What controls the second cooling fan on a 2000 1.8T A4?

[Quattro Krant]

Long story short, car is starting to overheat in stop and go as the second cooling fan isn't kicking on. If I turn on the heat or AC the fan kicks on but it will not come on to prevent the car from overheating. Some places tell me it is a "old style" that screws into the radiator, other places say it is that sensor in the lower radiator hose that you can sort of see standing over the car. I looked and looked and I can not find a screw in type that goes in the radiator, but, I didn't have the car on ramps or get under it, I just looked from the top. Does anyone know? Thanks for any input.

[mike-2ptzero]

Save yourself a head ache and buy a EFK to replace the stock belt driven fan.

[ModifiedA4]

two things cause it to kick on.

1) the A/C pressure switch

2) the coolant thermoswitch on the lower radiator hose.

i have a ton of experience with this circuit so ask away :)

[Quattro Krant]

Thanks for the offer. Saturday I drained the radiator and put some flush in it, drove it around and then drained it. I was really suprised by how much stuff came out, alot of rust/scale. So I filled it up again with more flush and drove it around again, doing about 2 hours of stop and go and it didn't overheat I had the temp in the car set to 76 and the fan on just 1 bar. My GF said that the car would have overheated on her a LONG time ago if she drove it that way so I'm assuming that solved the problem. I still have to drain the flush and put in the new coolant, but I want the water that I drain to look like bottled water before I put in the new coolant.

Now, a few questions for you. The fan that is on the pulley, does that fan ever actually "engage?" It just seems like it is loafing along and not really drawing alot of air through the radiator. Does the fan on the passangers side of the car ever kick on due to temps rising or does it only come on when you turn on the AC/heat in the car?

Thanks again.

[Quattro Krant]

Looks like all she needed was a good flushing. So happy that she just needed a little time and $25 worth of flush and coolant.

[Poopie]

Thanks for the offer. Saturday I drained the radiator and put some flush in it, drove it around and then drained it. I was really suprised by how much stuff came out, alot of rust/scale. So I filled it up again with more flush and drove it around again, doing about 2 hours of stop and go and it didn't overheat I had the temp in the car set to 76 and the fan on just 1 bar. My GF said that the car would have overheated on her a LONG time ago if she drove it that way so I'm assuming that solved the problem. I still have to drain the flush and put in the new coolant, but I want the water that I drain to look like bottled water before I put in the new coolant.

Now, a few questions for you. The fan that is on the pulley, does that fan ever actually "engage?" It just seems like it is loafing along and not really drawing alot of air through the radiator. Does the fan on the passangers side of the car ever kick on due to temps rising or does it only come on when you turn on the AC/heat in the car?

Thanks again.

this is the clutch fan. the internal clutch components engage when it reaches a certain temperature. It goes bad over time.

[jstryz]

Hello
dont know how old this thread is, but I will appreciate if you could share some of your expertise with me. I am kind of having the same problem with my MY2000 A4 1.8 LT
seems the aux fan is never turning on. If I drive without stopping the temp goes normal if I stop the air coming from the A/C stops cooling and eventually theengine temp start over heating normally is halfway to the scale it can move up to one line more. I went to a mechanic (VW actually) and told me the fan was damaged replaced the fan and still no luck seeing it turning on. now I am told it is the fan switch, now I purchased the fan switch but I do not seem to find where it is located, I got a screw tip sensor and I was told it is located by the radiator or lower hose, I can see the sensor(switch?) on the lower hose but it does not looks like the one I purchased. where should I start looking? are there two sensors or I just got the incorrect one? you said two things turn on the fan the A/C pressure switch and the thermoswitch on the radiator hose, this fan never turns on so now I am not sure it is the switch itself (unless the circuitry somehow runs but that switch all the time) can I just unplug the thermoswitch on the lower hose and shortcircuit the terminals to see if the fan turns on? a third party says that the over heating is not related to the switch and that it might be the fan clutchon the main fan. how can I check if the fan clutch is ok? Again your help will be greatly appreciated
Thanks

[walky_talky20]

I have an '01 A4 1.8T and had a hard time with thermal fan switch as well. Everybody told me it was the type that screws into the radiator, but it wasn't. The switch is in the lower radiator hose and has a 4 pin "D" shaped connector.

If your fan switch is in the lower radiator hose, this is the switch you need:
http://www.ecstuning.com/Audi-B5_S4-...itch/ES262920/

I had much better luck sourcing it when I searched for B5 S4 parts, as most B5 A4's use the screw-in type.

My problem was a bit different - turns out this blue fans switch is the same size and electrical connector as a coolant temp sensor (green) and the previous owner had switched it on me - the fan was running all the time. Here is my thread, it might help you out:
http://www.audizine.com/forum/showthread.php?t=234149

[prodrive]

If you convert your cluch fan with a EFK ,you can have a better control on temparatures ..
And ,you can have a switch to control AC fan ,when ever you want for extra cooling when needed.

This may help for you to understand the AC fan system. ;

[jstryz]

Thank you very much for writing. Yes. the switch is located on the hose itself not on the radiator I do not understand why all selling sites refers to that switch, the switch you refer looks more like the one I need have. Went through all your thread as well I do have a multimeter I think I can unplug the connector and test it. Thanks again for writing I think for 20 bucks I will order it replace it.
on other things how can I convert my clutch fan with EFK I am not familair with that.

[NUISANCE]

You can try this-

http://www.modifieda4.com/web/tech/fan-mod.html

[jstryz]

Hello again.
OK I bought the switch that was recomended ago, which in fact it is the one my car uses (8D0959481B) installed it but saw no diference, the fan is still not turning ON, and eventually the engine starts overheating, I checked the fuses and are OK, I removed the lower cover on the dashboard and tested the relay (only found one 373 I believe) and worked OK. Do you have a schematic diagram by chance? what else should I check?
I took my multimeter unpluged the switch and checked Volts between all four terminals on the connector and saw no voltage between either of them I was expecting to see 12V between some of them but since I have no schematics I can not move anymore.

help...

Thanks

[walky_talky20]

Hello again.
Do you have a schematic diagram by chance? what else should I check?

The diagram in one of the posts above is what you are looking for. It shows the dual thermal switch that you've replaced (F54/F18). You can test the wiring and relays by shorting out a few pins in the thermal switch connector. I don't know the exact pins in the 4 pin connector, but you can go by wire colors. Brown is ground. If you jumper the pin with the brown wire to one of the other pins you should energize the low speed fan relay(J26) and the fan should spin at low speed. Shorting the brown wire to the other pin should give you high speed through the high speed fan relay(J101). The 4th pin is unused on the 1.8T, I believe, and should have no effect. I did this test on mine and jumpering the different pins allowed me to switch the fan on the 2 different speeds.

First check the fuses shown in the diagram - fuses 42 and 51 I believe. If you still get no fan movement upon jumpering the connector, you should check to see that you are getting a good ground on the brown wire to the thermal switch connector. Then check to see that you are getting power and ground to the fan relays. Then test your relays and wiring from relays to the fan motor.

[jstryz]

Hello
It is working now!
I had pictures blocked at work so I went home and saw the diagram posted long time ago, well it turns out it was rust on both relays, for some strange reason the legs and inside has rust and was causing false contacts, I actually had hard time one of them as the wiring from the female connector went out as I pulled the relay. I replaced both of them.
Thank you very much for helping out!

[JK35]

Where is the fan relay physically located? I'm finishing the AEM install and would like to use "fan #1" trigger off the standalone, to trigger the existing relay until I decide what to do for a cooling system (which will DEFINITELY include the removal of the POS clutch fan spinning off my motor... but in the meantime, I DO need the electric fan. this seems overly complicated. I had expected to find a temp sensor which triggered a relay and turned the little fan off and on as needed. this looks like someone tried to reinvent the wheel (schematic is some help though ;) )


I am also looking for the fuel pump relay, as I want to control the in tank pump off of it rather than wiring in additional relays (wiring)

anyone have a diagram of the relay bank that is under the drivers knee pad? I would love to know what each relay is for without having to sniff out each and every one manually.


thanks

[JK35]

how about this... can anyone tell me where to find a diagram which shows what each of these relays are for?

[shownomercy]

Deleted a/c unit what can i do to get the fan working again?

[walky_talky20]

1: What do you mean by A/C unit? Be specific.
2: The fan is operated by the temp switch, 2 cooling fan relays, and a resistor on the driver side frame rail. The A/C system can over-ride and force low or high speed at any time, but the fan is not disabled without the A/C system. It would merely run off just the temp switch at that point.