Fan going constantly

[gas guzzler]

Have noticed the fan appears to be going round constantly, what causes this and will it cause a problem? Thought these viscous fans came on at set temperatures.

Andy

[Southernboy]

They are only supposed to work at high temp.... I think you need to check the viscous coupling mechanism...

[Del]

They do rotate constantly, but when the engine is cold the fan is just free-wheeling, at a lower speed than the engine, on just a bit of centrifugal force - at this stage one which is operating correctly can be stopped (care using hands!). The viscous coupling "locks" the fan at higher temperatures so that the fan runs with engine rpm. If the viscous coupling fails (as they do with age sometimes) they will either seize so that they are running at full speed 100% of the time (danger trying to test/stop one by hand!) or they free-wheel 100% of the time in which case a car might typically start to overheat in a summer traffic jam.

[Zed-dan]

I have this as an intermittent fault with my M54... sometimes it starts and the fan is as you say "free wheeling" and sometimes loud when idling and louder and louder with revs. even if car is cold/hot it is honestly random. help?

[Del]

I have seen a lot of talk on BMW internet forums that the "less expensive" viscous couplings, (as seen on ebay etc,) can be very noisy and making a sort of roaring noise. So yours may be just getting old and need replacement or a previous owner has fitted a replacement noisy one. I believe the original ones were made by Sachs. If you have noises coming from the front, it might be an idea to rule out a dodgy bearing. If you gain access to the front and remove the auxillary belt, you can spin each item in turn which has a bearing e.g, water pump, alternator, belt tensioner. They should feel perfectly smooth, quiet and have a certain amount of resistance so that when spun with the palm of the hand they do not turn more than say half a revolution. I've replaced my water pump, tensioner and alternator over the past few years either because the bearing felt slightly rough/grinding or in the case of the tensioner and alternator sounded a bit like a rattly roller skate and kept spinning