Etwenty1 45 Report post Posted February 3, 2018 After some advice on correct temps and aux fan operation please, I recently got my 1999 540i back together after after new hg and heat exchanger, replaced wp, thermostat, flush, new BMW coolant, new temp sensors in bottom hose and in thermostat, have got the temp showing on the obc. I thought the aux fan was supposed to come on at low speed at 91deg then full speed at around 112, or be on if AC is on is that correct? Mine doesn't start until 108 but does come on if AC is on. Is this normal or do I have a fault? Normal running temp is around 108 right? Quote Share this post Link to post Share on other sites
Eagle 1662 Report post Posted February 3, 2018 Both my old ones would hit 110 in certain conditions. They do run hot for emissions sake. No idea about fan conditions but it doesn't seem abnormal to me. Quote Share this post Link to post Share on other sites
Allanw 1071 Report post Posted February 3, 2018 Pretty sure the fans are mostly independent of engine temps, more to do with radiator returns temps - once the ECU determines the return from the radiator is climbing too high, it cranks up the Fans. Certainly, the M54's with Map control thermostats do that. Quote Share this post Link to post Share on other sites
dirtydoogle 383 Report post Posted February 4, 2018 it's 91 degrees at the sensor on the bottom hose, 108 is about right on your ktemp Quote Share this post Link to post Share on other sites
Etwenty1 45 Report post Posted February 4, 2018 Cheers guys, So after bleeding the system over the weekend it's holding at 108 on the open Rd, rising to 110 in traffic. I still get no low speed action from the aux fan over 91 deg unless AC is running. Quote Share this post Link to post Share on other sites