On 4/18/2011 9:41 PM, Steve Turner wrote:
Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...7_04012009.pdf


I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. I just came back from testing for that, and now I'm confused
again. I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... What do you
make of that?