Through arc seam tracking for robotic arc welding is used by many robot manufacturers and automated arc welding system builders to compensate for weld joint location variance relative to the taught path of the robot manipulator. The method typically requires the robot manipulator to weave the weld torch assembly across the weld joint and to subsequently monitor the variation in welding arc current as a function of torch position. New weld power supply designs incorporating various proprietary control mechanisms require changes in the feedback variable selection traditionally used for through the arc sensing. Understanding and quantifying the control techniques used by the subject weld power source is required in order to provide robust seam tracking. Experiments were conducted to assess the impact of weld power source control technique on feedback values of welding current and voltage relevant to seam tracking. Several pulsed output power sources with different control philosophies were evaluated and the resulting data is presented. The results of these experiments are presented along with conclusions regarding the optimum choice of feedback variable for through arc seam tracking.