water temp sensor... my temp guage is crazy
 

Hello, I have recently aquired an 1986 560sl with a seemingly faulty temp guage. When I start the car the temp climes rather quickly and consistently to the very top (120) of the red temp zone. When I check the coolant tank it is barely warm so I now the guage is wacko. I already changed the coolant temperature sensor in the front center of the engine but it did not help. Is this a common problem? What should I do next? Thanks, John.