y.

See if the fuse blows when in fan mode (No heat or AC). This would mean a problem with "G".

Next eliminate a wiring issue. Disconnect C and Y heading to the outdoor unit at the FURNACE.

If the fuse doesn't blow then it's wiring or thermostat.

Put things back.

With the thermostat set to off and fan set to off connect R to G and to Y at the furnace

If fuse blows, then wiring is defective. If not, then stat is defective.

A multimeter would be necessary for anything else you need to do.

Take the stat off, red to yellow for cool
Red to white for heat
Red to green for fan
Try all of those and see when the fuse blows.

You did not by chance hit the wire outside with the weed eater while trimming, this happens a lot.

Thanks a lot, will try.
You mean disconnect them, correct? And try them one by one at a time? Is this in a position while the face of the tstat off the wall?
I did check the low-vol lines, no weed eater damage...

Thanks for your help and responses.

I tried to say the same thing, but "safer". One problem that occurs if you do this powered with R and Y is you can pop fuses or breakers for the outside unit because of the built-in delay in the thermostat.

Anywho: for cool the stat controls the indoor fan and for heat the furnace controls the indoor fan.

R to G - makes the indoor fan run.
R to Y - Makes only the outdoor compressor to run
In order to get cool, you must connect R to Y and G. You may test independently.

R to W - Makes the heater and the indoor fan run. The indoor fan will usually have a start-up delay and will not turn off immediately.

Check my other post.

Problem still remains. Put a new tstat and fired heat. It runs great. As soon as tstat goes to AC, fuse blows. Checked all low voltage wiring in and out... all fine and no sign of chewed or damage.
Diagnostic LED flushes at AC setting as:
24, meaning as the panel reads: Secondary Voltage Fuse is Open check for short circuit secondary voltage (24 VAC) wiring.
2 rapid flushes followed by 4 delayed flushes.
I removed the tstat face, loosen all wirings and connected R to Y & G at the same time, turned on unit, fuse blew.

Carrier unit Model #: 58STA090---1114
Serial #: 4404A40864
Any suggestions?
Thank you,

Because this
I removed the tstat face, loosen all wirings and connected R to Y & G at the same time, turned on unit, fuse blew.

Happened. It's not the tstat.

It still looks like a wiring fault. Next you need to isolate, is it Y or G that's causing problems. Just like I asked you to do earlier.

Check Y & G independently.

Do you own a multimeter?

Take the yellow wire off y on the control board in the base meant, leave the yellow from the t-stat on the y and try it this will tell you weather the problem is in the outdoor or indoor unit

Thanks appreciate it.
So, assuming the problem is Y or G, what then... I just want to be a step ahead and resolve it when I found out of the faulty wire...

Do you have a ohm meter?

No I do not. I will be getting one. So, assuming I have one, what is the procedure?

Basically you need to figure out if it's Y or G. Rigth now, you know it's Y or G.

Next, you need to figure out where is the faulty segment.
Is it:
Thermostat to furnace?
Or
Furnace to outside unit.

If we assume there is a problem with "Y".

If you disconnect the wires (C&Y) at the outdoor unit, then it's not the outdoor unit. If you disconnect the wires at the furnace going to the outdoor unit and the fuse blows, then it's the wiring between the thermostat and the furnace.

Not a part of this discussion but would like to ask a question.

Why not just purchase a roll of 2 wire bell wire, disconnect the existing low voltage wiring at the control panel and at the contactor in the compressor unit. Temporarily replace LV wire with the bell wire and see what happens?

If your present LV wire happen to run through the attic, squirrels love the stuff.

Assuming I have a multimeter,
It not possible to check continuety on my LV from both ends of the Y or G line. Leads are not long enough. Is there any other way to check integrity of the wire with multimeter?

Thanks,

Your not going to be checking integrity, but rather shorts to other lines.

The most likely short is between Y and C. All of the other wires shorted together really don't cause any problems except odd behavior. Common is likely at ground potential and it's supposed to be.

Again assuming Y is the problem

A short from "Y" to ground or "Y" to C are most likely.

It's probably not G, if you can use the FAN position of the thermostat and the fuse doesn't blow.

Thus with power off, you can go to the furnace and measure the resistances from "Y" to every other terminal and the furnace case (ground).

You could disconnect the LV wire at both ends and then check for continuity between the two wires. Continuity would indicate a short between the two.

I have the same problem. However, after reading online forums, I tested for and found a short between Y and COM.

Any suggestions on what I should do next?
Is this because of a bad component on the board or another bad part?

Hi g33k86. Next step is to run new themostat wire. Most likely the bad wire is no fualt of any other components, just a bad wire. It happens. I would suggest running a new five wire stat wire. Unless you happen to have an extra wire in your current wiring that is not being used, then you could combine y and c, use it as one wire, and use the extra wire for the y, or c, which ever you choose.