HVAC Questions? Ask an HVAC Expert for Answers ASAP
Hello there! I'm Norman but you can call me Norm. Let's see what I can do to get you some answers today. I'm ready to get started whenever you are.
I hope you are doing well today.
I'm sorry to hear you are having troubles.
Hi Norm. Thanks. Hope you can help us out. Any ideas?
Let me look over the furnace information really quick and I'm sure I can help.
Have you checked the low voltage to the thermostat by measuring from C to R with the door switch held in, one probe on each?
If you don't have 24v there we know the problem is in the control voltage.
The light being off in the PG9YAB furnace is usually always attributed to a loss of main power or the low voltage fuse being blown.
You have confirmed 120v to the board according to your information and that the transformer is stepping down to 26v, which is good.
We just need to confirm the voltage is getting through the board to the R terminal which is how the call for heat is made, through the thermostat, then back on the W terminal.
We loss power yesterday for about an hour.
he is checking voltages now
When you said the fuse is good, did he check the fuse on the control board?
from c to r with door switch held in i'm getting mv readings, bascially nothing
Ok, so no go on control voltage.
yes, fuse on control board was good
Pull that fuse off of the board and ohm it out.
Lets be certain
he did and fuse reads fine
If the fuse is good and you still don't have 24v at the C to R measurement the board has to be bad. Very odd that it's not a solid LED, that normally indicates a control board failure.
Just to be certain, did he ensure the transformer 24v side is connected to the board, both wires?
yes, both wires are connected
could the power outage have caused the board to go bad?
When he measured the transformer and main power did he measure leg to leg or to ground?
leg to leg
I again have to stress that the low voltage fuse is normally the culprit with these symptoms, I know he said he checked it but just be 100% sure before we conclude the board is bad. I'm not there so I have to be cautious and advise certainty in this matter.
I hope you understand.
earlier he got the furnace to fire up but when he let go of the switch it shut down and it has not fired up since nor has the led come on
he will check fuse again
Yes a power surge can surely take out a control board. When he is completing all of his test does he have the thermostat set to HEAT with the temp up above the room temp and with the door switch held in?
thermostat is switched to heat, but there is no display
Ok, so it's not battery powered and we have no control voltage, that is why.
ok, he checked fuse again reading is 000.1ohms
Have him hold the door switch in and check from C terminal to each side of the fuse terminals.
One probe on C one probe on each post, one at a time.
from neutral he gets 125 to each with fuse in
125v on the fuse post?
We are talking about the car type fuse correct?
Little tiny fuse plugged into the control board, may be yellow
I'm thinking he is checking the wrong fuses
He is checking the main fuses to the furnace.
correct. I actually had one leg of my meter on the 120VAC Neutral and the other leg of my meter on the fuse base and I got approx 124VAC. From C to the fuse pose mV.
btw the car type fuse is the correct size 5A.
Ok, we need to measure from the C terminal on the low voltage connections.
Use C for one probe. Then touch each post with the other.
10-4 stand by. THX!
With the door switch in.
with the door switch in I read from C to each terminal r,w,g,y, then reversed my polarity and got the same results. Readings start at about 30mV and decrease down to virtually nothing. Should I try this again with the thermostat removed?
No. I need you to put the fuse in on the control board.
Then place the black probe on C down with these connections.
Then place the red probe to each side of the fuse post.
Right where the fuse plugs in
I had the fuse in during my previous readings.
I understand but I need to know if voltage is even getting to the fuse which is before the R,W,G,Y in the circuitry.
w/black probe on C nothing on either fuse post (mV)
With the door switch in?
Ok, one more measurement
Find L1 which is the hot wire past the door switch that connects to the board.
Place one probe there, then another probe on the White neutral wire that connects to the board from the main power in.
With the door switch held in and the wires connected to the board.
L1 is the terminal on the board.
from l1 to neutral, 115, w/primary side of transformer removed
Connect the transformer for me then measure it again.
Have you been conducting these test with the transformer removed?
where should he measure at? L1 back on
Ok, I want everything connected to the board. Do not remove anything for any test.
Measure from L1 on the board to the White neutral wire that enters the furnace and connects to the board, with the door switch in.
can't get to l1 w/wire on.
Should be able to jam the probe in the back of the terminal
but from newutral to black transformer lead post w/wire in place, 120mV
neutral to fuse post 124vac
Nuetral to fuse post doesn't tell us anything. Two separate circuits.
Ok, with all wires connected. Now measure from XFMR terminal on the board (black wire connected) to the transformer Neutral (white wire connected), with door switch held in.
approx 120mV from both transformer post and L1 to neutral with all wires in place
If you are reading millivolts then you have no main power
Check the main power entering the furnace.
measure from leg to leg.
White to black
from neutral to L1 wire with door switch in, 115volts
Ok, that's more like it.
Now I need to be 100% certain you are doing this right.
With all wires connected to the furnace, and the door switch held in
There are 4 wires to the transformer
2 are primary side (black and white)
2 are secondary (blue and red)
Measure from black to white, then blue to red.
i have paper clip. on my positive leed. measuring from neutral to L1 and newutral to transformer primary(black) with door switch in, 27vac
Black to white should be around 120v
red to blue should be around 24v
Some how we are getting mixed communications.
would this be easier over phone
Not allowed, site policy
Ok we said the transformer has 4 wires.
I need him to put the black probe on the black wire.
the red probe on the white wire.
Hold the door switch in and measure.
tell me this one reading
Still with me?
yep, just one second.
with everything connected and door switch in. from neutral to line in, mV. when I remove black wire(feed), press door switch, I get 115volts. It just doesn't make sense.
Because you are reading only one leg
You don't have main power into the furnace.
Have you checked your breakers in the electrical panel?
Yes Power to furnace (black wire) fine to ground/ Neutral when I press in door switch. 115VAC
Hang in there with me.
I am. Conducting the test in the order I give them helps me stay with you and helps us follow a flow.
okay what should I do?
REALLY appreciate your help!
Ok keep all the wires connected to everything.
Where the main power enters the furance find the white and black wire. They should be wirenuted to the furnace wires in a junction box on the furnace wall.
Measure behind the wire nuts from White to black. Door switch does not matter here. one probe on black wirenut, one probe on white wirenut.
Alright not the black wire is broken by the door switch correct?
now the black, sorry typo
Ok keep one probe behind the white nuetral wirenuts, place the other probe on the furnace side of the door switch with it held in.
at the switch
this makes no sense. reading from neutral from one side of door switch and pressed in. led heartbeat ok. and furnace kicked on.
Loose wirenut in the junction box.
same situation as before, if he lets go of door switch, furnace will shut off
Or failing furnace door switch
It will always shut off when you release the door safety switch.
that's its job
the door has to be on for the furnace to continue to run.
I get that, and I agree about the door switch, however "everytime" I press in door switch and take voltage readings on L1 wire with it removed from L1 terminal I get 120VAC
as you should
it's when you release the switch that the circuit opens.
I know that the door has to be on but if I let go of it it will shut off (again it's doing it's job), this is where I was earlier and when I released the switch and placed the door back on it would not fire up or the LED would not light.
Sounds like the door switch isn't engaging all the way.
tape something to the door where it makes contact with the door switch to ensure its pushing the switch in all the way.
At this point the mrs is holding the switch in while I type. Could it be something on the board or the L1 block?
If it works with you sitting here holding it but it doesnt when you put the door on, the problem is the door switch isn't being engaged.
If when you put the door on it was trying to fire but wasn't, I'd say it could be something else. But the issue you describe sounds like the switch.
I would agree if it occurred every time however it does not fire up every time I hold it in. nor does it re-fire if I let it go and press the switch in again. I can/will try taping something to the switch to press it in further.
Sometimes these switches can be faulty and work sometimes and not others. This would cause it to work some times and not other times.
I'd replace the door switch, simple cheap fix.
Now if you were wiggling wires around and it kept powering up then off again. I'd say a post on the board could be bad or a loose wirenut/
Door switches fail all the time!
Will do. for now I'll jumper the wires out "safely" and place the doors back on. Is there a restart delay that I need to be concerned with?
Some boards have built in delay. Usually if they try several times and fail they will lock out. But with you just pressing the switch in and letting go you should be okay. just kill main power to the furnace and jump the switch out. By time you do that the furnace should just fire right back up after you re-power the system.
Thanks will try that now.
Sounds good. Let me know if it works good after you do that.
Unfortunately I have to get ready to step away from the computer for a bit. Any chance you've almost got it ready to fire up?
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I am back now, in case you need further help.
working now, thanks for all your help!