Ralph Mowery wrote, on Mon, 22 Dec 2014 14:43:24 -0500:
Do you think this might be the air-flow switch, bolted into the side of the air chamber?
I tested it and it seems to be a short (which is probably what it is supposed to be).
It might be a temperature switch though... or a fusible link?
Tony Hwang wrote, on Mon, 22 Dec 2014 15:58:34 -0700:
The flame sensor is supposed to have 50 microamps, but I couldn't measure that low with my Fluke 75 DMM:
So I removed the flame sensor to take a look at its condition:
That flame sensor was in surprisingly good shape, as is everything:
Even so, I steel wooled the small amount of oxide, just to be safe:
Sounds properly polaraized. The big reset is usually to switch off the AC power for several seconds.
Stormin Mormon wrote, on Mon, 22 Dec 2014 11:24:38 -0500:
I double checked all the grounds, and they're good.
Might have been someone, suggested low air flow, and a bad limit switch. Trader, was that you?
Stormin Mormon wrote, on Mon, 22 Dec 2014 19:37:57 -0500:
While I have an AC on/off switch:
I don't seem to have that big red button inside the blower compartment:
Yet, the manual implies that there can be an "AUXILIARY LIMIT SWITCH (WHEN USED)"; but mine doesn't seem to have that switch.
With a new filter, and open vents, should be much improved air flow. Lets see who spots the non sequitor.
The thing with two rods, sleeves, and what looks like a coin cell, is a thermal cutout. It "should" be self resetting. Continuity is good. Does not measure air flow.
Stormin Mormon wrote, on Mon, 22 Dec 2014 20:16:27 -0500:
Seems reasonable.
I don't know what measures air flow.
This is a "pressure sensor", with a black tube connected to the air chamber; but I don't know how to test it yet:
Stormin Mormon wrote, on Mon, 22 Dec 2014 19:39:38 -0500:
Seems reasonable.
My hypothesis, at the moment, is that all the vents where shut which (maybe) caused a "limit switch" to open up?
If only I could turn that limit switch back on.
I wish I could find a "master reset" which doesn't seem to exist. I've pressed all the limit switch buttons I could find already.
Here's the latest video after cleaning the flame sensor and hooking up the DMM in DC amps mode at the 300ma range with the flame sensor hooked in series with the DMM:
As before, the blower started up & then the induced draft motor spun up, & then the igniter lit up orange, & then there were flames which soon shut down. The flames happened three times, all the while the LED blinked #34. When the flames died the third time, the furnace shut down & the LED blinked #14.
- Control will try three more times before a lockout #14 occurs. If flame signal lost after trial for ignition period, blower will come on for 90 second recycle delay. Check for:
- Oxide buildup on flame sensor (clean with fine sandpaper).
- Proper flame sense microamps (.5 microamps D.C. minimum).
- Gas valve turned off. - Manual shut-off valve. - Low inlet gas pressure.
- Green wire MUST be connected to furnace sheet metal.
- Inadequate flame carryover or rough ignition.
14 IGNITION LOCKOUT- Control will auto-reset after three hours. Refer to #34.
Stormin Mormon wrote, on Mon, 22 Dec 2014 20:11:18 -0500:
I'm not sure I understand ...
non sequitor === an inference that does not follow from the premises; specifically : a fallacy resulting from a simple conversion of a universal affirmative proposition or from the transposition of a condition and its consequent.
- a statement (as a response) that does not follow logically from or is not clearly related to anything previously said.
Anyway, I did *try* to run the "component test" as described here:
COMPONENT TEST To initiate the component test sequence,shut OFF the room thermostat or disconnect the "R" thermostat lead. Briefly short the TEST terminal to the ?Com 24V? terminal. Status LED will flash code and then turn ON the inducer motor. The inducer motor will run for the entire component test. The hot surface ignitor, blower motor-heat speed, and blower motor-cool speed will be turned ON for 10-15 seconds each".
I first removed the RED wire from the (R) terminal block screw, but when I held down the door switch and then at the same time I touched the COM terminal to the TEST terminal, nothing happened:
So, I belatedly realized they must be talking about removing the RED (R) terminal at the thermostat itself, so, put back the Red wire onto the (R) terminal at the furnace, and then went down to the thermostat and removed the two red wires from that thermostat.
Then, back up at the furnace, I added this orange wire to the COM terminal so that I could hold down the door switch and then touch the orange wire momentarily to the TEST blade on the control board:
I'm not exactly sure what happened in this component test, but, a bunch of things went on, and that was it. For example, the inducer motor ran, and the igniter lit for a few seconds, but, I didn't see anything else happen that told me any data.
Maybe I missed something in the so-called "component" test?
Danny D. wrote, on Fri, 19 Dec 2014 23:56:58 +0000:
Here's where the Bryant 373LAV horizontal furnace stands tonight.
Blower starts, inducer motor starts, igniter starts, flame starts, flame shuts down. Three times the igniter ignites the flame but three times, the flame shuts down within seconds.
All the while it displays code #34. At the end of the sequence, it shuts down and displays code #14.
Video:
Schematic:
Codes:
- Control will try three more times before a lockout #14 occurs. If flame signal lost after trial for ignition period, blower will come on for 90 second recycle delay. Check for:
- Oxide buildup on flame sensor (clean with fine sandpaper).
- Proper flame sense microamps (.5 microamps D.C. minimum).
- Gas valve turned off. - Manual shut-off valve. - Low inlet gas pressure.
- Green wire MUST be connected to furnace sheet metal.
- Inadequate flame carryover or rough ignition.
14 IGNITION LOCKOUT- Control will auto-reset after three hours. Refer to #34.
Ok, so now the question that might shed some light on what's going on is, what changed that allowed the furnace to get this far? Previously, it would come up with a flashing LED trouble indication as soon as power was put on, without firing at all. Now, it's firing, then shutting down because it thinks it hasn't lit.
I think that may be relevant, because initially you had continous LED flashes that the chart says corresponds to reversed polarity. While the polarity isn't reveresed, I wonder if it will also give that code if the ground is missing? I ask that because the flame sensor is single wire and depends on a ground path back to the controller board. Is it possible there is a bad ground connection between the controller board and the furnace metal?
Other than that, the most obvious and direct possibility is that the flame sensor is bad. If you'd had this code, this behavior from the beginning, that would be the logical place to start. But what's odd is how it behaved very differently initially, unless that was something to do with you having the safety switch open, how you were starting it, etc.
trader_4 wrote, on Tue, 23 Dec 2014 04:44:15 -0800:
I agree with you.
The manual says: "NOTE: If the polarity is not correct, the STATUS LED on the control center will flash rapidly and prevent the furnace from heating. The control system also requires an earth ground for proper operation of the control and flame sensing."
So, *both* the old and new LED indications point to a suspected lack of "earth" ground (among other things).
Plus, the constant flashing stopped just about when I started testing the power connections (although, I didn't *change* anything). I wiggled everything. But I left the wiring as it was before.
Yet, suddenly, it stopped flashing constantly, and, instead, the error migrated from a power problem to short cycling (which now implicates either the flame sensor or the flame sensor control circuit).
Yet, the flame sensor has only one wire, so, ground is clearly the case.
I'm thinking the same thing. A bad ground could cause the flame proving electrode not to have its requisite 5 microamps to 6 microamps. In the schematic, there are two "Note #5" comments, each next to the "GV" which is the gas valve
Note #5 says: "5. This wire must be connected to furnace sheetmetal for control to detect flame."
I'll need to check that, remove it, sand it, clean it, & re-test.
I suspect ground is critical for the flame-sensing circuit, because the DC current is only 5 or 6 microamperes, which is so low, I'm going to have to figure out *how* to test it using coils of wire wrapped around an ammeter.
Looking at the troubleshooting flow chart, it does now imply that either the flame proving electrode is bad, or that the control to that flame sensor is bad.
I agree with you:
Danny D. wrote, on Tue, 23 Dec 2014 13:21:56 +0000:
Found a service manual online, by the way:
Danny D. wrote, on Tue, 23 Dec 2014 14:36:38 +0000:
Here's a better link to the service manual so you can see the schematic and troubleshooting flowchart:
If the ground is affecting the flame sensor, then it't the ground path from the metal cabinet/frame of the furnace to the control board. That's where I'd be looking. There must be a ground wire that goes from the control board to the frame and the frame is also grounded to the incoming AC.
it is difficult to measure the 5 uA using a VOM.
I think you are on the right track.
Check all the grounds This means the controller ground to the AC supply and ALSO the furnace chas sis ground to the AC supply.
Check that the flame sensor insulator is clean so that there is no partial short to ground of the flame sensor output whick is a very small DC voltage or current realtive to funrace chassis ground.
Check that the flame sensor probe is not very dirty but I don't see how tha t would casue the original symptom, but culd casue the symptoms you have no w.
Check that the flame sensor probe does not touch metal but is properly touc hing the flame.
The flame sensor probe acts as a tiny battery and the controller needs to s ense that current. THe flame sensor current is relative to the furnce grou nd which needs to be connectd to the AC ground and to the controller ground . I guess also make sure the burner jets are also grounded to the chassis. The flame is grounded to the burner jets actually.
Mark
Looking up videos on troubleshooting a Bryant 373LAV flame sensor, these guys clean it with a brand new five-dollar bill:
This looks like a similar furnace to mine, with a similar issue, and this guy does not recommend cleaning the flame sensor:
After replacing the flame sensor, he shows the whole cycle:
These guys use #0000 fine steel wool to clean the flame sensor:
All say not to touch the sensor with fingers, which I didn't know when I first had cleaned it.
But, interestingly, *all* (of those) simply clean or replace the flame sensor without *any* of them testing it for the 5 or 6 uamps of control current. (I'll google separately for videos *testing* the flame sensor circuit but this is already long.)
Normally, I test things before replacing them, but, none of the technicians in the youtube videos I've found bothered to test it before replacing it.
Even so, I might buy a spare flame sensor and igniter, and keep it next to the furnace. Any suggestions as to good suppliers?
good..
see Fig 12. Note #5 in the lower right corner.
This wire must be connected to furnace sheetmetal for the flamesensor to work.
THe note is for the return wire of the gas valve.
I'm not sure what they mean exaclty, but something there needs to get to furnace sheetmetal ground for the flame senor to work.
Mark
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