Hello All,
Glad to join the forum. Purchased my first diesel and Mercedes a few weeks ago: 84 Turbo Diesel. Despite the fact that this car was operating perfectly I went ahead and ordered my Haynes manual a couple days ago. The car, not wanting my investment in the manual to go to waste, decided to throw me my first mechanical challenge today -- a few days before the arrival of said manual.

Symptoms:
Starts fine and operates at around 85 degrees while driving at any speed. Within 20 seconds of sitting at a stop light the temp gauge rises quickly. It gets to about 110 degrees and seems to stop around that area. If I place the car in neutral or park and give it about 1500 rpms the temperature goes right back down to the 85-90 degree range. I also have that condensation showing above the passenger-side vent below the windshield. No smell of anti-freeze or odd smell of any kind. I see no fluid leaks from hoses in the engine compartment and the engine areas don't feel particularly hot. When the temp rises the auxillary fan does NOT come on. In fact, I don't ever recall hearing it come on since I've had the car.

Suspects:
I've googled around and browsed MB forums and it seems I could possibly have 2 issues hitting me at once: heater core being responsible for the windshield condensation, and either bad aux fan or bad engine fan clutch.
If its a bad aux fan (ostensibly it should have come on when the temp reached 110?) then its curious that this problem only cropped up today after having driven it for 3 weeks. If its a bad engine fan clutch my understanding is the tell-tale sign involves a freely spinning fan when the vehicle is warm but turned off. My fan has some resistance when I spin it.

Questions:
-if the engine fan clutch is shot should or shouldn't it spin freely when warm and the engine off?

-if my temperature cools down when parked and putting my foot on the accelerator (not allowing a normal idle) would this further support the idea that the engine fan clutch is bad -- premise being it needs the increased rpm's to cool the radiator to the degree it used to be able to cool it when it was simply at idle?

-Can someone confirm that the auxillary fan should have come on when the temp got that high?

-When I "played" with Volvo 240s we shunned the Haynes manual in favor of the Bentley manual (which I thought was excellent). Is there a better manual for 300 Diesels?

Any other feedback and comments are surely appreciated. Apologize for being the parasite with such a long post. Oh, one other oddity this car has. There is a large hose leading from the engine block to the interior that has a valve installed. The previous owner said he had a mechanic install that to help control how much hot air to let into the interior of the car. So, I suspect it isn't quite a correct situation for the climate control system to be in, but don't know enough yet to say its a contributing factor...

Thanks in advance,
David