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.