Good news. I managed to trigger my auxiliary fans by holding down the auto and recirculate buttons simultaneously for 10 seconds. I can cancel the fans by holding the buttons down another ten seconds. This says most of my auxiliary fan hardware is working.

I am just wondering why when I drive around in hot bumper to bumper traffic the fans don’t come on. Maybe I’m not hitting some criteria like appropriate Freon pressure?

I was ready to Hotwire the fans and build my own controller with some mosfets and an arduino to handle PWM. My car has some kind of intermediate MY 1997 update where they went to a pulse width circuit instead of the resistor and blue/green relay system. Somewhere in my car, likely under the dash or in the false firewall there is a PWM speed controller for the fans. They went away from this system a couple of years later. The W210 had it too for a short time.

Well now I know I can just hit the buttons and manually start the fans when I’m in traffic so that’s cool. Literally cool.

I do wonder why my car doesn’t turn the fans on when my coolant temps go into the high 90s or when I activate the AC. Something may be broken in the CCU. Or maybe it is doing what it supposed to be doing and I don’t know how it is supposed to work.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles