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JND 05-25-2012 09:13 PM

Trane XE78 Furnace coupled with Trane 1000 AC
Trane XE 78 furnace coupled with Trane 1000 stopped working all together after running the AC one time. Everything worked overnight but stopped by the next morning. Wife heard a noise come from the AC unit (she says) that night. The furnace has the LED indicator that has no flash. No blower fan either. We used the furnace all winter, by the way, with no issues. I have checked incoming power to both units and all is fine there. Panel pressure switch is making also. Seems the AC unit is the culprit but logic tells me that if there was an issue with it then the breaker would have done its job and tripped before reaching the furnace. Both breakers for each unit were on when inspected. Control board is a Whites Rodgers 50A50. Ran a resistance test on the thermo and checked out. I also tried to jump the blower fan straight from the thermo by connecting red and white no fan. Had power to the transformer to controller but no power out of controller. What would cause it to blow?

REP 05-26-2012 01:36 AM

One thing is by jumping the red and white your furnace should have come on unless you are using your own coloring system.

JND 05-26-2012 09:09 AM

Just the standard colors. Red and white - furnace & green for the AC. I am a little confused about the blower running if it comes after the controller electrically. Essentially I would be sending 24v back to a dead board. Looking at it, the blower would get a signal from the board even though it's high voltage it is still being monitored by the board correct? Still, I don't know what caused the system shutdown. I did those test just to occupy the simple ideas. Also I do not see any direct shorts in the system. Another thing that bothers me is that there are not any fuses for the low voltage after the xformer.... Logic is that high voltage needs to be broken from inline before reaching low voltage, xformer or not. Redundancy seems lacking. Where would a fuse be? Checked the wiring harness for an inline fuse and also checked in the moler plug for one but nothing. AirTron put this dude in 20 yrs ago but fuses have been around far longer than that? Any ideas would likely be entertained before I go replacing parts that are bound to blow again without finding cause.

REP 05-26-2012 03:53 PM

You have a very standard furnace and a/c unit.You should have 5 wires from the furnace to the stat and two wires from the furnace to the outdoor unit.
On a call for cooling the stat will send power to a relay to send power to the outdoor unit which should power the contactor which allows line voltage to power the outdoor fan and compressor and at the same time run the furnace fan motor.
There is a wiring diagram on the furnace blower door panel.Once you take the front cover off the furnace you must tape the panel switch down in order to use a meter to check each item that should get power.

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