Abstract: A carwash includes a single thru-beam or retroreflective photoelectric sensor that can determine the position of a vehicle's left side, right side, front end and back end. To accomplish this, the sensor is coupled to a spray arch that travels around the vehicle while spray washing the vehicle's exterior. In some embodiments, the sensor responds to a beam projected from an emitter that also travels with the spray arch, whereby the emitter, the sensor and the spray arch move in unison. The beam is preferably projected at an acute angle to a horizontal plane so that the sensor can detect the presence of the vehicle without the spray arch having to get too close. To avoid over twisting the sensor's electrical cable, the arch alternately rotates CW and CCW to limit cumulative rotation in any one direction. In some cases, the single sensor responds to beams from multiple emitters.