Radiator Fans Run After Car Is Off

[LT1FirebirdSLP]

2002 Audi B6 A4 Sedan AWD with the Automatic. About 266k miles on it.

In January, the temperature dropped to -16F one night with a windchill of -40F. The next morning, when I started the car, it started to overheat which was odd. Turned the car off and then gave it another 24 hours. The following day, it was quite a bit warmer, car came up to operating temperature and did not overheat but very strangely, when the car was done being driven and turned off (key out of ignition) the fans would run for 10-15 minutes and vary speeds (not racing constantly but would instead run very slow for about a minute, then race for about 10 seconds, then medium speed for 2 minutes, etc). This kills the battery every time.

I plugged in my VCDS and pulled codes with absolutely nothing remarkable showing for the cooling system, or engine management overall.

Given the ease of changing out the fan relay (located under the ECU under the hood) and how common the fans running after car is off issue is resolved with this part, I did that just to see if that was the simple fix. It was not. The fans still run after the car is off and the battery still dies.

I've done some more tests and here are some other observations that I have made:

1. If you pull fuse #9 (Radiator Fans) when the car is off and fans are still running, they CONTINUE to run.
2. If you pull fuse #29 (Engine Management) when the car is off and fans are still running, they STOP. Plugging the fuse back in does NOT cause the fans to run again until the car is started. The unfortunate side effect of this is that is causes the EPC light to come on.
3. If you disconnect the radiator fan power supply harness behind the driver's side headlight while the fans are running and car is off, the fans stop. Reconnecting this harness will cause the fans to start again.
4. If you disconnect the ECU (main harness going into the ECU) while the fans are running and car is off, the fans stop. Reconnecting it will not cause the fans to start again until the car is started.
5. If you pull the fan relay (under the ECU) while the fans are running/car is off, the fans stop. Reconnecting the relay will NOT cause the fans to run again until the car is started.

I checked a variety of measuring blocks via VCDS to see if something weird was going on with the ECU/Fans/Thermostat but both the Radiator Outlet Temperature Sensor and Coolant Temperature Sensor seem to report valid values through out a drive cycle, starting with a cold vehicle. Thermostat duty cycle seems to be reasonable based on the given engine temps. Air Conditioning on/off has no effect on any of the observed behaviors.

It's an old car, without a doubt, but I would really like to diagnose this and get it fixed. I'm never a fan of just firing the parts cannon at things so if anyone has any thoughts on what the issue might be I'd love to hear some input. I can drop some drive logs in the thread if that is helpful as well, just let me know what measuring blocks you want to see.

[EuroxS4]

Could be a bad FCM could be a cracked head also, could be water in the ebox.Take your pick

[LT1FirebirdSLP]

Could be a bad FCM could be a cracked head also, could be water in the ebox.Take your pick

It could be a bad FCM. I'd have to figure out how to check the existing one. Replacement FCMs run ~$400 so buying one just to check doesn't make sense.

Cracked head doesn't seem likely. I'm not down on power and there is no noticeable coolant consumption. Oil consumption is consistent with a vehicle this car's age. I could do a compression test but I'm not sure how that ties in with fans that run after the car is turned off.

Ebox? Guessing you mean water in the box that contains the electronics (i.e. ECU, relays, etc. under the hood). Yes there has been water intrusion problems in there in the past. I've gone through it exhaustively, cleaned contacts in the fan relay holder, hit everything with electrical contact cleaner, the works, with no change in this behavior. Everything is currently dry and sealed.

[Gin+]

Sounds like the FCM is functioning correctly to me, essentially you cut the signal by either pulling the relay or disconnecting the ECU and the fans stop.

I believe that relay (under ECU) should open when you cut the ignition. I'd venture down that route if it were me.

[VinnysS4]

Ebox? Guessing you mean water in the box that contains the electronics (i.e. ECU, relays, etc. under the hood). Yes there has been water intrusion problems in there in the past. I've gone through it exhaustively, cleaned contacts in the fan relay holder, hit everything with electrical contact cleaner, the works, with no change in this behavior. Everything is currently dry and sealed.

Sounds to me like this is more likely the problem. Water ingress into the ecu box is very common and as you have seen, will cause the fans to turn on. Just my .02c


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[codemode]

I’ve seen this already. It was a sticking relay. It looked okay, I even bench tested it with a load and it worked. When I opened it up, it was all corroded.

[LT1FirebirdSLP]

I’ve seen this already. It was a sticking relay. It looked okay, I even bench tested it with a load and it worked. When I opened it up, it was all corroded.

That's what my intuition was as well which is why I replaced it entirely with a new one. But unfortunately the behavior persists. The fact that the fan stops when the ECU is cut leads me to believe this is more ECU driven but I feel like that would be testable using VCDS. I'm still researching how to go about that.

[LT1FirebirdSLP]

Sounds like the FCM is functioning correctly to me, essentially you cut the signal by either pulling the relay or disconnecting the ECU and the fans stop.

I believe that relay (under ECU) should open when you cut the ignition. I'd venture down that route if it were me.

Yup, this is my suspicion as well. Will be checking this.

[EuroxS4]

Except when those FCM's fail they back feed voltage into the fans regardless of what the relay is doing. Please check FCM.

[LT1FirebirdSLP]

Except when those FCM's fail they back feed voltage into the fans regardless of what the relay is doing. Please check FCM.

Interesting. Ok, noted. I'll take a look at testing the FCM.

[JDCinATL]

I had a similar problem, FCM replacement was the answer, along with large fan replacement. I was having A/C issues, noticed big fan not running, small fan would spool up, the shut off when ECU saw improper R134 pressure from lack of airflow. FCM from Rockauto isn't too expensive, going on 2 summers of medium use now!