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Old 06-19-2004, 10:52 PM
Registered User
Join Date: May 1999
Location: Oklahoma City, OK, USA
Posts: 28
Aux fan cycles on/off

My 1991 190E 2.3 aux fan has recently started cycling on and off pretty frequently, like every couple of seconds. It doesn't always do it, but will once the temperature has gotten to aobut 80C. It was operating fine, coming on at 100 C like it should, but then started operating erratically. It does it without the A/C operating, so I don't think it's the high pressure switch. I measured the resistance of the coolant temp sensor, and it was about correct. Any suggestions as to the cuplprit? It seems as though it should be the coolant temp sensor, but it seems to check out. Thanks for your suggestions!

Ted Beer
'91 190E 2.3
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Old 06-20-2004, 10:40 AM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
Check the CT sensor [ high speed fan] connection...clean the
If the ECU sees an open connection there , it defaults into high fan circuit.
If connection is good, You could have an intermittent bad sensor...

Last edited by Arthur Dalton; 06-20-2004 at 11:06 AM.
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Old 06-22-2004, 10:57 PM
Registered User
Join Date: May 1999
Location: Oklahoma City, OK, USA
Posts: 28
A little more information on the aux fan cycling. I pulled the connector on the coolant sensor, and to my surprise, the fan continued cycling on and off. I'm sure I have the right 4-pin, but without being connected, I would have expected to have the fan not run at all, or to run all the time. I'm pretty confident that the sensor is fine. It has all the correct resistance values over a range of temperatures. I don't understand where the ecu is getting the signal to turn the fan on and off. I can hear the relay pulling in and out. Any other ideas?
Ted Beer
'91 190E 2.3
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Old 08-13-2004, 05:20 PM
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Join Date: Jun 2003
Location: Southern Cali
Posts: 81
Did you ever figure this out?
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Old 08-14-2004, 12:49 AM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
The ACC temp sensor that you should be looking for is 2 pole blue not the 4 pole black at the rear of the head.

'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi
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