Been repairing HVAC equipment for 3 years now and have finally hit one that stumps me. It is a Bryant 355AAV FAU paired with a Bryant 180ANA CU. Upon arrival the fuse on the FAU is blown, so I disconnect the stat and CU and ohm out the low voltage 4 wire connection to make sure no wires are shorting to each other or ground. Upon completing this test I put the stat back on the wall, replace the fuse, and power up the system. No problems, so I plug in the 4 wire connection to the CU and immediately the fuse blows. I disconnect the 4 wire to the CU and test for 24v from the onboard transformer and there is no voltage, even though both legs of high voltage OK. I replace the transformer with a regular 110v transformer, which powers up the board, but as soon as I plug in the 4 wire, boom, another fuse blown. At first I figure it is out of phase, a problem that I have had when twinning FAU's, so I move the hot lead of the transformer to the other leg of the incoming high voltage, same thing happens. I even try wiring in another 230v transformer, which just runs off both legs with no neutral but same thing. Can't figure this out, losing hair and have a very unhappy customer. Any help would be greatly appreciated. Already tried replacing CU circuit board but same problems...

To late for me to help, but you got tricked. The 24 VAC xformer in the HP unit is not for power. It's to isolate a logic signal.

There are phasing issues you have to pay attention too.
The ABCD bus and that 240/24 transformer.

I'll come back later.

Looks like we have some other issues... even with the ABCD cable disconnected and the transformer disconnected on the CU there is some "mystery voltage" of about 30v that I am picking up on the C and D pins(red and white). We elimated the possibility that it is coming over the ground or common wire, and also elimated the CU ECM fan, will be doing more trouble shooting with the designer of the machine tomorrow or Friday...

So, with the xformer disconnected and the ABCD bus disconnected, your reading 24 V on C & D on the board?

I suppose the voltage goes away when you remove the 240 VAC power to the AC?

I'd then start looking for leakage currents.

The likely culpret is the compressor and/or condenser fan. The contactor itself is yet another possibility.

You have the schematic for this, correct?

Phase shift. Check each transformer low volt leg to ground to obtain the hot and neutral legs then wire accordingly. Phase shift also happens on the high voltage side which is a utility problem that usually is what starts the transformer problems. This usually happens when the unit has run for a long time (years) and then develops a problem.

It turned out to be a bad solonoid that is built into the two stage compressor. When ohm'd to ground found it was shorting out. New compressor is on the way. Looks like this is the only duty for the onboard transformer. The 24 volts on C and D power the board and contacter.