Picasso instantly running fan

Hi all,
Longtime lurker here but never needed to register as you've solved my problems ( up til now ) with your archive of previous posts.

My 2009 C3 Picasso VTR+ engine fan cuts in within 5 seconds of the vehicle starting, continues to run throughout the journey (long or short) then runs for 5-6mins after the engine is turned off.

Recently had a new water pump and pulleys, belts etc fitted, took it back and explained ive never had the fan run like this, mechanic insists system has been bled, and said the fan was working and stopping ok but it was running again when i got home. I know people will say take it back again but i know if i take it back im going to either be rude, angry or both. He's had his chance so wont be getting anymore business from me.

Fan does not run when i turn the ignition on, only when the car is started. The blue thermometer on the dash is on when i start the car so assume the engine temp sensor is ok, but something is telling the fan to run when the engine is running and then telling it to stop after a set period of time after the engine stops.

I know its probably something quite simple, but not sure what area to start,. I've made a tall header tank, filled it to above bonnet height and ran the engine til hot and didn't get any air bubbles out of the cooling system. The interior heater seems to work much better than before so i dont think theres air in the system.
Is something likely show up if i get an OBD reader to plug in to see whats telling the fan to run ?

Many thanks for any ideas. Adam

I have not seen a fault like this but I might be able to nut it out.
Has this only happened since taking it to the mechanic? Not before or after? So you might assume it's something he did?
The engine ECU runs the radiator fan. There are two parameters it monitors to do this.
One is the engine coolant temperature and the other is the air conditioning high pressure line.
I don't know if your car has air conditioning?
There are also a number of different types of fans. Some run by relays and some have PWM (variable speed) controllers.
I assume the instrument display is the later type that does not have a readout for the temperature display, just the over temp light?
I know that a faulty coolant temperature sensor will bring the fan on but you should get a temperature light on the instrument cluster at the same time.
Sorry but I'm not familiar with your car, so a blue "thermometer" means the coolant temp is OK? The engine ECU tells the instrument cluster what it "thinks" the coolant temperature is. So if the dash says the coolant temp is OK, in the very least the engine ECU think the temp is OK and the fan shouldn't run continuously.
If your over temp light does not come on I don't think it's a failure of the temp sensor and they tend to fail outright rather than give a false reading.
So, if your car has air conditioning, I might suspect that the pressure sensor is faulty or there is a problem with the connector or wiring.
I will disconnect the high pressure sensor on my car and see what happens.

Hmm. Nope, not the aircon high pressure transducer. Removing the connector does nothing. The engine ECU also runs the aircon compressor, so maybe it will ignore the pressure transducer until it gets a request to run the compressor???

However, removing the connector from the coolant temperature sender DOES replicate your fault exactly!!

So on the surface it looks like your coolant temp sender is faulty. BUT you are not getting an over temperature warning on the instrument cluster.!!!!

As I said, there is only one temperature sender that provides info to the engine ECU and then the engine ECU sends that info off to the the instrument panel. So I don't understand how the engine ECU "thinks" there is an over temp and runs the fan but doesn't pass it on to the instruments!!??

Check the green connector and wiring on the thermostat housing.

It's possible the temperature sender is reading "high". High enough to run the cooling fan but not high enough to bring the over temp light on?? It's a bit of a long bow to draw, but it's all that I've got. As I said, these things usually fail out right, not sort-of-fail.

Thanks so much for the detailed reply Ozvtr, and for unplugging the sensor on yours to test it.

I have never used the aircon on my car and suspect it would likely need recharging if i were to try, so i was hoping it may be due to lack of pressure in that line as you mentioned. Shame ! Would have been as simple as getting jt recharged.

Ill check the green plug at the weekend as you suggest and let you know if i find/change anything.

Again, many thanks

Ok, well the temperature sensor is green with a black plug ( green / red wires) seems ok, plug is on solidly and wires look ok. I if remove the retaining clip and pull out the green bit am i going to lose coolant ?

There a large box on the top corner of the rad ( hidden by the air intake box). It has a wire bundle from the loom coming into it on q plug then another plug with wires going lower to the fan. Is there something in this box like a sensor or relay ? I cant find any info.

Also, stupid question i know but where is the thermostat on these cars ? Ive looked it up on the Eurocarparts site and i can see anything that looks like it !

Hopefully i have attached a pic of the plug and box to show its location.



Many thanks to anyone reading.

nasamad wrote: ↑Sat Dec 04, 2021 11:18 am Ok, well the temperature sensor is green with a black plug ( green / red wires) seems ok, plug is on solidly and wires look ok. I if remove the retaining clip and pull out the green bit am i going to lose coolant ?

Yes, but if the engine is cool and the cap is on the expansion tank, not much should come out.

nasamad wrote: ↑Sat Dec 04, 2021 11:18 am There a large box on the top corner of the rad ( hidden by the air intake box). It has a wire bundle from the loom coming into it on q plug then another plug with wires going lower to the fan. Is there something in this box like a sensor or relay ? I cant find any info.

There are a number of different types of radiator fan assemblies. Some use electronics and some use relays. Yours uses relays. The big connector you have pulled out there is the power connector. There is a smaller connector next to it which controls the relay. You could try disconnecting the small connector and see if the fan does not run.

nasamad wrote: ↑Sat Dec 04, 2021 11:18 am Also, stupid question i know but where is the thermostat on these cars ? Ive looked it up on the Eurocarparts site and i can see anything that looks like it !

It's in that plastic housing where the temperature sensor is. The thermostat is part of the whole thing. It's not a separate part!