I replaced old thermostat with Honeywell wifi and added C wire from furnace to new thermostat. I left the jumper wire on Rc and R in place and wifi would not come on line. Checked C wire and have 28vdc from C to R. The C wire is grounded to furnace just as it's supposed to be. The old thermostat does not have a C wire or Rc terminal. It is a Trane XT300C thermostat. What am I doing wrong?