imac 820-3478A have 12v and all led off

[mon2]

On the bench it's 100k ohms, but on the board it's 0 ohms.

Please check on both sides of the 100k ohm resistor on the logic board. No power to the imac. Maybe you are measuring the 100k ohm resistor leg that is going to ground?

[okianeko]

both side are 0 ohm

[mon2]

Carefully flux and remove this part off the board. Then check the resistance to ground on the pcb pads only on the logic board ( no resistor ). The fault may be a solder blob across the pcb pads.