Today, I noticed a humming noise that seems to be coming from somewhere under my dash board. It sounds like maybe a motor or servo of some kind. When I turn the cockpit blower fan completely off, it goes away. Then, when I turn it back on (either in just fan or a/c mode) it takes a few minutes for it to reappear.

Any ideas?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Fan motor bearing my guest!

Aux. Water pump for the heater is another possibility.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.

Thanks. I recently replaced the blower fan and regulator, so I'll check out the aux heater pump. Does that operate without calling for any hot air? The problem occurred with the temp selector in full cold. The humming seems to be coming from under the dash, right above where the temp controller is positioned.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

my guess is the little aspirator motor right behind the glove box to the right.. I draws air across the temp sensor.. Its located right under the very right hand vent. Its easy to take apart and oil up.

Bill

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Bill

Backyard Shadetree Mechanic, 30 years running.

1987 300TD
1988 300E (sold, sniff)
1999 S320
1981 Volkswagen Rabbit Truck (Diesel)
2001 Ford Expedition
1988 Suzuki Samurai
On my list to buy: R129, for me, and a R107 for my wife.