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Discussion Starter · #1 · (Edited)
Hello, I have a Day&Night (Carrier) model 518A048 furnace and A/C unit. Need some help to get it wife just got laid off, and I'd really, really like to avoid calling someone in if possible...

The other day we noticed it got cool in the house and that, although the thermostat was calling for heat, the furnace was not blowing warm air. just blowing as if on a "low" fan setting.....Hmmmm what could this be? So I went out to the garage and the furnace circulation fan was running, but the furnace was not burning/running.

Also noticed that the fan kept running, even if the thermostat was set to "off".

After unplugging the 110V cord from the wall, I removed the cover from the control board assembly (the board to which the thermostat wires attach) and looked for fuses - there are none.

So I bypassed the safety interlock cover switch, now fan is running again, Tapped on the two relays w/screwdriver handle, to see if there was anything intermittent going on with them...nope. So I got out my DMM and checked to see if the transformer secondaries were putting out any voltage, and there was no 24V as expected.

I'm thinking, Aha! That's my problem, ye olde transformer has simply given up the ghost, after 23 years of faithful service....:yes:

I congratulate myself on my troubleshooting prowess, and order up a new transformer (luckily it has been warm and we could do without heat for a few days) and just installed it. Furnace behaved same way.

Checked the secondaries and I have a solid 25V.

Also set thermostat to call for AC, no response from the AC compressor. I let it run (fan running) for a few minutes, thinking that maybe the circuit needed to time out, reset, or something, and after running for a few minutes, I heard a faint "Pop" ...and then decide to check the transformer secondaries...dead. :(

So whatever it is, the transformer is not the cause of the problem (good thing I picked up a spare - I'm putting an inline fuse on the next one!)...

So, I am wondering what else it could be....I'm thinking that there is a good chance that the board is toast...and there is a dead short on it that is killing the transformer....Are these boards even available any more?

Any HVAC gurus out there who have experience with this equipment?

Thanks!! Rob

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3,630 Posts
that there was no respose to the call for the AC it could of been a time delay thru the stat and or the board in the furnace...but when it cleared you popped the secondary on the TR if it was the board it would of popped right off the bat from the call for cooling.redo the TR fuse the secondary and before repowering it disconnect the 2 low voltage wires out at the condenser that come from the furnace.set the stat for cooling go outside and read 24Vs on those 2 wires(shut the line disconnect off for the condenser)your only looking at the control circuit closure for now don't need the compressor/fan banging on ..just concentrate on the contactor closure.JUST DO THIS first with the 2 wires hangig in the condenser for the 24Vs you disconnected from ....set your meter for audible OHMS go to the copper line with one lead and check both wires for grounds..and make sure the contactor is free moving with a test push on the spring for the wiring up the 24Vs from the stat cooling call,connect one wire and touch the other to the remaining one the contactor should bang in.if it pops the fuse right off the bat the coil might be shot for the compressor..get back with the results

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Discussion Starter · #3 · (Edited)

Biggles, thank you very, very much for your advice. Here is what I did.

1. Replaced the transformer and added a 2A fuse on one leg of the secondaries.

2. At the control board terminals, Disconnected the two low voltage wires which run outside to the contactor on the condensor (to eliminate the A/C side, just for now)

3. Powered up, set the thermostat high to call for heat, bingo, furnace lights/runs. We have heat!

Now for the AC...

4. Outside, At the condenser, I removed the power interconnect plug, then disconnected the two low voltage wires right at the contactor. (I also did check the contactor, and it seems free/contacts move well).

5. Back at the control board at the furnace, I re-connected the two low voltage contactor wires.

6. powered back up, Set the thermostat to call for A/C. Went outside, checked for 24V at the wire ends, and I have no 24V. (I also went back and checked the two terminals on the board, and no 24V there either). The terminals that the contactor wires are connected to are the "Y" and the "C".

I power down, and now with the wires disconnected and free at both ends, I check the wires with my dmm, and find out that the two wires are not shorted to gnd but are shorted to... each other! :eek:

Certainly wasn't expecting to see that! (maybe a mouse or something has knawed/mangled this wire where it runs inside the garage wall somewhere?) Can't imagine what else would suddenly cause a physical problem with this wire...

with the shorted wire disconnected, I checked voltage between the "Y" and "C" terminals while the thermostat is calling for A/C, and there was no voltage there. Checked the fuse on the secondary and (as expected) I had blown it (from having the shorted wire connected). Put a new fuse in, powered back up, set the thermostat to call for A/C, and now I have 24V at the board terminals "Y" and "C". Whoo Hoo!

So, tomorrow evening after work I will pick up a length of new low voltage 2-conductor wire and run a new wire out there between the board and the contactor (It's about a 35 foot run)....Will update again tomorrow!

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