I had an earlier post about neither the furnace or a/c working... well the problem was a blown fuse, and I have no idea why I didn't find it earlier. Anyway, I replaced the fuse and the furnace heat now works, but when I go to the a/c, the fuse blows every time. My furnace takes a little 5 amp buss fuse. Any clue as to why this is happening? Would it be safe to try to use another fuse with a higher amp rating?

I had an earlier post about neither the furnace or a/c working...well the problem was a blown fuse, and I have no idea why I didn't find it earlier. Anyway, I replaced the fuse and the furnace heat now works, but when I go to the a/c, the fuse blows everytime. My furnace takes a little 5 amp buss fuse. Any clue as to why this is happening? Would it be safe to try to use another fuse with a higher amp rating?
Changing the amp rating could cause overloading and overheating of the wiring. Not a good idea. I would check the amp draw of the air conditioner. You may need to put it on a different circuit, or take some other things off that circuit to try to stop the fuse from being overloaded. The best solution would be to have an electrician check it out. If not, or can't afford it, try contacting your local Fire Prevention Bureau, some of those FF can really help you out.

You must be referring to the low voltage fuse and not the line voltage fuse. If the 5 amp fuse only blows when you switch it over to the cooling mode, you probably have a problem with the condensing unit. Check the small 24 volt wires going outside to the unit. If they appear to be in good condition you probably have a problem inside the condensing unit. Depending on the make and model, it could be the coil on the contactor, time delay relay, or the circuit board itself or the reversing valve coil. (if it's a heat pump)

Which fuse does it blow when you turn on the AC? The low voltage fuse or the high voltage fuse?

You must be referring to the low voltage fuse and not the line voltage fuse (I'm assuming low voltage as when it blows, I have no 24V power to the thermostat). If the 5 amp fuse only blows when you switch it over to the cooling mode, you probably have a problem with the condensing unit. Check the small 24 volt wires going outside to the unit. If they appear to be in good condition you probably have a problem inside the condensing unit. Depending on the make and model, it could be the coil on the contactor, time delay relay, or the circuit board itself or the reversing valve coil. (if its a heat pump)

I've got exactly the same problem though mine is only a 3 amp fuse. It's the one inside the unit. I know the compressor can turn as if I take the cover off and manually activate the plunger switch, it will turn on. The same compressor has had a problem where I've had to clean this same switch a bunch (10-15) times to get the unit to activate properly (it's 4 years old). Could this constant draw of power trying to turn on without actually turning on have fried something in the outside? Normally I can hear it buzzing with electricity, but now it doesn't even do this. I hear the thermostat click, then at most 15 seconds of blower activity, then dead, blown fuse.

Thanks in advance for the help!

You have something in your control circuit that's blowing the fuse. Check the small 24 volt wire that goes out to your outside unit for any shorts. If that appears to be in good condition, you most likely have a problem with the coil on the contactor, the time delay relay, or the control circuit board being bad. If you have a spare fuse, you can pull both wires off your contactor and turn on the ac. If the fuse still blows it's not the contactor and you'll need to look at some of the other items. If the fuse doesn't blow, change the contactor.

Thanks JackT for the quick response! That's a great suggestion. I'll try that as soon as I get home from work tomorrow and that makes a ton of sense (the contactor). In my marvelous electrical jargon I've referred to the contactor as the plunger switch which has been sticking for the past year and a half or so. Perhaps all that current draw finally killed it? My solution had been to clean it with electrical cleaner and then bang the side of the unit and it would then draw in but you could hear the buzz of the switch trying to draw in. I've checked all the wiring and don't see anything that looks ugly in there. Would it be sufficient to simply pull the electrical connection from the box next to the compressor or do I need to go into the compressor and pull the actual wires off the contactor?

JackT has been posting good advice to this thread. If you have an intermediate connection outside, pulling the wires off there, would isolate the problem as being before or after it. Go for the easiest check first. If the fuse doesn't blow, then go into the contactor itself.

I'd pull both of the wires off the contactor and then try it. Make sure you pull off both of them off just in case the coil is grounded out. Good Luck

You guys are the best! I pulled the 24V from the contactor, replaced the fuse, set to cold and viola, no blown fuse. Now to go get that contactor, it's supposed to be 92 today! Fortunately my upstairs unit is working, though it was a block of ice yesterday (don't let your wife put your cat box under the intake).

Success! I couldn't find the identical contactor in-stock so I found a square D equivalent of the GE I took out @ Grainger and viola, the compressor fired right up.

aterbrys0528, Don't use a bigger fuse. If the fuse blows when the ac is turned on you probably have the same problem as Hokiez had above. You shouldn't blow a 5 amp control fuse unless you have a short in the control wires feeding your outside condensing unit or you have a bad control. I would suggest you follow the same steps that Hokiez did, unhook the wires outside or pull both wires off the contactor and see if the fuse still blows.

Glad we could help you Hokiez,