trader_4 wrote, on Tue, 23 Dec 2014 04:44:15 -0800:
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 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 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?
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."
https://www.flickr.com/photos/98287134@N02/15460746143/
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.
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.
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.
https://www.flickr.com/photos/98287134@N02/15894320489/
I agree with you:
1. Today I will clean all the grounds.
2. I will check that "Note #5" wire listed in the schematic.
3. I will re-clean the flame sensing electrode (just in case).
4. I will figure out how to measure 5 or 6 microamperes
(and it can be as low as half a microampere).