Hi, first post here. Sorry if it’s been covered before. 
 

I have an Octavia II estate. 1.9tdi pd. 

 

Randomly today the radiator fan was on when I left work. Full speed so very noisy. As soon as I turn the ignition on it goes off. Temp gauge shows normal at 90 degrees. 
 

Wondering if there’s any common issues that come up? It’s been a very wet few days but the fuse box doesn’t seem too wet. 
 

I have seen somewhere something about the ambient air temperature sensor could be faulty making it stay on? Had a look the loom seems fine as does the sensor. Probably worth a change for a few pound. Otherwise am I looking at a relay replacement or fan change? 
 

I fix aircraft rather than cars for a living so any help would be appreciated. 

DPF regeneration? Was there a strange burning smell outside the car? Does it happen every time?

As far as I’m aware it isn’t a DPF engine as it’s 2010?  There’s no lights coming on suggesting dpf regen. 
I’ve had the ambient temp sensor off, cleaned, seems ok. 
Live data wise the coolant temp sensor seems good, slowly rises to operating temp. 
It goes on every time I turn the ignition off after a small delay and goes off as soon as I turn the ignition on. So I guess it’s going fail-safe or shorting when there’s not Ecu input or possibly a relay problem. 
 

Next step is changing the fan out I guess and seeing what happens. 

There is no light to show a regen is in progress, but you are right I don't think the 1.9 engines had DPF? 

It’s currently driving me mad. The only way I can isolate the fan is disconnecting the battery or pulling fuse F25 which is labelled as ABS pump. Although I’m sure the fuse box diagrams are vague as to what they power purposefully. 

Are you sure that's the ABS pump fuse? The fuse box diagram is for LHD cars so if your car is RHD it will be mirrored on the vertical axis, this has caused me confusion before

Is it the engine cooling fan or the aircon condensor fan(s)?

There’s one large cooling fan on the back of the radiator. I’m pretty sure it’s the only one on this car at least 

22 minutes ago, slow_nick said:

Are you sure that's the ABS pump fuse? The fuse box diagram is for LHD cars so if your car is RHD it will be mirrored on the vertical axis, this has caused me confusion before

I’m pretty sure, nearly caught me out before on the dashboard fuses when sorting an indicator fault. I do wonder why they don’t change the documentation when it’s for the British market haha. 

I'll have a look at some wiring diagrams later and see if I can figure out anything helpful. 

Are you sure it's not fuse 24 you're pulling? that one goes to the radiator fan control unit, which would seem logical.

That would be a great help thanks. 
 

Definitely 24 as it’s the big 40amp fuse. Doesn’t make any sense to me. 
If I leave the fuse out and run the engine I get the PAS light and the handbrake light flashes. 
 

What is confusing is that the fan will still run once the engine is on, even with the fuse pulled. Not sure if there’s a separate circuit for turning the fan on once the air-conditioning is running maybe? 

I guess you meant to write definitely 25?

What happens if you pull 24 (10amp)

Fan action can be triggered by coolant temperature, or refrigerant pressure, or both.

I did indeed mean 25. Typing on phones 😓 haha. Not too sure just yet. I will try as I pulled random fuses before to see what worked. 
 

Refrigerant as in a/c? I’ll check the pressure on there too then. 

There have been fan running issues on Briskoda before, regarding Fabia's. At least one of them was related (Or seemed to be) To wet weather and perhaps soaked plug sockets/electrics. I remember Mr Crow having an issue. Not sure if it got rectified but it may be worth a trawl. I would think Skoda may have used similar tech across the models and it was a similar age. Just a thought!

1 minute ago, Jordn76 said:

Refrigerant as in a/c?

Yep

It does seem strange that it only happened in the wet. Seemed fine before. I still need to get the time to get it up and under-tray off to have a look at the plugs to the fan. 
 

I’m sorting of hoping it is a case of contact-cleaner. Water displacing plug lubricant and good to go. 
 

I’ll check the air-conditioning pressure and pull F24 and see what happens. 

Check whether fuse 24 is actually blown too.

Fuse 24 is good, had a look at all the fuses and none are blown. Waiting for the fan to start up again now as it’s taking longer than normal. 

Currently, I can’t get the fault to replicate. It’s been a dry afternoon. Leading me to believe maybe water ingress into the fan plug or controller? 
 

I’m in South Somerset so a lot of country lanes and puddle splashes. 
 

I’ll leave the fuses in and see what happens as I’m sure it’ll be back when I don’t expect it. 

It might make sense if you find that the permanent live (fat red) connection is shorting - when wet - to the switched live (thinner blue/red) inside the fan connector.  That way, when engine is on, the fan module thinks everything is OK, but when engine is off, the fan control module will see a (false) switched live and think it should be getting instructions from the engine ECU, which it won't be, so it'll go into an emergency spin-up.

Sounds like that might be the ticket to me. Still waiting to get the car up to have a proper look under there but I’ll let you know what I find. 
 

You’ve been a great help so far. I’m a mechanical engineer by trade so wires and electricity have always been black magic to me.