HVAC circuit board problem

7 years ago

This is kind of an ongoing saga because since about 2 years ago this started- the 16 pin connector on the circuitboard(Lenox 50a65-120-04fyi) does not hold contact to the board. i wiggle it, hold it to one direction and it runs fine or else the fan starts but the glow plug doesn't heat up thus no gas\heat.

I tried widening the male and squeezing the female pins didn't solve the problem.

But here is the major issue why i post - wiggling the wires this morning, downstairs the thermostat is not getting any power.

At the furnace, upon plugging in the power, the blower and exhaust fan starts and the 2 LEDs are blinking(running normal according to chart) as if normal operation.

Can part of the board that sends out power to the thermostat be broken yet LEDs imply it is working properly or do i have a bum thermostat?

TIA.

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Comments (8)

mike_home
7 years ago
Intermittent connections can cause all types of strange behavior. Therefore anything is possible.

Before you start sapping thermostats, I suggest you measure the voltage between the R and C connections on the board. You should get 24V AC between those terminals. If you don't then you have a board problem and it should be replaced. If you do get a good voltage reading, then measure the voltage at the thermostat to verify the thermostat is getting power.
upup_and_away
Original Author
7 years ago
last modified: 7 years ago
I checked the transofrmer is putting out 24v ac and there is 24v going t the thermostat. I found out the blue wire broke about 6" from the board. I don't think this was one of the wires running the heater but fixing this, the thermostat is running again.

For the ongoing problem, I went through all 20 pins and widened the male end and crimped the female ends and upon reconnecting power, it seems to work ok in that the heater starts up.

The problem i now have is that the blower fan will not turn off- blower is set to auto and the house(at least the downstairs this covers) is at desired temp and shows the furnace is not running but the blower is.

Now what do i look for???(btw, the lights blink implying it is working normal)

TIA.
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mike_home
7 years ago
Set the thermostat so it is not calling for heat. Then measure the voltage between the C terminal and the G, W, and Y terminals receptively. The G terminal controls the fan independently. In most furnaces the fan will also turn on if the Y terminal is energized. All of these terminals should have zero volts. If they are then it is a board problem and not the thermostat.

If it is a board problem then the relay that turns on the blower motor is either stuck closed, or a connection problem on the board is causing it to close and never open. The blinking lights are unlikely to indicate if there is a connection issue or a stuck relay.

Did something happen to this board that caused the connector pins to bend and wires to break? If you are up to replacing it, you can buy a new board for $90 and have it shipped to your house by the next day. If this board has had physical damage, then I suggest replacement is the best long term solution.
upup_and_away
Original Author
7 years ago
I tried a quick test before leaving for work where i set the thermostat to off even removed the thermostat itself. maybe i didn't wait long enough but the fan never stopped. I'll check those voltages when i get home today.

Last night on the furnace board, both lights were blinking fast(=running heater) so it didn't report any problem there.

As i mentioned in the orig post, it is a long saga where troubleshooting, my looking around lifting wires made a spark noise and the heater exhaust fan started leading to glow plug heating up etc etc. This after ordering a replacement that didn't solve the problem. With the weight of the wires puling it down, i put straps to take the weight off the connector this time. This seems to fix the wire contact problem but now the problem with the thermostat\fan....

Thanks again.
upup_and_away
Original Author
7 years ago
I got my voltmeter and found a few pins have 24V(with thermostat off).

Earlier, 1 wire(yellow wire) from the AC compressor connected to the yellow wire(sorry i need to go back up to see what letter is printed next to it) came loose when i was poking around. Ever since i connected the yellow wire from compressor back to yellow the blower has been running ever since.

Disconnecting it and the heater\blower has worked ever since.

What pins should the ac wires connect to?

Thanks.
weedmeister
7 years ago
AC compressor is usually Y or yellow. Heat is usually White or W. G is green and Fan. Blue depends. If you have a heat pump, blue can be the Reversing Valve. If not, sometimes installers steal ground and use Blue.
Elmer J Fudd
7 years ago
Can you call a competent HVAC tech? Seems like you really don't know what you're working with.

Good luck.
upup_and_away
Original Author
7 years ago
last modified: 7 years ago
Thanks weedmeister. u just confirmed the pic i took of the wires before is correct. I guess the board is bad where if i connect the AC compressor something is stuck on that the blower won't turn off. I left it unplugged for now(as i said it works properly with it disconnected) and deal with it next year for summer.

Hopefully the straps holding up the wires so they don't hang off the connector will permanently fix the problem of the intermittent connection of the exhaust fan pin.

Thanks.