An off the car wheel unbalance measuring system has a rotationally driven wheel mounting shaft supported in a framework pedestal together with a pair of force transducers mounted in the pedestal and in mechanical communication with and spaced along the shaft. The angular position of the shaft is monitored with respect to an angular reference position at a predetermined number of angular increments during each full revolution of the shaft by a shaft encoder. The encoder provides a reference pulse once each shaft revolution and a plurality of angular shaft position increment pulses each shaft revolution. The angular increment pulses are in the form of a dual signal the components of which have a predetermined phase relationship. The shaft reference pulse and the angular increment pulse are required to provide a reference indication as well as a predetermined number of angular increment indications in each shaft revolution. The encoder outputs are also observed to determine that the dual signal phasing is correct and the reference indication is further used to ascertain that the shaft speed is within a predetermined speed range.
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