Weird after-run fan behaivor

[Luuk813]

Hi all,

After replacing the headgasket on my 2.3E (NG) Audi Cabriolet I noticed that the 1speed / after-run fan speed was always running, even with the ignition off. The only way to make it stop was to disconnect the batteries. Since I had other priorities, I just let it run for a while, and disconnected the battery when I was done with the car...

After some time the fan stopped running, so I wanted to confirm that the issue was fixed. I heard somewhere that you can trigger the after-run fan by connecting the two wires of the thermal-sensor on the back of the valve cover together. When I tried this however nothing happend, so I bought another afterrun-relay to test with, thinking this was my problem.

When putting in the new relay, the fan instantly started running again, and didn't stop till the battery was disconneced, just like my first relay initially did. I asssume that it running for way longer then designed for broke the old relay, and the reason for the fan running all the time was not in the relay to begin with.

Now I'm wondering on how to troubleshoot this situation further. It seems like something is making the after-run fan run all the time, but it is not the relay. Is there anyone who knows how the thermo-switch on the top of the block/back of the valve cover functions? Should it 'close' when the air temperature surrounding it reaches a certain temp? And should there be a voltage going to it from the 2 connectors? I currenlty have non, and no resistance between the connector spades either...

Hope anyone is willing to help me with this one, since I wasn't able to find alot of information on the workings of this after-run system. Most of the info out there seems to be for turbo cars that have an water-based thermoswitch for the after-run fan & pump (which mine doesn't has at all).

[Tractor Dave]

Not sure how after run fan works on NG engine but if its similar to S2, there are two relays. The second relay is under the steering wheel and controls after run pump and fan. Have a look and see if you have something there. The S2 relay has a zendor diode which fails and the fan runs all the time when it does.

[Nuvo]

The three pin thermo switch is at the bottom of the radiator, this controls the 2 speed fan. This will most likely to have failed short circuit. Don't forgot to use a new sealing ring if you need to replace it.

[Luuk813]

The three pin thermo switch is at the bottom of the radiator, this controls the 2 speed fan. This will most likely to have failed short circuit. Don't forgot to use a new sealing ring if you need to replace it.

I failed to mention this, but I recently replaced this sensor (for a genuine BEHR one), so that shouldn't be it. Just to confirm, even with that sensor disconnected the fan still runs without ignition turned on. If I remove the after-run relay, the 2 speed fan comes on correctly once the engine gets up to temp, so I think we can remove this sensor from the equation.

[Luuk813]

Not sure how after run fan works on NG engine but if its similar to S2, there are two relays. The second relay is under the steering wheel and controls after run pump and fan. Have a look and see if you have something there. The S2 relay has a zendor diode which fails and the fan runs all the time when it does.

Didn't know there might be a second one, so thanks for the tip. Do you perhaps know if it's flipped on the LHD car's? As in, should it still be under the steering wheel, or under the glove box? Your reponse was most likely based on the RHD cars you have in the UK right?

[Tractor Dave]

Not sure. Mine is rhd.

[Luuk813]

I went to check for the relay panel and it is indeed under the steering wheel on a LHD car. I however don't have relay 17 (Relais für Nachtlaufpump / run on relay) or relay 18 (Relais für Küherlüfter / relay for radiator fan) in there. There's also no cabling running to relay spot 17 or 18, so it is clearly designed to go without it these. The relay you mention Tractor Dave only seems to be there when you have a afterrun pump like the S2. The after run setup on NG's seems to be a more basic version.

[Tractor Dave]

Ok. Now you need to go back to basics. You have replaced the thermo switch, you have replaced the relay so you mush have a wiring fault. Something was likely crushed or damaged when the head gasket was replaced. Just a matter of testing continuity on all the relevant terminals and checking for shorts.

[newsh]

You changed the head gasket, have you got some sensor connections plugged in wrong? I did something similar on my Ur Q years ago, swapped 2 sensor wires and got some very odd symptoms.

Also check for moisture and or tracking under the relays in the fusebox.

Pull the rubber boot back on the otter switch ( bottom of radiator) to check there's no bare wire shorting.