if the light beams from cells 34c and 34d are either both broken or unbroken no rotation will occur from the preceeding it will be appreciated that the locator unit 32 can rotate at the same time that the unit is moving up or down however instruction to the locator unit 32 to rise takes priority over an instruction for it to lower advantageously cells 34c and 34d are set further apart than cells 34a and 34b in order to facilitate rotation of the locator unit 32 prior to the units up or down movement thus producing more accurate motion as the locator unit encounters sharp 90 u00b0 profiles for example the steep rear windows in some automobiles the horizontal and vertical distances between the cells can be changed to achieve whatever tracking accuracy is desired and to produce the proper distance of the spray bar 24 from the surface being washed while other dimensions may be selected it has been found desirable to arrange the cells above the locator unit 32 so that cells 34a and 34b are from about 10 20 inches apart while cells 34c and 34d are from about 12 20 inches apart the upper and lower rows of cells i e 34a 34b and 34c 34d respectively are arranged so they are substantially parallel and generally from about 3 8 inches apart the closer the spacing the more accurate the tracking the use of a third photo electric cell centered between the top to cells 34a and 34b increases the accuracy of the unit but is unnecesary the third photo cell is more accurate for objects such as luggage racks or pointed bumpers or any more sharply pointed protusion from the vehicle fig4.