Audi 100 TDI Coolant Temp Gauge Replacement

Hello everyone,

I've been having this feeling for a while now that something isn't right with the temperature gauge on my Audi 100 C4 TDI.
In the morning, when I drive to university, the temperature gauge barely moves at all, even after driving 10 kilometers. Sometimes it will move a little bit, up to 50 degrees, and then drop back down again.
I then checked to see if the radiator was warm, but it's cold. The upper hose of the radiator is warm, but the lower one is COLD, and near the engine, it's lukewarm.
Just recently, while on the highway, even after driving for 30 kilometers, the display didn't change at all.

Are these signs that the thermostat is defective?
How bad is it if I drive like this? Will the engine overheat?

Sometimes the display works as well. In those cases, it settles at 90 degrees and everything is okay.

Thank you very much for your answers.



Best regards, Ismail.

Hi,

This is more likely to indicate that either the temperature sensor (the lower, larger sensor on the hose, which comes out of the cylinder head on the side) or the instrument itself is faulty.
It's highly unlikely that 'Amt Thermostat' is the correct term used in the description.

Depending on how you drive, this is perfectly normal. If something were wrong, both the temperature gauge AND the thermostat would be malfunctioning because the radiator would remain cold.

Hello,

Do you have climate control with automatic features, or a climate system with manual dials?
If you have a climate control system, you can display and compare the water temperature via channel 51. This value is controlled by sensor G110 in the heating circuit. Otherwise, you need to use VAGCOM to determine whether the problem lies with the sensor or the display.

Best regards,

Depending on the driving style, this is perfectly normal. If something were wrong, both the gauge AND the thermostat would have to be malfunctioning because the radiator remains cold.

He writes, however:
'Just recently, while on the highway, even after driving for 30 kilometers, the display didn't change at all.'

So, this definitely isn't normal and it must be either the sensor or the control unit.

The thermostat is likely working correctly as long as the radiator doesn't feel hot shortly after the engine has started cold.

P.S.:
The suggestion about using climate codes is not bad.
Unfortunately, there are vehicles (like mine) where the display in the climate control system stubbornly and completely linearly rises to 90 degrees after the engine starts and remains stuck there. This happens automatically and completely independently of the driving conditions. Therefore, it doesn't always provide an accurate reading.

Markus H. wrote:

He writes, however: "Just recently, while on the highway, even after driving for 30 kilometers, the display didn't change at all."

Sorry, I missed that.
The question is whether the thermostat was left open.

Ismail wrote:

I then checked to see if the radiator was warm, but it's cold. The upper hose of the radiator is warm, but the lower one is COLD, and near the engine it's lukewarm. Just recently, while on the highway, even after driving for 30 kilometers, the display didn't change at all. Are these signs that the thermostat is defective? How bad is it if I drive like this? Will the engine overheat?

To be on the safe side, especially on longer journeys, turn the heating up to full blast.

If the heater (with air conditioning off) is producing unusually (extremely) hot air despite a cold radiator, then the thermostat probably isn't opening anymore. However, even with a functional display, it should at least show 90° or more.

If the heating system only produces lukewarm water and not hot water, there should be no immediate risk to the engine.

Hello everyone,

I'm new to this forum and I'm experiencing the same problems.
For my Audi 100 C4 (AAT), manufactured in May 1994, like Ismail; At...
However, the heating system in my Audi is also not working.
It's only blowing cold air; this is a major problem right now, especially during the winter.
Furthermore, I've noticed that the temperature changes when driving uphill.
is increasing (approximately 5 km). After that, the display shows a temperature of around 80°C.
The heating is also blowing warm air. Then, when I checked the coolant level...
It was noticed that there is something in the coolant that looks like engine oil.
Perhaps someone can help me with this.

THANK YOU

Best regards,
Markus
Bitte gib mir den deutschen Text, den du übersetzt haben möchtest.
Audi 100 2.5 TDI AAT R5, manufactured in May 1994.

Hi,

In your case, a faulty thermostat seems plausible, although when my thermostat was defective, the heating system still functioned almost normally.

If you see black or dark streaks in the coolant that settle at the top, try wiping them away with a cloth. It is most likely oily residue containing soot.
Unfortunately, it means the head gasket is damaged.
The only hope is that your engine has an oil-water heat exchanger. If so, it could be that component – and it would be much cheaper to replace.

EDIT:
'In my experience, the AAT seems to be very sensitive to a faulty thermostat. I drove with it like that for about six months (due to disagreements with the workshop), and shortly after the repair, the head gasket failed. Thank you!'

Markus H. wrote:

Der AAT scheint meiner Erfahrung nach sehr empfindlich auf ein defektes Thermostat zu reagieren. Ich bin ein halbes Jahr lang so rumgefahren (wegen Streitereien mit der Werkstatt), und bald nach der Reparatur ging dann die ZKD kaputt. Thank you!

Hmm, in which direction was the thermostat faulty?
Did the engine get too hot, or did it stay too cold?

Der AAT scheint meiner Erfahrung nach sehr empfindlich auf ein defektes Thermostat zu reagieren. Ich bin ein halbes Jahr lang so rumgefahren (wegen Streitereien mit der Werkstatt), und bald nach der Reparatur ging dann die ZKD kaputt. Thank you!
Hmm, in which direction was the thermostat faulty?
Did the engine get too hot, or did it stay too cold?
Hello,
'It's not because of excessively cold coolant that a head gasket fails!'
Best regards,
Günther.

@Ulf:
The thermostat remained open in my car, which meant the coolant stayed cold.


Hello,
'It's not because of excessively cold coolant that a head gasket fails!'
Best regards, Günther
Not?
My assumption is that this will not result in a consistent temperature balance within the engine. Instead, the temperature will fluctuate significantly depending on the driving profile, as it will no longer be regulated.
And what about the different temperatures of the engine block and cylinder head, which puts extra stress on the cylinder head gasket?
Anyway, I drove quite a few trips of around 600 km with a faulty thermostat back then, and I believe that wasn't good for the head gasket.

If my assumptions are incorrect, please provide clarification...

Regarding the question of 'MRI,' the oil in the coolant can actually only be caused by the two reasons mentioned above, assuming we rule out a hairline crack.
What is the mileage?

Hello.

I now have 257,000 kilometers on the odometer. I've just replaced the thermostat, and now...
The heating system is working perfectly again.

Best regards.
Markus.