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Old 08-18-2001, 01:38 AM
JimSmith JimSmith is offline
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,596
MB Medic, Question About "Blue" Switch

MB Medic,

I have been following this thread with interest since I have a 1991 350SD that has been heating up with the A/C on and heavy load (driving up a long hill at highway speeds or lower) or sitting idle in traffic. At other times it runs at its normal 90 or so degrees C. I noticed only one fan was running when I lifted the hood while it was hot.

So, when I saw the comment here to disconnect the blue switch leads and the fan(s) would run in fast speed, I gave it a try and only one ran. The car was cold, and not running, I only had the key in the ignition turned all the way on. I gave the driver's side fan, which was not running, a tweak with my hand and it started. I stopped and started the fan (attached and unattached the plug on the blue switch) and it no longer needed any assistance to start. I have concluded the temperature switch is ok from this data, for the moment. I will let the car heat up and check the switch with a multimeter in the morning.

I will give the car a test drive tomorrow to, but I was wondering what the logic was for this circuit with a normally closed position for the temperature sensor switch. Is the relay energzed so when it fails the fans come on and stay on? I noted this was different than my 190E 2.3-16, which has a normally open switch in the temperature sensing position, and that car has "used" two of these sensors in 199,000 miles.

Thanks for the information. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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