First and foremost, I am fairly versed in wiring just about anything that can be imagined. That said, I am curious as to whether or not anyone has ever tried to wire an external thermostat, with probe, to the auxillary fan? If so, where did you get the thermostat from? At what temperature did you program it to come on?

Some great ones on ebay

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

The type with a probe stuffed between the fins of the radiator are worthless. Those are just repurposed hot water heater components not designed for an automotive application. A better solution. Is to install an automotive temperature switch in one of the spare ports in the head. It works reliably every time at the exact same set point, unlike the hot water heater device.

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CENSORED due to not family friendly words

617A electric fan - your setup

All for $1 or there about.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

If you have functioning AC, don't give up the refrigerant trigger for the aux fan.

Sixto
MB-less

If I were going to do this, I'd put in a 30A PWM controller like this one:

30A Max 40A 50V 1500W Motor Speed Controller PWM HHO RC Controller Black | eBay

reverse engineer to a certain extant to remove the pot and replace it with circuitry that could take a voltage signal from the analog pin of an Arduino. Put an extra temperature probe in the head and there you go -- full variable speed control of the fan to keep the temperature in a set range.

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RenaissanceMan Labs: where the future is being made today.

Garage:

2017 Chevy Colorado Diesel (nanny state emissions)
2005 Volvo S40 T5 AWD, 77k
1987 Mercedes-Benz 300D turbodiesel, 4 sp auto, 156k - 28.7 mpg
1996 Tracker 4x4, 2 door, 16v, 3 sp auto. 113k - 28.6 mpg

WARNING: this post may contain dangerous free thinking.