Radiator fan is on all the time and revs itself up

[Dake]

I recently changed the radiator, hoses, and coolant sensors on my car. Where I live it has been over 90 degrees F most days this summer. In the morning the fan will kick on as soon as I unlock the car before I even start it. It will do this little "rev up" thing before eventually kicking into full speed. Both fans run at this point. After driving the fan will stay on max speed for a good 15 or 20 minutes. It's the loudest thing in the neighborhood at night.

I used to haul 2 motorcycles behind my car and the fan never ran that hard.

It's almost like the car wants its coolant to be like 60 degrees or something. What are the possibilities here? Could I have gotten a bad sensor? All the wiring and pins look good and clean and there are no other signs of a bad ECU.

[JustinKing]

Simple fix, only one thing could be wrong, faulty FCM (FAN CONTROL MODULE)

Here's the part:

https://www.ebay.com/itm/174340787660

Part number should be 8E0959501AB

Verify number on ur fcm its located top right of radiator when looking down at engine. Little silver box with many wires going to it very common for these to go out guarantee you you fix that buy that part flop it in your fans will work absolutely perfect I just did it to my car

[V70R]

Which motor do you have? Which sensors did you replace? Access to a decent scanner that will pull VW data codes?

[shoemaker617]

before buying any expensive parts i would pull up the plenum cover and see if its full of water. If so you want to pull the engine control module cover up and make sure theirs no water inside ecm box

Sent from my SM-G973U using Tapatalk

[Gekko]

I think VCDS can show current temps from the sensors. If you can do that I would highly recommend checking them and see what it looks like. If you’re lucky it’s a shot sensor. If those look fine it’s probably what was already mentioned. ECU going for a swim from clogged drains or the fan module is shot.

[Dake]

Sorry guys I took a vacation there. I doubt the plenum would be full of water considering this happened at the end of a 600-mile drive in dry weather just this weekend but I will check the ECM box. I don't have access to a tester but maybe I can find someone around here who does. It started immediately after replacing that sensor which is either a huge coincidence or a bad sensor in my mind. My engine is a 1.8t.

Half the time the fan will just do little revs without kicking fully on.

I was camping this weekend and had to pull the battery negative because the fan was on full blast for over 15 minutes in a crowded campsite with no signs of stopping.

[V70R]

Did you by chance change the thermostat assembly including the sensor when replacing all these parts? I’ve seen a few Mahle and OE Audi tstats over the years where a failing thermoswitch causes the fans to engage just as you are describing, however the only way to tell is via VCDS or scanner that allows you to pull soft codes and access measuring blocks.

Regardless, let us know if you’ve replaced the thermostat assembly to help narrow things down for what’s going on with your B6

[Dake]

Bumping my own thread for a new question. Yes @V70R I did change the thermostat about a year ago with a new Mahle unit. I also replaced the coolant sensors at the front and rear. The car has been in dry storage for 3 months so we can eliminate the idea of water in the plenum / ECU. Plus I have only had my ECU cover off once and that was 10 years ago. This failure isn't consistent with a shorted fan relay.

I can start the car, run it for less than 2 minutes, turn it off, and the fan stays on for over 10 minutes. However it does eventually turn off. I'm starting to think I got a bad coolant sensor for the front because this problem began right after I changed the radiator and the lower front coolant sensor. I'll replace it again to see if that's the problem before I spend $$$ on an FCM. In the meantime I inspected my FCM. It has part number 8E0959501D. From topics I've read there is no way to inspect this thing for anything more than obvious damage without access to VCDS.

Question: I can't see a way to get in there and splice wires to replace the FCM without removing the entire front carrier. Is there an easier way to access it?