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Douglas , HVAC Technician
Category: HVAC
Satisfied Customers: 3089
Experience:  Manufacture Rep for Major Brand, Technical Trainer, NATE
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control board of lennox furnace

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3A fuse in control board of lennox 80uhg3-75a-2 keeps blowing when I connect t stat wires or jumper 'r' and 'w' wires are ccurrently disconnected in control box  and no action in the furnace

Hi, I'm Douglas and I am here to help you resolve problems with your heating and cooling equipment.

I see someone else jumped in at some point there.

Did the fuse blow as soon as you connected it, or did it have a period of time before it blew?
Customer: replied 3 years ago.

I was talking to you yesterday. if turn off power, then connect lv wires and turn on power, its almost instantly popping. also, as soon as jumper it pops

Ok... this is somewhat difficult to tackle. If you look on the control board there should be a large molex connector with maybe 9-12 wires on it. With the jumper you have from R-W in place, power off, new fuse and that harness unplugged from the board, when you power it up, will it blow the fuse again? (this may help us determine if the short is in the board itself or in one of the external circuits leading to the limits etc).
Customer: replied 3 years ago.

fuse did noot blow

Ok you said you have an old meter, does it have an ohms setting (omega sign) and if so, put it there and touch the leads of the meter together, the needle shoud swing to the other side. ( we need to know the meter works).
Customer: replied 3 years ago.

yes it works

great, with the furnace power OFF, do you know where the main limit is? Its usually a 1.5" x 2" fiber card with two wires going to it, mounted on the fire wall.

With your meter set to ohms, place one lead on one of the terminals of the main limit and the other to ground. no need to remove wires. It shouldnt have any continuity to ground, in other words the meter shouldnt register anything. If it does, we are closing in on the short. You can leave the molex disconnected for this test.
Customer: replied 3 years ago.

definate continuity to ground on that limit

Ok, now pull the wires off the limit and check it again.
Customer: replied 3 years ago.

wired off and still grouunded at that limit

Both wires off?

Ok... now, there are probably two screws that hold that in, pull the limit out and first check the hole to see if ther is any foil back insulation that may be touching the limit rods or something.
Customer: replied 3 years ago.

it looks to be cut neatly and out of the way

any chance the limit was in crooked or touching the back to the metal cut out? With the wires off, touching one terminal and ground, you had continuity?
Customer: replied 3 years ago.

it was upside down...I turned it around and now it isn't grounded anymore

Argghhh.. go ahead and put it all back, leave the jumper in place and try it now.
Customer: replied 3 years ago.

harness in or out?

in, lets try the furnace with the jumper first and if all is good, pull the jumper out and reconnect the thermostat wires
Customer: replied 3 years ago.

ok didn't pop the fuse...combustion blower started but then shut off and got the fast alternating led's

Ok... I have to find that manual again...
Customer: replied 3 years ago.

it says its either main ground improper, under voltage, or broken ignitor/ open ignitor circuit

Whats the resistance of the ignitor? Unplug it and check with your ohm meter, it cant be over like 19 ohms.
Customer: replied 3 years ago.

stupid analog...looks to be sitting right around 20

Ok switch it to Volts AC and measure from the Neutral line (white wire on power line) to ground and see if it registers any voltage.
Customer: replied 3 years ago.

no voltage from neutral to ground...when check this ignitor again, the needle doesn't even seem to move. maybe its pooched

could be, but its odd it read before.
Customer: replied 3 years ago.

not used to rests at about the 20 ohm mark...don't think it actually read before

I like analog, but digital is easier. I like the ability to pick up something quick, the digitals buffer and one can miss a blip.

Anyway. Can you pick up an ignitor tomorrow?

IF nothing else, I think we took care of the fuse blowing problem. it took a while
Customer: replied 3 years ago.

should be able to...I think a new ignitor might be the answer...thanks so much douglas!h

Ok let me know if I can be of any additional help
Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 3089
Experience: Manufacture Rep for Major Brand, Technical Trainer, NATE
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