10 ton trane RTU question

[oldrookie]

Try to make this ar short as I can. Service call: No power to unit. Observations: Thermostat blank. ( energy stealing/flash memory type) Breaker not tripped. jumped test terminals at unit. Everything running. Went back to t-stat swapped with known working one from another unit-did not work. Jumped R/G/Y1/Y2 @ t-stat base. Came on and ran. Voltage check: no common @ t-stat sub base. ??? Duct detector was totally dark. Wanted to condemn detector; but ran new detector wire to make sure. Bingo. Back in Kansas Toto. Now, for those who do not know; the detector leads do not break t-stat control wires externally. Remember this is a TRANE. ( I hod Cross as I type name) They connect to the TB on the RTU. Question: Did the unit intelligence take away the common or was this a product of the detector fault? Please do not advise me to read the schematic. After countless attempts; I have come to the conclusion that I am simply not bright enough to read a Trane schematic. Any help?

[too tall terry]

i would think it was in the fault of the detector myself,as LTB does hold a common,but what do i know?