a vehicle wheel alignment sensor unit is configured with an absolute rotational position sensor to provide an absolute rotational position of a mounting shaft relative to the vehicle wheel alignment sensor unit enabling resumption of operation and recall of stored runout data subsequent to an operational disruption such as a system power loss . an internal power source maintains the integrity of the sensor memory for a definite span of time during momentary system power losses such as battery changes or during overnight shutdowns , permitting the mounting shaft runout compensation values to be maintained and utilized upon the restoration of system power .