Coolant Temperature Sensor AFN

Hi everyone,

Here's the problem I'm having with my Audi/VW Passat: After driving at a brisk pace for a few minutes, the temperature gauge on my instrument cluster seems to shift from its 'stuck' reading of 90° back to the far left – it seems like the needle prefers being there. I initially suspected the thermostat and have already replaced it, so that can be ruled out. Now I suspect the temperature sensor.

My questions about this are:
1) Does anyone have a part number and cost code readily available?
2) Where is the donor located?
3) Has anyone else experienced similar problems?
4) Besides the displayed reading, what other side effects can a falsely reported coolant temperature have?

hi!
I've just encountered the same problem...
'Have you checked your car's error memory yet? I don't want to give you any advice because I don't know the details myself, but my temperature gauge is also giving me problems after I converted from a 1Z to an AFN engine in my Golf. Have you found out anything new about what might be causing it?'

Hello,

I have the following vehicle: a Skoda Octavia from 1998, with a 1.9 TDI engine producing 90 horsepower. The engine code is AGR and it has 130,000 kilometers on the odometer.

'Occasionally,' the temperature gauge in my instrument cluster drops from its usual reading of 90° towards 'cold.' During more demanding driving or after a few minutes, the gauge returns to normal. This drop can go as low as about 25% of the maximum indicated value. It also happens on longer drives, where you would expect the engine to be properly warmed up. The error memory shows nothing, and the temperature sensors for the instrument cluster and the engine control unit show nearly identical readings (within about 3-5°). I haven't taken any steps yet to address this issue.

However, what's interesting is that this phenomenon only started after the water pump/coolant was replaced by a workshop (I had it done during the 120,000 km timing belt change). Furthermore, about 2 months after this work, I had to replace the coolant level sensor with its expansion tank. Both the sensor and the inside of the expansion tank were covered with a continuous layer... which is suspicious. The workshop might have made a mistake by using the wrong type of coolant.

My suspicion is that the coolant temperature sensor might also be coated with this layer, and that this could be causing the fluctuations in the display. However, what speaks against this is the phenomenon mentioned above, which seems to occur only sporadically.

I'm curious to see what the outcome of this action will be.

Regards,

Jens.

It could be the thermoswitch located inside on the driver's side near the radiator, or it might be the thermostat. Even a new thermostat can sometimes fail. There are also thermostats that open earlier, which results in a lower temperature.

Otherwise, the instrument cluster itself might be faulty; a common issue is a defective voltage regulator, which can usually be replaced relatively easily and inexpensively.

In my case, both the voltage regulator and the thermostat were broken, but also the warning light for the water level was constantly on.

'The thermal switch costs around 20-30 euros and is easy to replace. If water leaks out, you need to top it up. You should also change the fluid periodically. When I replaced the thermostat, I flushed the entire system.'

Due to excessively low temperatures, increased consumption may occur.

About zspeedster.

So, this AI no longer has a voltage stabilizer. Instead, the analog signal from the temperature sensor is converted into a digital signal by an A/D converter for use by the display instrument.


About Fireball.

If you have a VAG-COM diagnostic tool, monitor the temperature in the instrument cluster housing using your laptop. If the temperature remains constant, I suspect the problem lies with the instrument cluster itself.

I had a similar problem with my Golf IV, except that in my case, all the dashboard instruments only worked intermittently.

About zpeedster.

So, this AI no longer has a voltage stabilizer. Instead, the analog signal from the temperature sensor is converted into a digital signal by an A/D converter for use by the display instrument.


I'm sorry, I confused it with the AFN engine in the Golf 3 or Passat 35i.

Hello Octi98,

I'm experiencing the exact same problem with my Passat! I already replaced the thermostat, but the issue still persists!
What was the actual cause in your case?

THANK YOU!

Titas.






Hello,

I have the following vehicle: a Skoda Octavia from 1998, with a 1.9 TDI engine producing 90 horsepower. The engine code is AGR and it has 130,000 kilometers on the odometer.

'Occasionally,' the temperature gauge in my instrument cluster drops from its usual reading of 90° towards 'cold.' During more demanding driving or after a few minutes, the gauge returns to normal. This drop can go as low as about 25% of the maximum indicated value. It also happens on longer drives, where you would expect the engine to be properly warmed up. The error memory shows nothing, and the temperature sensors for the instrument cluster and the engine control unit show virtually the same readings (within about 3-5°). I haven't taken any steps yet to address this issue.

However, what's interesting is that this phenomenon only started occurring after the water pump/coolant was replaced by a workshop (I had it done during the 120,000 km timing belt change). Furthermore, about 2 months after this work, I had to replace the coolant level sensor with its expansion tank. Both the sensor and the inside of the expansion tank were covered with a continuous layer... which is suspicious. The workshop might have made a mistake by using the wrong type of coolant.

My suspicion is that the coolant temperature sensor might also be coated with this layer, and that this could be causing the fluctuations in the display. However, what speaks against this is the phenomenon mentioned above, which seems to occur only sporadically.

I'm curious to see what the outcome of this action will be.

Regards,

Jens.

Okay, the engine temperature tends to decrease in colder ambient temperatures and during steady driving (or even with the accelerator disengaged) due to its high efficiency. However, I believe it should still produce enough waste heat to keep the coolant circulating and maintain the engine temperature. The engine temperature should ideally drop slightly below the thermostat's closing point, but not much further. A possible exception would be if the accelerator is completely disengaged for several kilometers, with an outside temperature of -20°C and the heating system set to maximum.

"Mine actually dropped quite a bit back then because the gauge only started moving down from 90° when it reached below 70° (checked with VAG-COM). That clearly indicated a thermostat that wasn't closing completely. However, if the temperature remains at or above 80°, I would first replace the temperature sensor. It's usually a sensor with 4 connectors and dual functionality: one for the engine control unit and one for the instrument cluster. The sensor should cost less than EUR 20.00 and is relatively quick to replace."

This residue in the container might be present if the switch was made from G11 to G12, and not all of the G11 had been removed. However, usually G12+ or G30 is used, which is also red but mixable. I don't believe this residue is causing those temperature fluctuations; the sensor would also be affected by it, and a VAG-COM test didn't show any discrepancies between the engine control unit and the instrument cluster. Both should decrease at the same rate.

Hello community,

So, I'm now observing the exact same phenomenon in my Passat 3B AFN from 1999.
The temperature only reaches 90°C after approximately 7 kilometers on a country road, and then drops below 70°C when driving on the highway (at around 140 km/h). The temperature does not increase back to 90°C even at that speed.
The temperature only rises to 90°C within 1 minute and stabilizes when I drive slower than 100 km/h or come to a stop.

The first thing I did was replace the thermostat, but it didn't work.
Yes, I know that a diesel engine doesn't have enough energy left to "heat up" when driving without a load and at lower outside temperatures, but I don't expect to be driving without a load when traveling at a constant speed of 140 km/h, and that hasn't been the case in the last 5 years.

This topic has been open for quite some time now, but it seems there is still no solution.

A possible solution might be found in the power supply system. Could it be that the voltage regulation isn't working correctly at higher speeds?

CS

It's kind of strange. Someone swore that your thermostat isn't closing properly (I know, it's already been replaced). At high speeds, the radiator is cooling too much to maintain the desired temperature. That's precisely when the thermostat should be partially closed...

I don't think it's a power supply issue. I don't believe there's a manufacturing defect. What about high and low temperatures; are there significant differences?

I don't think it's a manufacturing defect.
The strange behavior was identical with both the new and old thermostats. And I don't think I can reproduce a fault that has developed over the vehicle's 200,000+ km mileage during installation.

The temperature fluctuations don't always occur; they happen sporadically.

Please provide the text you would like me to translate from German to English. I will then give you only the translation, without any explanations.

Hi,

Perhaps your thermostat isn't working correctly after all.

Besides a faulty thermostat, this can also be caused by foreign objects in the cooling system. If the thermostat tries to close but gets stuck due to a foreign object, it won't close completely, which leads to a further drop in temperature.

Drain the coolant, remove the thermostat, and thoroughly flush the system with a water hose while the engine is cold.

Regards,
Sure, here is the translation of the text from German to English:

"Micha"

Okay, this afternoon things were completely different. On the highway (about 20 km) at 140 km/h, the coolant temperature was fine. Then, on a country road, the coolant temperature suddenly dropped well below 70°C; it was a downhill stretch.
I then stopped and touched the coolant hose. It was never 90°C. So, the temperature gauge is accurate.

What surprises me is that nowadays everything in the engine is controlled electronically, but the good old mechanical thermostat is still state-of-the-art. Or is there something else in the cooling system that regulates temperature control?

Thank you for your help, CS.

"In my case, there is no additional control."

Why should you replace something that works well? Only if you have something better. I'm glad it's not an electronic component in this case.

Replace the part again and flush the system thoroughly, ideally with as few hoses connected as possible, and test each component individually and in both directions.

So, I think I found it.

Your advice about flushing everything is appreciated, but I couldn't understand how something could get stuck inside the thermostat during the drive, and if the engine cools down, the foreign object disappears?

I just replaced the temperature sensor, which my mechanic friend said is known to fail frequently. It costs €24 from VW and €18 as an aftermarket part.
So far, the temperature is consistently at 90° again.

CS

cs wrote:

Then I stopped and touched the coolant hose. It was never 90°C. So, the temperature gauge is correct.

Aha .

When the thermostat closes, the lower hose is always cooler because there's no circulation through the radiator, but the fluid in the radiator still gets cooled down.

It's likely that using a VAG-COM tool would have helped identify the cause of your problem more quickly.