S4 fans running on for long time?

[calvin]

Hi all,

Just done a long drive in my 98 S4, from Lowestoft to Horsham (155 miles) - faultless.

It was between -2 and 1 degree all the way, with snow and plenty of cold.

My thermostat is (I am sure) dodgy as I rarely see it hit 90 and sometimes not move at all, but I keep an eye on it and the oil temp comes up to around 80 afaik.

Anyway, turned off the car after a gentle last few miles and a minute or two to cool the oil in the turbos and the fans came on and ran for about 15-20 minutes...! (they have finally just gone off)

I havent driven the car for long distances for a while (due to other Siena related issues) but I dont recall it doing this - is my coolant sensor on its way out or what else causes the fans to run when the engine off? I did restart it and the water was showing 70 and the oil no more than 70 - any ideas?

ta!

 

[aragorn]

Sounds like the afterrun system, but as standard it doesnt kick in until 110c or something silly.

I fitted a lower rated switch to craigs which kicks it in at around 95-100c

Might be worth fitting a new one, or at least unplugging it and seeing if it still does it? The switch is further along the rear coolant gallery from the main CTS.

 

[calvin]

Cheers Aragorn - will have a looksee and p'raps get a friendly VAG'er to come and hook up VAGCOM and see what temp its reading / any history etc.

what am i looking for? a switch / sensor plugged into one of the coolant hoses?

 

[aragorn]

on the back of the inlet manifold, running between the two cylinder heads theres a black metal coolant gallery. On the driver side of this gallery, is the main coolant temperature sender, on the passenger side, is the afterrun sender.

Afterrun is designed to run then fans and an electric circulating pump, to move water round the engine after shutdown, to prevent any locallised heat build up. As standard it kicks in at 110c or something silly high. If yours is kicking in with the coolant at 70c theres something wrong with the sensor.

Next time it does it, pull the plug off the sender and see if it stops. If so i would imagine the sensor is dead.

 

[Matt82]

with mine, the aircon fan will spin after a drive. 5-10 minutes maybe. i thought it was just the after cooling thing.

the water temp gauge should get to 90* pretty sharpish and sit there resolutely.

if you change the stat, at least you can drive the car hard withuot feeling guilty

 

[aragorn]

Matt: does yours have the afterrun mod?

Tis quite common for people to change the standard silly temp sensor, for a much lower temp one from some VW. Just looked it up and apparently the stock item only kicks in at 112c (part number 357 919 369F). The common upgrade is a blue sensor from a vW transporter, (part number 021 919 369) which kicks in at 93c and shuts off at 88c.

With the stock one, i'd never heard it once. With the new lower switch, then it does fire up the fans after a decent run, but certainly not for 20mins, and the fact its running on for so long with already low coolant temperatures, indicates a fault to me.

 

[calvin]

Thanks Aragorn,

After a few more very short runs my fans are now running for ages after each switch off.

Found the blue sensor (main temp sensor) and unplugged that first just to check, nothing happened. (just wanted to be sure I had the right one!)

Found the green one buried under the expansion tank, and unplugged that - immediately stopped the fans. Plugged it back in after 5 mins and the fans didnt come back on, so am assuming a relay or something has stopped trying to check the sensor?

So, looks like my afterrun sensor is kaput - does changing it mean re-bleeding the system, or can I get away with it without. And (sorry to be daft here) is it easy to remove? I can see that the main CTS would come out by pulling out the retaining ring, but I cant see one on the afterrun sensor. Whats holding it in?

Thanks in advance!

 

[aragorn]

afterrun sensor should have an identical retaining ring.

pull the clip, remove the sensor and fit the new one.

Make sure you retrieve the oring and get a new oring with the new sensor.

Its worthwhile getting the part i mentioned above from the transporter.

On craigs S4 we didnt even lose a drop of water, just swapped it over and that was it.

 

[calvin]

Was having a clear up in the garage today and found a spare coolant sensor from my VW Transporter Camper (T3) and it looks just like the one I am yet to change on my S4 (work has been busy busy!!) to fix the problem outlined above - so have been frantically searching through here to find this thread..

The part code on this is 025 906 041A and its also marked "EH2 12V 125 max".

Obviously this is not the same as the part Aragorn recommended above to fix the problem, but it got me thinking about what the sensors are doing - does anyone know what the markings tell you? ie what temp does this one operate at?

I am going to get my faulty thermostat sorted, this sensor, the water pump checked and changed and a thorough look over the cooling system I think, to be sure I can rest easy that its all working as it should!

 

[aragorn]

025 906 041A isnt a switch, its a coolant temp sensor.

Ie does the same job as the 4 pin item on the offside of the S4 motor, but for a different engine/ECU.

 

[calvin]

ah, I see - so the part i need to swap out is actually a switch? - I assumed that it was also a sensor / sender unit that told the ECU to turn on the afterrun (ie fans and water pump).

Thanks anyway.

 

[calvin]

Having had some work done on this, I am now confuddled!

I had to have various other issues sorted on my car including thermostat and cambelt - all done now. However, attempts to fix my afterrun fan system have failed.

The specialist concerned assures me they replaced the afterrun switch/sensor (back of the block, rhs) but my fans still run on after the engine is turned off, even after a relatively short run.

If I disconnect the switch / sensor (!?) by pulling the wires, the fans stop.

They suspect the fan control unit, but at nearly £200 (Audi prices) I m reluctant to try it without being absolutely sure!

I have thought about Aragorn's suggestion of replacing the stock item with a lower rated one as above, but surely that will just turn the fans on at an even lower temperature?

Oh, by the way, the fans do go off, but after a good 5-10 minutes minimum, even in the cold.

How does this circuit work please, and what components are there in the circuit, so I can get my head round what is going on and what has failed?

 

[aragorn]

I'll take a look at the circuit diagram for you in a bit. I didnt realise there was a "fan ECU"!

It should be obvious if they have changed the switch, the new one will be shiney and clean...

My suggestion makes the afterrun system more effective, but yours is broken, theres a difference! Stock afterrun doesnt kick in until 112C, which is completely useless, and i very much doubt your engine is getting to that temperature at all, never mind after a short run. Swapping it out for the 92c sensor means it works over a much more sensible range.

 

[calvin]

Hi Aragorn,

Did you manage to look out the circuit diagram for the "Fan ECU" or whatever it is? (pretty please?)

I have had the car checked and they are sending me a new switch in case this one is faulty from new - I noticed tonight that the fans ran for a minute or two, then turned off, then started again...!

confused, I am!

 

[aragorn]

Sorry mate just looked up the diagram now, and found something interesting.

There are three themoswitches.

One is the switch you've replaced, and VAG call that the "Thermo-switch for continued coolant circulation"

There is also however two "Radiator fan thermo-switch" (well it says two, its actually one sensor with two switches inside)

The "ECU" is probably near the fans themselves, but it appears to also be connected up to the AC system, presumably as the AC uses the same fans.

I'd try and find and change this other sensor, its probably in the radiator or one of the rad hoses?

The only thing that doesnt make any sense, is why the fans would stop when you unplugged the afterrun switch....

 

[calvin]

Its fixed!

After all the work I had done by VAGTech, I still had this issue, and agreed with me that it could be a faulty sensor / switch still (the fans would go off if i unplugged the switch in the collant gallery at the rear of the block) - so they sent me out a new one.

5 mins work, whipped out the old switch, installed new with new O-ring, and to date, no fan run-on at all.

I investigated the upgraded VW Transporter one Aragorn mentioned, but at (ANOTHER!) £25 I thought I'd stay stock for now.

Thanks for your help, now onto the last remaining issues - my "clonk" (feels like the subframe is moving slightly!) and rattle (sounds like the heatsheild and in that area, but isnt), then hopefully I will have a nearly faultless car again (first time in years!)

Cheers all :icon_thumright:

 

[calvin]

By the way Aragorn, I think you are right and the Aircon uses the same fans as I was confused for a bit looking for a different set - they are big enough to do the jobs judging by the small dogs that get sucked up as they walk past!