Unless MB uses a different knock sensor... knock sensors detect engine pings (not temp). If engine ping is detected, it will retard ignition timing to protect the engine. Take my W124 230E (though not equipped with any knock sensors) as eg, with ignition timing at 15 deg BTDC. When coolant temp remains below 100C (ambient 35C), there's no audible engine pings. Once coolant temp reaches 100C & above, engine ping steps in. When coolant temp reaches 110C to 120C, the engine ping is really very bad... even pings when car is stationary with D selected. My point is, although the knock sensor does not retard ignition timing by measuring temp directly, engine ping is related to engine temp (apart from other factors such as quality of fuel, intake air temp, compression ration, etc.) such that as temp goes up, so does engine pings (hot combustion). Essentially, when temp goes up, ignition timing is RETARDED, not advanced. If its the other way around, you'll be replacing your engine block, pistons, etc. in no time!
Originally posted by psfred
Visco clutch locks up before the aux fan runs in high speed. Don't short the engineering expertise of MB -- their cars are way ahead of everyone else. Not so true now as it used to be, but still true. Overheating is a maintenance issue, they DON'T over heat when in good condition. 120C is not overheated, and the timing is not retarded when hot, it is advanced to speed the engine up! Knock will retard the engine (use premium, the best you can find, in hot weather for this reason). If the temp gauge sits at the mark above 80C all the time in traffic, your radiator is crudded up or the visco clutch isn't working.