[pauleyman]

I'm at my wits end now.
A little backstory. WOrking on a 2014 york g8c10016muc12a. It had a control board from source 1 labeled 1012-83-9651b.
It was flagging a pressure switch code. I checked the rubber line and replaced the pressure switch. Ran for a day. Cleaned out the orifice the rubber line plugs into and no real change. I have since figured out the board had a connection issue. I was able to repeatedly make it fire if I just pushed on the board. You immediately heard a relay come in and everything worked fine until I let go. It was repeatable so I decided it was a bad board.
I have installed a white rodgers model 50m56u-843 universal board with a black cover. Apparently, there is also a white cover older unit with the same model and it's slightly different. This unit apparently also requires a shunt installed if you're using a york furnace and I have done so. It wouldn't fire before that. The main fan wouldn't come on without the shunt.
So now it fires and seems to cycle fine but it consistently shuts off after a few minutes and flags a high limit code. As soon as it shuts off then the main fan runs cool down cycle it shuts off then it immediately cycles again. I suspect after some time it will shut off completely. I can't see sensors failing so I'm wondering if I still have some cause leftover and I had multiple problems but only solved one with the new board. I hate to throw parts at it but I'm thinking my next step is a new high limit switch as they are fairly cheap. It also flags a weak flame code for a few seconds when it first fires but it is not weak. I did install the new ignitor that came with the board as a kit. I did not use any of the adapter harnesses in the kit as it was not necessary.

 

[surferdude2]

Could be a dirty flame sensor. Worth a shot... easy to do. The code may be incorrect.

Otherwise, check air flow and flue restriction.

Last choice, replace hi-limit.

 

[de-nagorg]

Here is a tip from an old Electronics repairman.

When dealing with a suspected faulty board, look it over carefully for a "COLD SOLDER JOINT".

And re-solder all cold joints that you find.

In automated technology they always create a cold joint, that causes this intermittent function.

Dollars to doughnut holes, you have a cold joint on the original board, and .01 cents would fix it.

A cold joint is evident where the component wire looks like a bullseye in the hole in the board where it should be soldered in.


ED

 

[pauleyman]

I was headed for flue restriction next. But what about airflow? Dont know how to check that

 

[surferdude2]

ED has a point, it might be easier to fix the old board. I've done lots of that kind of work... I always liked to find the ones with some repeatable pressure issue... easy peasy... that's what pencils with erasers are for, pressing on boards. Other than that I fixed the ones ED gave up on. lol Give me a hair dryer and a can of freeze mist and I can save the day.

 

[user_12345a]

Furnace control boards should never be repaired because they're life safety devices.

You have no idea if the traces and components are still good after a solder joint failure.

So now it fires and seems to cycle fine but it consistently shuts off after a few minutes and flags a high limit code. As soon as it shuts off then the main fan runs cool down cycle it shuts off then it immediately cycles again. I suspect after some time it will shut off completely. I can't see sensors failing so I'm wondering if I still have some cause leftover and I had multiple problems but only solved one with the new board. I hate to throw parts at it but I'm thinking my next step is a new high limit switch as they are fairly cheap. It also flags a weak flame code for a few seconds when it first fires but it is not weak.

Use a meter to see if the limit is actually opening.

If it is, you may have connected the wrong speed tap wire to the heating terminal on the board.

Too low blower speed can cause high limit to trip.

The factory heating speed is not always correct as each duct system is a bit different.

The safety device is probably doing its job so don't replace it.

Check temperature rise if limit is actually opening with no obvious cause - instructions here: https://structuretech1.com/diy-furnace-test/

Check voltage, polarity and ground.

Check flame signal current - put meter in series with the flame sensor and switch to DC microamps. Cycle furnace and post result back.

 

[pauleyman]

I can do that. If memory serves black wire is cooling. Blue is heat and red wire is parked. I'll check voltage on the limit tomorrow.

 

[user_12345a]

Did you wire blower in accordance with schematic or use the same heating speed as what was used before?

Voltage across the limit should be 0v when closed and 24v roughly when open.

 

[surferdude2]

Furnace control boards should never be repaired because they're life safety devices.

You have no idea if the traces and components are still good after re-soldering something.

I beg your pardon... you must be thinking of how your soldering looks. I have no problem soldering on boards and making it look better than some factory board immersion flexing machine did. YMMV LOL

 

[user_12345a]

I beg your pardon... you must be thinking of how your soldering looks. I have no problem soldering on boards and making it look better than some factory board immersion flexing machine did. YMMV LOL

Not worth the risk, likely voids safety certifications.

OP bought a new board regardless.

 

[pauleyman]

Did you wire blower in accordance with schematic or use the same heating speed as what was used before?

Voltage across the limit should be 0v when closed and 24v roughly when open.

I just used same heating speed as what was used before. I took pictures so it was easy to duplicate. I could switch the wires just to test a higher airflow? I am checking the switch itself today and I will verify temperature. The high limit is either faulty or it's doing it's job and has been suggested I have an airflow issue. Possible flue, fan speed, restriction or even bad fan motor. Given I changed nothing from the previous board I don't see how it could be any airflow issues except possibly the flue. If this board has slightly different settings for temp reading it could shutoff where the other board didn't. Flue might make sense though and could have been contributing to the previous pressure switch problem though I doubt it given the symptoms and repeatability of the board connection on the previous board. Pressure switch tested good and did exactly what it's supposed to do. Back to the attic I go.

 

[user_12345a]

Flue restriction causes pressure switches to not close, not high limit to open.

Airflow shouldn't have changed in your case so the codes may not be correct.

You have to verify.

Did you by any chance buy the board off ebay? There are bad sellers there reselling crappy used boards as open box.

 

[pauleyman]

Flue restriction causes pressure switches to not close, not high limit to open.

Airflow shouldn't have changed in your case so the codes may not be correct.

You have to verify.

Did you by any chance buy the board off ebay? There are bad sellers there reselling crappy used boards as open box.

Good point. No I bought the board at a local HVAC supply house.
I'll report back later today. Thank you for all the replies.

 

[pauleyman]

Flue restriction causes pressure switches to not close, not high limit to open.

Airflow shouldn't have changed in your case so the codes may not be correct.

You have to verify.

Did you by any chance buy the board off ebay? There are bad sellers there reselling crappy used boards as open box.

I didn't get very far today except to check the continuity of the high limit switch. When the unit shuts off the switch is open so it simply appears it's doing it's job. That being said it should've been doing this on the old board so what has changed? I've not changed anything else. I verified the cool and hot leads to the motor and they are correct. It's a cheap enough part I thought maybe I should just change it anyway. I really don't understand why the board hasn't shut down completely after multiple cycles of the high limit tripping. It is my understanding that is usually what happens. One or two trips no big deal but multiple it finally shuts down for safety but his has gone through many many cycles like this. I just watched it run and it keeps doing the same thing until the thermostat is satisfied. flame looks good. Main fan starts and shuts off like it should.

 

[user_12345a]

possible the furnace had this problem all along and you never noticed - it can still heat with the burners cycling on and off, just not efficiently.

Measure the temperature rise and clock the gas meter.

for gas meter clocking you need the btu value of the gas, all other gas appliances get shut off.

meter clocking - https://www.hvacrschool.com/clocking-a-gas-meter-its-not-that-hard/
I would not use the smallest dial though - if you do average 5 turns.

 

[pauleyman]

I dont think it did. I watched it cycle with the old board.

 

[user_12345a]

Impossible for a board replacement to cause high limit trips if the same speed tap is being used and new board has good relays, is delivering proper voltage to the blower motor.

This is assuming the new board is 100% compatible and has the same fan on delay as old.