They should only come on when the temperature is very high (well above 80C). I suspect the sensor controlling them is not working properly. You might want to check here K6JRF Auto Page . Tells you how to lower engine temperature (not relevant) but also excellent info how the auxilary fans are controlled. Cheers

http://www.k6jrf.com/MB_CTS.html advise him that it visits these very interesting pages. http://www.k6jrf.com/MB_S500.html For the problem of high temperature the topic nº 18VERYSPECIALPLACE