Stormin Mormon wrote, on Wed, 24 Dec 2014 10:24:11 -0500:
In this case, the flame is spreading from the top burner (where the igniter is mounted) to the next two burners, where the burner furthest from the igniter (which is the bottom burner) has the flame sensor.
They designed the system that way, according to what I read, because the burner furthest from the igniter is the burner most likely not to light, if there is a propagation issue.
In my case, all three burners light just fine. In fact, I am clueless how the flame moves from the top burner to the next two burners. I guess the flame leaps down from the top burner to the middle burner due to the unburned propane gas initially dissipating? And then the same thing, I guess, happens to make the flame move to the bottom burner where the flame sensor is mounted. In this picture below, the left flame sensor is the bottom one that I cleaned of rust, down to the bare metal.
makolber wrote, on Wed, 24 Dec 2014 06:03:12 -0800:
I have a Fluke 75, which can only get to around 1ma:
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However, I borrowed an Alpha 2360, which can read to 1/10ua.
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When I tested the AC voltage, I put a lead on ground and another lead on the white wire from the control board to the flame sensor.
To test the DC current, I put one lead in the white wire from the control board & the other lead on the flame sensor spade connector.
In that picture above, you can also see that I've added a black jumper so that I could short the flame sensor to ground, in the middle of the 7-second flame.
All I can say is: a. The sensing of the flame failed. b. The flame sensor is getting its nominal 90VAC from the control board. c. There is no current whatsoever when measured in series with the flame sensor using a meter capable of measuring 1/10ua.
Disregard my previous question about the black jumper wire.
I presume you measured continuity from the end of the sensor wire to the electrode?
IDK how much you can see, but can you see that the flame sensor is in the flame, ie that the flame is uniform, OK at the sensor?
Did you try testing the meter with some other small current load, to make sure it doesn't have a blown fuse, etc?
The only thing I can add from all this is that given that you're measuring normal voltage from the board to the sensor, then the possibility of a bad ground would seem to be eliminated.
About the only thing left would be that we don't understand the failure mechanism with the flame sensors. The few descriptions I read, the physics were not well described. I wonder if it's possible something changes on the surface of the metal over time, heating, etc that causes it to stop working? I guess we'll find out when the new one shows up. But given all that you've found so far, my vote would still be the sensor. You apparently have 100V on it, no current flowing.
Regarding the current, I know it's supposed to be a few uA. But I wonder how they define that? Peak? RMS? If it's peak, then it's going to be substantially less RMS, because remember the flame rectifies it, so you only have a half-wave, so you have two adjustments to get to RMS. One that it's a sinewave, the other that it's only half of a sine wave.
That is typical scenario when flame sensor does not do it's job. Did you happen to check DC voltage(5V) the logic needs? One time there was a TSB out on those board. One high Wattage resistor overheating causes resistor lead solder joints on the board crack causing all kinda funnies. I had to reinforce the joints with lots of solder and drill holes on plastic cover. In a situation like that multimeter often can't catch intermittent glitches. O'scope is only way to visualize it. Nowadays there is pocket size basic digital scopes. Sensor rod shoild be positioned right in the flame.
Some thing called a flame spreader, which metal channel goes from burner to burner. Been a while since I had one of them apart.
At the risk of sounding like a grinch, I'm so completely tired of that ## furnace. Of course, I've been sick for a couple days, and not my usual cheerful self.
Tony Hwang wrote, on Thu, 25 Dec 2014 07:50:31 -0700:
I don't know where to look for that 5VDC yet.
I don't know where to find a TSB.
All I know about the board is that it has this number on it: 1012-940-J HK42FZ009
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And one supplier told me the replacement is this:
325878-751 control board conversion kit.
The LH680014 flame sensor rod only goes in one way and it's in the flame right in the middle, as can be seen on the bottom burner of this picture, with the burner removed:
trader_4 wrote, on Thu, 25 Dec 2014 03:32:44 -0800:
MERRY CHRISTMAS!
I haven't seen that question about the black jumper wire yet, but, I used it for two different purposes at two different times:
To ensure a ground from the burner metal to a chassis ground bolt
To short out the flame sensor in the middle of a flame to test it that would kill the flame immediately.
(Neither effort changed anything.)
You know, I did *not* check that, and I should have, although it sure looks nice and tight and clean. But, I will check that, although, in effect, when I checked for the 90VAC voltage, that tested the wire.
I can't see much with the burners in place, but with the burner removed, the flame sensor is "in" the middle of the flame:
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This is a great idea, because the meter isn't mine, so I just assumed it worked. The meter read 104VAC when I tested the flame sensor wire to ground, but, that was with the leads in different holes, and that would be different fuses.
So, it could be that the meter is blown, so, I will try to rig up a micro-amp test of some sort. (I'd hate to open it up because it's not mine, although if the fuse is blown, I'd replace it as a courtesy to my friend).
Do you think so?
But, didn't I make my own ground because I tested that AC voltage at the white wire from the control board to the chassis ground?
So, if there was a bad ground back to the control board, didn't I actually circumvent it with the second test lead?
I'm still confused how to test the control-board ground.
Other than cleaning it, which I've done now with emery cloth, I also can't imagine the thing ever failing. It's just too simple and sturdy. So, while I am hopeful that the new sensor will solve the problem, I don't really think it will.
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All the videos I watched had no current with no flame and then it went up to 4 or 5 ua in the flame, but none said how it was measured.
I don't think the Alfa 2360 meter has a choice though, of average, peak, or root-mean-square. It just has a 400ua scale:
trader_4 wrote, on Thu, 25 Dec 2014 03:11:27 -0800:
I used the black jumper for various purposes in the tests.
I used it to "double" the ground from the burner metal plate to a chassis bolt at a green ground wire at the top of the chassis.
I used it to "short" the flame sensor to ground (with the nominally 90VAC white AC wire still connected to the flame sensor) to see if that shut down the flames prematurely (which it should do in a working circuit).
Typical Carrier part number. There is only one high Wattage resistor, about middle center of the board. I have old spare board kicking around here at home or out at Cabin. I'll dig it out and take a look. If it is not here, I am going out to cabin tomorrow. Tony
Sorry to hear about the heater still not working. That's no fun to be cold, indoors. I've done that enough times.
You'd already know that a calorie (food measure) is actually a measure of heat. So, please feed the kids lots of high calorie foods to help keep them warm.
I've worked on a dozen or so furnace over the years. I've had to replace one or two flame sensors, it's a relatively common part to need to replace. I hope that works for you. When yours comes in from the parts place. I do hope you will let us all know how it works out?
BTW, a long reach nut driver with a magnet in the socket, is your best friend at moments like this.
Ok I assume when you measure the AC voltage the meter is in the AC volts mode. And the meter is in the DC current mode when you try to measure the small DC current? And the leads are plugged into the meter correctly? On some meters you have to plug the leads into different jack for current measurments.
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