On 4/2/2011 3:23 PM, dpb wrote:
On 4/2/2011 2:10 PM, Steve Turner wrote:
A couple of months ago the heater quit coming on in my upstairs Trane
XB80 HVAC unit. It's been off warranty for about 2 years, and I am
enough of a do-it-yourselfer that I'm comfortable with simple repairs. I
found that the LED on the control board was not lighting up, and
subsequently found that I was not getting 24V output from the 115V-24V
transformer. Sure enough, the 24V circuit was open, so I found a local
replacement transformer (not the exact form factor, but same specs) and
things worked fine after I replaced it. Now, I have the very same
problem (only this time it's the A/C that's trying to come on; not that
it matters), and once again 24V is missing on the output side of the
transformer. This time the 24V circuit has continuity, but the 115V
circuit is open. Of course, I can replace the transformer AGAIN, but I'd
sure like to know what could be causing this problem... Any ideas would
be appreciated. Thanks.

Cheap transformers comes to mind???

That is what I think also. Google yields a slew of hits for 24 Trane
xformers.

Transformers can short inside (winding to winding) and they will wind up
smelling burnt. That may burn one of the leads leading in. The fact that
you have one with a primary failure, and another with a secondary leads
to thinking they are designed at the margin.

Someone that knows Trane might come along and offer an opinion. I
rarely saw opened transformers, and when I did it was on the input side.
Sometimes they put in fusible links in lieu of a fuse elsewhere, but
this would be audio gear...

Jeff