On Monday, December 22, 2014 10:36:41 AM UTC-5, Danny D. wrote:
Stormin Mormon wrote, on Mon, 22 Dec 2014 07:54:59 -0500:

If you're safe with a VOM, here's the technique:

I'm perfectly fine with 120/220V and can easily work with my
Fluke 75 on the furnace. I just need to know what to check,
bearing in mind that it's not at first obvious where the power
comes in yet.



I've never seen a furnace where it's not obvious where the power
comes in. I'm beginning to agree with Stormin.




You can get into the small box, and check for polarity and
if the ground is connected.

I can't imagine that the power polarity has reversed, on its own,
but, as you note, I should check it (since the flashing is hard
to understand and one interpretation of the flashes might be that
the power polarity is reversed).

Of course, that only matters for DC, right?

No, you can reverse the hot and neutral on AC. But as I said
before, it's virtually impossible for that to have happened and
I doubt anything to do with the power to the furnace is the problem.


So I'd need to know *where* the DC is coming from, since the line
voltage is AC (yes, I know a neutral isn't the same as a hot, nor
even the same as a ground, but, in terms of polarity, switching
a neutral with a hot shouldn't matter for AC motors or step-down
transformers (which I presume exist near the circuit board).

I hesitate to suggest people open up safety boxes and work
with live power, some folks can't do that safely. Prefer not
to go into a lot of detail on an open forum.

I have no problem whatsoever working around power.