Originally Posted by Lancerdad34
I have a 19 year old Trane XE 80.
nothing running and my Honeywell programmable thermostat said "system on".
I checked for 24 volts in the wires going into the thermostat and got nothing.
I got banned, twice, from that forum. . .
You're due about four more years out of this unit with the oldest I've heard of running for 80 years. Mine was born in '82 and I have replaced the inducer motor & its circuit board, air handler motor, gas valve, flame sense switch and a control board relay. And, had to sand the control board edge connector contacts that were coated with calcium from a water leak (probably from the AC drain).
I don't want a new furnace because I will not be able to repair these computer-controlled, touchy & complex gadgets.
If the display works but no 24vac then the thermostat is running on its battery.
Check if the 24vac transformer in the furnace
is putting out. If no 24vac there, check that the transformer's primary winding is getting its 120vac.
Depending on what you find, you will be
checking upstream or
downstream of the transformer
or replacing the transformer (but they are pretty hardy).
You could post a schematic found on the inside of a furnace panel to aid us in efficient troubleshooting of this system but most DIYers lose their nerve when they see the schematic.