Right side I/O board preventing start up

Several weeks ago, I had to change the battery for a second time in my late 2012 i7 Macbook Pro Retina (A1425). After fitting the new battery and hitting the start button, the Magsafe connector lit green, then briefly orange before dimming and dying completely and the computer showed no sign of life. Measurement of the voltages on U700 showed that the chip had become damaged and effort to replace the chip resulted in a few of the board contacts lifting under the heat required to desolder. So, I bought a replacement second hand logic board. Upon start up, all appeared fine except that I had forgotten to couple the 6 wire plug connected to the right side I/O board to the logic board, hence no wifi device was detected. After connecting the plug, the computer would no longer start: no fans, no chime, black screen and no key combinations (SMC reset, PRAM reset, Shift, Option R, Command D, etc. made any difference. I have since discovered that the computer can be started if the right side I/O board is again disconnected and an SMC reset is performed followed by a hard reset (connect the Magsafe while the start button is pressed). I do not have the circuit diagram for the I/O board, so cannot check the voltages that should be present on the 6-wires that connect to the mother board. Some of the wires show continuity with each other, which may indicate that they are shorted. If anybody has an idea of what could have gone wrong with the I/O board or can give me the voltages that should be present on the 6 wires together with their continuities I would be most grateful for the help.