Om642 turbo actuator signal fault

[boxer 01]

Hi I've gotta admit I've also been posting on the other mb forum about my fault. The story is for the last few days my w211 has been going into limp mode with a 2510-1 boost pressure regulator positioner signals fault. I have had a foxwell scanner plugged in and the actuator after a few minutes just goes to 27% open and stays there till the engine is turned off then it resets and is ok for a few minutes before going into limp mode again. I thought it was the actuator so it was sent away to be refurbished and on refitting the same fault is still present, I have checked ground and live on actuator plug and they are ok but I don't know where the signal wire comes from and I could do with knowing if there is some other component which might be affecting the signal to actuator. Any help would be appreciated.