I have a trane furnace I have power up to the junction box where is the step down transformer located and how do I test it sure could use a electrical diagram of this system where do I get one TRANE gas fu

Look on the back of the blower access door for the wiring diagram.

Do as hvac100 said and the call your friendly neighborhood HVAC company and have that Trane replaced with either a Carrier or a Goodman. :p