Abstract: A radiator cap for closing the filler neck of a vehicle cooling system includes a numerical display assembly embedded in the top surface of the cap. A temperature probe extends from the bottom surface of the cap and is connected to the display assembly, the assembly being powered by a solar cell battery in the cap. Upon securing the cap to the filler neck the temperature probe senses the temperature of the associated cooling system, this information being delivered to the display assembly for conversion into numerical form. The cap enables a user to ascertain the cooling system temperature at the filler neck site.