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1991 190E 2.6 Auxiliary Fans don't run

My 1991 Mercedes Benz 190E 2.6 overheats when I sit in traffic. The auxiliary fans don’t turn on when the car heats up. The fans do work. I’ve seen info on the net about sensors, relays, resistors, and switches. Most of it is about the 2.3. How do I troubleshoot this? What are the important parts? Where are they and what do they look like?

First off, I’d replace the thermostat if you haven’t done so recently. I believe on this car, the fan switch is in the radiator, so if there’s poor flow between the radiator and the engine, you can still overheat even though the radiator itself never gets hot enough to turn the fans on. A weak water pump could have a similar effect by not circulating the coolant enough when the engine is at idle.

With all those questions, you need the repair manual! Seriously!
The MB is different enough from other cars in the control, and powering, of the auxiliary fans, that you definitely need the wiring diagram and the repair manual. Yes, I did look at the wiring diagram. That’s not enough. One needs some text, from the repair manual, to explain some things.
If you don’t have the manual to explain how the the engine cooling system works, how are you going to make it work correctly?