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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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Old 08-30-2014, 01:10 PM
Lovemy540 Lovemy540 is offline
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Location: Olathe KS
Join Date: Mar 2013
Posts: 2
Mein Auto: 97 540i
Aux fan driving me crazy

Been having some sort of issue with the fan. Read hundreds of pages on the interwebs with none matching what is going on. I am under the impression that the aux fan should stay engaged when the a/c is switched on. Sometimes it runs for a bit and then shuts off. Then will restart. Run for about a minute and stop. And will repeat this over and over. It also kicks on in high mode sometimes with key in run position, with car not running. Usually after a day worth of driving. Next morning it will act normal. Like it never happened. I have checked all three speeds via shorting out connectors. All work. So fan is fine. I am replacing coolant temp sensor today as advised by my indy friend. Said it was a cheap shot and wouldn't hurt to replace anyway. Today the fan stopped while driving and a/c air from vents got hot, then it must've kicked back on and it seemed to run okay until I got home. That is when I saw it kicking on and off. Is this a symptom of bad resistors? A bad relay? Just my bad luck? Any help would be awesome. My wife is starting to use phrases like "we should just sell it" and "you shouldn't have got that car anyway." So I need to figure it out before something really expensive breaks. Again any help would be great...
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Old 08-30-2014, 01:53 PM
edjack edjack is offline
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Location: San Jose, CA
Join Date: May 2007
Posts: 9,091
Mein Auto: '97 540i 6 speed
You need to understand the aux fan control circuitry before you can troubleshoot it.

The fan is controlled by relays and temperature and pressure sensors, any of which may be intermittent. PM me with your email address, and I'll send you a description of the IHKA, including a description of the aux fan circuit.

Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.

Last edited by edjack; 08-30-2014 at 01:55 PM.
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Old 08-30-2014, 10:45 PM
Lovemy540 Lovemy540 is offline
Registered User
Location: Olathe KS
Join Date: Mar 2013
Posts: 2
Mein Auto: 97 540i
Looked at the info you sent me. Still a little confused to be honest. As all the parts that signal the fan to activate seem to be working as far as I can tell. Which doesn't say much for me. The fan just stops and starts with the a/c light on. The most confusing part is the fan running on high with the key in the run position. Which is also intermittent. Its what made me think it might be the temp sensor, since the coolant temp doesnt come down immediately and the fan will run on high til cold, unless i turn key off, in which case it stops. I do appreciate the material on the systems. Just slightly confused by it.
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