[BillyHill]

I hope someone can help me. I have searched all over and haven't found an answer to my situation.

I have a 1975 Payne model 394GAW000075. It used to work fine but one day recently only the blower was running and no heat.

I disconnected the 110 v and back on again and only the blower worked still. The blower is running even with the thermostat off.

I have an old 2 wire thermo and tried replacing it with a new one but the same issue happens with the new or old thermostat. So I don't think that the thermo is the problem.

There is no pilot on this model, it lights automatically each time it's told heat is needed. Except now the blower just runs and nothing else.

When it was working the blower would shut down once temp was reached, then start agaian for the heat cycle and the burners would light.

I hope I have been specific enough for help to be offered. I have checked for loose wires and everything looks fine. Thanks in advance!

 

[JScotty]

Sounds like you have a bad limit. The limit measures the temp inside the furnace & if it gets too hot it shuts down the burners and forces the fan on. If you have a volt meter, check both sides of the switch, if you read 24v (assuming the furnace isn't actually hot) the limit is bad.

 

[ben's plumbing]

take a picture of unit with doors off so we can see it ..ben sr

 

[BillyHill]

Thanks for the replies. Here are a couple pics. If these don't show what you need please let me know.

Unfortunately I dont have my voltmeter here. I can get one here tomorrow though.

 

[BillyHill]

Here are a couple more pics if they help...the wire with the little fuse going to the limit looks like maybe it got hot....can i bypass the limit just to see if it will light? or should i replace that fuse? I havent seen any reset button or switch anywhere but I assume there must be somewhere.

Thanks again for any advice offered.

 

[BillyHill]

I just bypassed the limit by jumping the two wires together and there was no change. I can cycle the blower on and off as normal with the thermostat but still no attempt to light....I used to hear the ignitor clicking quite a few times to light but now not even once...

Ideas?

 

[Marty S.]

That fuse is thermal. If there was flame roll out it got hot and did it's job shutting the furnace down. Flame roll out is usually from a major crack in the heat exchanger. You are beyond DIY and need to call in a pro IMO.

 

[BillyHill]

I hope its not that major....may be though. I tried putting my voltmeter probes on the limit wires with the furnace running and got no reading.

Am i testing the limit right? Or do I need to test one wire at a time with the black probe grounded somewhere?

 

[JScotty]

I hope its not that major....may be though. I tried putting my voltmeter probes on the limit wires with the furnace running and got no reading.

Am i testing the limit right? Or do I need to test one wire at a time with the black probe grounded somewhere?

No you're testing it right, checking from each side to ground can be misleading as sometimes power can backfeed. If you read no voltage when reading across a switch (meter leads on each side of the switch) that indicates that either you don't have power to that switch or that the switch is actually closed. An open switch will read the voltage present. So if you read across that switch & see 24v that would indicate the switch is open.

If you aren't reading voltage across the switch I would then try testing to ground as that will verify that voltage is present. If you read 24v from the switch to ground (anywhere on the unit case will usually do) that would tell you that power is present at the switch and since you are getting nothing across it the switch is closed and your problem is somewhere else. If you read nothing from the switch to ground then you don't have power & you will need to track down where you're losing it.

 

[BillyHill]

Thanks jscotty.

ok, so I tested it again running, at the limit with both leads on and got pretty much 0, fluctuates to a fraction of a volt and back to zero again

tested with one lead on one wire and other lead grounded, same result

followed the limit wires to the circuit board, tested there and same result

at sec-1 and sec-2 got same result as at lim-1 and lim-2

what is sec-1 and sec-2?

while there tested different locations on the other side of the board, and I got 120 vac at 3 different locations...

what else should I test? or is the board bad since something there isn't distributing power correctly it seems?

Thanks for helping!

 

[BillyHill]

I read somewhere if I don't have 24 volts on sec1 and sec2 that the transformer is bad.

Does that sound right based on all my testing so far?

 

[JScotty]

I read somewhere if I don't have 24 volts on sec1 and sec2 that the transformer is bad.

Does that sound right based on all my testing so far?

That very well could be. I believe the sec 1 and sec 2 are the secondary leads from your transformer (the 24v side) follow those wires back, if they go to the transformer then that's what they are. There should be 2 wires coming out the other side of the transformer(those would be the primary side), check to make sure you have 115v at the primary side of the transformer. There may be terminals on your board that feed that which would be the easiest way to check, they would probably be labeled pri 1/pri 2 or something like that.

If you have 115v at the primary side & nothing at the secondary side, verify to make sure there isn't some kind of fuse on either side. Sometimes they put an automotive fuse in the secondary side to keep your transformer from blowing if there's a short. If there's no fuses your transformer would probably be bad.

 

[BillyHill]

Thanks again Jscotty!

I do have 120 at primary, no 24 volt out though, so....

I just bought a new transformer from a local parts house for $16 out the door!

I will reply back shortly with either success or failure.