[FirmwareDeveloper]

My Furnace recently stopped working and the Furnace Fan stayed on constantly. I replaced the original Fan Control circuit board (HH84AA011) with a ICM271 drop in replacement board. Still nothing came on, not even the constant fan. I check the 120V-24V transformer and found no 24V output so I replaced the transformer. Now when I turn it on the 3AMP fuse blows instantly and the fan comes on (I understand this is a normal function of the board if the fuse blows). If I temporarily bypass the fuse nothing happens. I was told to suspect the Limit switch. Any other ideas?
One limit switch wire (red) has a component wired into it. (i.e. wire-component-connector-limit switch terminal) any idea what this is and if it is necessary?

Thanks

[hvac1000]

All parts are necessary for safety. It sounds like you have a short in the low voltage wiring and if I were you I would check to make sure all of that low voltage wiring is OK and not shorted before I would do anything else.

You have already spent some$$ trying to fix this and if the wiring does not show the problem I would contact a professional before you cause more $$ to be spent guessing at what the proble is.

[acetc]

I don't think the problem is the limit switch, you may have ruined the transformer by jumpering it out. Unplug furnace.In order to find your short or ground I suggest you disconnect the low voltage wiring to the thermostat at the board in the furnace (small wires connected with screws)Replace transformer and fuse then plug furnace in at wall receptacle , hold door switch in manualy, while monitoring the amp draw with a amp meter around each thermostat wire, reconnect wire ,one at a time starting with the "C" (common) Then the "R" then the rest. As you reconnect you will find the circuit that is shorted. If it is the yellow wire it is possibly the contactor coil in the condensing unit, if it is the white wire, it is possibly in the heating componets (gas valve, combustion blower controls, etc.)Remember the amp draw should be around 1.0 amps or less.
You can also trouble shoot without a amp meter by watching for the fuse to blow. (make sure you have plenty of fuses)
The problem may also be at the thermostat, if you have a common wire to the stat it may be touching one of the other wires.
Then there is the possibility that the thermostat wires are shorted together between the furnace , stat or condensing unit which can be checked with an ohm meter when the furnace is unplugged.
I hope this helps, Mike.

[FirmwareDeveloper]

Thank you for your responses, I appreciate it.

I have check the continuity of all wires I have access to with an ohm meter and they all seem OK. My transformer is good (120V in 24V out). I did the 1 wire at a time troubleshooting and found that the fuse only blows when BOTH the yellow (from thermostat) and the Dark Green (from the outside condensing unit ) are connected together at the Y terminal on the circuit board. Does this also indicate the contactor coil?
Yellow alone or Green alone do not cause a problem.

Thanks again

[acetc]

The coil in the contactor is most likely the problem, it is probably burnt out. Check the wires (low voltage ) out side to make sure they are not bare and shorting out . A lot of times a dog will chew on them and short them out. Mike