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Old 10-04-2001, 02:16 PM
Snow bum
Join Date: Jan 2001
Location: Here and there . . .
Posts: 238
Strange problem w/ aux. fan and climate control

While driving my 91 300D recently I noticed that the recirculate switch was illuminated although I had not pressed it. I pushed the rocker to turn it off, but it remained on. This was as I was pulling up the street to my house. When I pulled into my driveway I could hear my auxillary fans running.

The engine temperature was reading just above 80 C. The fans do not normally turn on until about 107 C, and the engine usually runs at about 90 C. I idled the engine for a while, playing with the climate control to see if it would have any effect on the fans. It did not. I finally shut the engine off, but the auxillary fans still remained on for about a minute. After they shut off I restarted the car and everything was back to normal.

Last night I took the car out for a little theraputic drive. It was chilly outside (about 52 F), so I was running the heater (economy mode). After driving about three miles the heat stopped coming out of the floor vents and cool air started coming out of the dash vents. The recirc switch was on (though I hadn't pressed it), and so were the aux. fans. Engine temp was just above 80 C.

I shut the car off and restarted, the problem persisted. I drove for another three or four miles, shut the car off and restarted, and the problem went away. It came back about two miles later.

Why are my aux fans coming on and why is my climate control randomly blowing cold air in recirc mode? I did a search but could not find anything. Any help will be much appreciated.

The MBs:
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K
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Old 10-04-2001, 05:59 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Sounds like the system is going into the fixed operational mode(FOM). Ashort or open circuit in any of the sesnsors is a likely problem. The most common problem is a shorted coolant temp sensor on the engine. IT is a BLUE 2 pin sensor! 008-542-45-17 WITH A LIST OF $30.
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Old 10-05-2001, 09:36 AM
Snow bum
Join Date: Jan 2001
Location: Here and there . . .
Posts: 238
The temperature gauge seems to be reading the correct temp. Does the computer use a different sensor than the gauge?
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Old 10-05-2001, 10:06 AM
Posts: n/a
Originally posted by Ben300SD
The temperature gauge seems to be reading the correct temp. Does the computer use a different sensor than the gauge?
The temp switch for the climate control/fans is not the one that sends a signal to the gauge.
Randy D.
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Old 10-05-2001, 11:41 AM
Snow bum
Join Date: Jan 2001
Location: Here and there . . .
Posts: 238
Thanks for the advice. The problem is intermittent, so I am going to carry a multimeter with me until it happens again and test the sensor. If it turns out to be the temp sensor then I will order it from PartsShop, which carries it for $16.
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