Welcome to the I-fixed-one-but-here's-another world of old Mercedes.

The first thing you should check is the radiator cap, probably best done simply by spending 14 bucks for a new one. Liquids boil at higher temperatures under pressure, so if your cap can't hold pressure, you will boil at 100.

Then I would make sure that your thermostat is working right. Again, most easily checked by replacing. Or you can throw it in a pan of water on the stove and see if it opens.

Be sure that the engine fan is turning at a good clip when the engine warms up.

Next is to make sure that you are getting ground from the temp sensor. It may not be working. You can do it easily by connecting a test lamp to a positive source, touch the probe end to the tip of the sensor with the engine hot, and see if the light comes on. Or you can use a multimeter on the ohms setting between the tip of the sensor and the housing. Hope I am not insulting your knowledge here.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe