rmmagow

If the temperature drops and stays lower with the heat turned on it is not going to be a thermostat or a temperature sensor/gage that is the problem. In all likelihood it is the main fan thermostatic control that is not working, and it is possible the auxiliary fan could be faulty. Debris plugged or bent over fins on the air flow side of the radiator can contribute to the viscous style clutches not working, as well as degrade the heat exchange capacity of the system.

The radiator might be having some difficulty passing fluid due to corrosion products building up also but the same water passes through the cabin heater cores and if they are not plugged I would not see a reason why only the water passages on the radiator would be plugged. The heat exchange capacity of the interior heater is pretty small compared to the radiator and if turning that on makes a difference it is a signal that the system heat exchange capacity is down significantly for some reason. The major controls on capacity are the thermostat, then the main fan, followed by the auxiliary fan assuming the radiator water flow passages are ok.

If the main fan is not turning fast enough, or the air passages of the radiator are plugged with dead bugs and bent over heat exchange fins between water channels, you cannot get rid of the heat, especially at idle or low rpm operation. At higher speeds if the heat exchanger fins in radiator airstream are clogged or bent over, you still cannot get enough air through, and on the viscous clutched models the clutch will not engage because it works on the temperature of the air coming over it from the radiator. Not enough heat exhanged and the air will not be hot enough as it flows over the fan hub with the viscous clutch parts.

So, I would do a careful inspection of the air flow side of the airconditioning condensor and the radiator first. If either is plugged up there will be heat exchange capacity losses that could be easily corrected. Next is the main fan clutch. Check its operation, along with the auxiliary fan operation.

Good luck and I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)