Just had the exact same problem on a '97 S500. Temp was very slow coming up and rarely got above 60 degrees centigrade. A new thermostat fixed it. Now it warms up faster and the gauge goes up to a hair width above 80 degrees centigrade and stays there. The climate control is much better as well, especially on days like today when it's 8 degrees Fahrenheit!

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire