Cooling fan system


Registered User
May 31, 2016
Reaction score
Just got my 8l back on the road, after a sudden coolant loss due to overheating, which also cracked the head, and the hotside on the k04.

When diagnosing it I tested and replaced the cooling fan.

Questions I have are:

Currently the cooling fan only runs if the engine is running. It appears to be switched correctly at 97 and 102 degrees, but if I turn the engine off, the fan stops. The turbo coolant pump does run though, with the engine off.

Is it correct that the cooling fan doesn't run after turning the engine off?

Is there a way of forcing the cooling fan to run, by pulling a fuse or anything?

I have a spare fan control relay box, but I'm unsure of how it is supposed to work.

Any knowledge or link would be Much appreciated. It appears the cooling system electrical side is little talked about online.

If the temps are around 90 degrees when you turn the engine off the fans won't run... they only run if temps are 96/97 ish when you turn the engine off... the fan switch does the basic control, the fan control unit is so the AC and ECU can switch fans in as required which is why they run at slow speed when the AC is on

Thanks, as always, Tufty.

Looks like I need to try a new radiator fan switch then. Temps were around 100 when I switched it off and fans stopped.

Any idea what supplies power to the fan control module after ignition is cut? Although if the coolant pump is running, there must be power?
Fans are switched by the fan switch in the rad... after run is controlled by the fan controller

Fan control is feed off the 30amp fuse on top of the battery IIRC but the fact the fans and after run works suggests thats fine...

Have seen a few fan switches go... fairly common

  • Like
Reactions: Tonka777
Awesome, thanks for the info. I'll have a pike around on the fan controller then and see what it looks like inside.

What's a normal operating temp?

I'm monitoring the 49c temp like a hawk. Hovering around 97 when driving, 100 when I stop at lights, but straight back to 97 ish with airflow.
Any idea what would make it run warmer? New stat etc fitted

Coolant sensor looked like it was hitting the inside of the flange when I refitted it. Could be an aftermarket sensor been fitted
aftermarket sensors could cause issues... would need to check value in VCDS for both the engine and instruments module to see if there is any variance...

Mine sits at 90 pretty much all day long when driving normally... rises a little if I press on a bit but not silly... most I tend to see after a bit of ragging is 96/97 but drops down after that

Does the 49c trick read from the instruments or the coolant sensor?

The way it's modulating around 96 degrees I'm suspecting the Vernat stat I put in might not be opening correctly.

Going to pick up new sensors and a new stat and flush the system next weekend.
Don't know which one tbh... never used it myself so never looked into it..

Sounds plausible that the stat is playing up but validation using VCDS is still worth while doing if you can

  • Like
Reactions: Tonka777
Vagcom is showing that once the car is up to temp, the 49c trick is reading 4 or 5 degrees higher than reported coolant temp.

I assume the 49c is reading from the dials, which is fed from one side of the coolant sensor, while the ecu reads from the other side of the coolant sensor.

Hopefully a new sensor will be more accurate
So new coolant temp sensor, radiator fan switch and genuine stat fitted. Still ran a bit warm. Messed around some more, and noticed that the bottom hose was hardly getting warm.

Then I read in the Bentley manual that the stat has to sit with the brace parts vertical.

Tested both the non genuine and genuine stat on the hob, and both opened and closed correctly, although the genuine stays open a little longer <88 degrees.

Fitted the stat in accordance with the manual and all is good again.

49c trick still reads 4 degrees higher than vagcom though!
I have an 82 degree stat in my car. The later BAM TT was fitted with these. I swapped mine to an 82 degree stat from 88 degree. Works fine.
  • Like
Reactions: Tonka777

Similar threads