Got a bit of a doozie of a problem.
This started out intermittent..Sometimes from a cold start the fan would immediately go to 100% speed. And oh boy let me tell you, that fan is LOUD. And it seems hard on the car. I actually raised my idle in VCDS to help somewhat in the middle of troubleshooting this because 850 watts is no joke, you can tell its a heavy load.
When you would shut off the car the fan of course normally stays on just for a couple of minutes... which is normal. But in this scenario, it still does run at full speed with the car off. So after this happened a couple of times, I would actually disconnect the fan after shutting the car off. Just to preserve some battery + the lower voltage in this situation prob isn't good.. I'd hate for something to burn up.
I had checked VCDS a few times and kept seeing electrical malfunction on the fan control circuit on the engine module. What I did not know is -- me disconnecting the fan would generate this code. After my testing today I realized it would not generate this code until I unplugged the fan, which in hindsight makes sense.
Knowing this is an uncommon issue -- I got myself a junkyard fan assembly. Kept it on hand waiting for this problem to re-appear.
This problem ended up stopping, and actually went away for a couple of months. This week though it came back and it's done it every time I've started the car. So my commute to work has been: go out to car, start it, open hood, connect fan (which then goes full speed). Get to work, open hood, disconnect fan, then turn off car. This is getting pretty old quickly, so tonight I replaced the fan assembly. Not too hard -- other than the bolts to the left (driver's side) intercooler pipe. But got through it. 2 hours later I go for a drive just to make sure everything is working annnnnd fan is running full speed
So I guess where do I go from here? Car is outside of warranty I believe (134k miles), although the dieselgate stuff was done on it in 2019. But I'm pretty sure this wouldn't even fall under emissions at all. I don't know if I trust the VW dealership here either. This is a problem that will probably take a diagnostics deep dive as I don't think just having a scan tool is going to help, since the car literally thinks nothing is wrong.
I have limited knowledge of CAN bus networks, but my current assumption is that the radiator fan perhaps lost connection to the engine control module? So it's just running at 100% speed to be "safe" because it can't read anything from the ECM. I just think it's weird that this scenario doesn't generate a code. Anyone knows if it's supposed to?
Attached are some screenshots from when I dug a bit more closely into VCDS tonight. I'm open to ideas if anyone has any!
No codes after plugging in fan (which is confirmed to be running full speed) and clearing codes, going for a 10 min drive:
![Image]()
Me confirming the car has the correct coding for the cooling fan. Which it seems to. It's the single fan that's regulated. (Not so regulated in my case LOL)
![Image]()
Couple of measuring blocks just to see..I think the ECM thinks the fan is running @ 10% speed? Which I promise you is not true. (need to add an extra 0 there, buddy!)
Also lol @ the difference in temp between radiator outlet and engine outlet. The thermostat may have closed though when I was on this screen. Might be why.
![Image]()
And finally I went to the Output tests.. I saw there was one for the fan control circuit. And there was the action of "off". I performed this action and NOTHING HAPPENED. So either A. this test is broken. or B. I may have confirmed my idea that the engine module cannot talk to the fan?
![Image]()
This started out intermittent..Sometimes from a cold start the fan would immediately go to 100% speed. And oh boy let me tell you, that fan is LOUD. And it seems hard on the car. I actually raised my idle in VCDS to help somewhat in the middle of troubleshooting this because 850 watts is no joke, you can tell its a heavy load.
When you would shut off the car the fan of course normally stays on just for a couple of minutes... which is normal. But in this scenario, it still does run at full speed with the car off. So after this happened a couple of times, I would actually disconnect the fan after shutting the car off. Just to preserve some battery + the lower voltage in this situation prob isn't good.. I'd hate for something to burn up.
I had checked VCDS a few times and kept seeing electrical malfunction on the fan control circuit on the engine module. What I did not know is -- me disconnecting the fan would generate this code. After my testing today I realized it would not generate this code until I unplugged the fan, which in hindsight makes sense.
Knowing this is an uncommon issue -- I got myself a junkyard fan assembly. Kept it on hand waiting for this problem to re-appear.
This problem ended up stopping, and actually went away for a couple of months. This week though it came back and it's done it every time I've started the car. So my commute to work has been: go out to car, start it, open hood, connect fan (which then goes full speed). Get to work, open hood, disconnect fan, then turn off car. This is getting pretty old quickly, so tonight I replaced the fan assembly. Not too hard -- other than the bolts to the left (driver's side) intercooler pipe. But got through it. 2 hours later I go for a drive just to make sure everything is working annnnnd fan is running full speed
So I guess where do I go from here? Car is outside of warranty I believe (134k miles), although the dieselgate stuff was done on it in 2019. But I'm pretty sure this wouldn't even fall under emissions at all. I don't know if I trust the VW dealership here either. This is a problem that will probably take a diagnostics deep dive as I don't think just having a scan tool is going to help, since the car literally thinks nothing is wrong.
I have limited knowledge of CAN bus networks, but my current assumption is that the radiator fan perhaps lost connection to the engine control module? So it's just running at 100% speed to be "safe" because it can't read anything from the ECM. I just think it's weird that this scenario doesn't generate a code. Anyone knows if it's supposed to?
Attached are some screenshots from when I dug a bit more closely into VCDS tonight. I'm open to ideas if anyone has any!
No codes after plugging in fan (which is confirmed to be running full speed) and clearing codes, going for a 10 min drive:
Me confirming the car has the correct coding for the cooling fan. Which it seems to. It's the single fan that's regulated. (Not so regulated in my case LOL)
Couple of measuring blocks just to see..I think the ECM thinks the fan is running @ 10% speed? Which I promise you is not true. (need to add an extra 0 there, buddy!)
Also lol @ the difference in temp between radiator outlet and engine outlet. The thermostat may have closed though when I was on this screen. Might be why.
And finally I went to the Output tests.. I saw there was one for the fan control circuit. And there was the action of "off". I performed this action and NOTHING HAPPENED. So either A. this test is broken. or B. I may have confirmed my idea that the engine module cannot talk to the fan?
