C4 AEL Coolant Temp Display Malfunction (Solution)

Hello everyone,

'For some time now, my coolant temperature gauge in the cockpit has been showing increasingly lower readings. I haven't worried about it too much, as it's supposed to be somewhat inaccurate. The car is running well and continues to run well. It couldn't be better. I have fun driving it every day. The oil temperature gauge is working and providing plausible values.'

But now the water level indicator is no longer showing anything, and that's not how it's supposed to work.
Okay, what have I tried so far?
Reading OBD diagnostic data via the climate control system.
Channel 51: Engine Coolant Temperature (ECT) in Celsius.
Display: 50 (Engine warm)
Display: 50 (3-pin connector disconnected from the sensor).
Display: 50 (3-pin connector disconnected from the sensor, and the 2-pin connector above it as well).
Display: 50 (another sensor connected to the 3-pin connector, not new - should still work).

Grrr: Not very informative. Why does it always say the same thing? What's wrong here? Can anyone help?
Does '5 0' now mean 50°C or 5.0°C?

Best regards, Kai, from Hohenahr/Central Hesse.

Please read out the data using VCDS!

How do I obtain VCDS?

Quote:

How do I get VCDS?

This is what's available in the shop ---> http://shop.dieselschrauber.de/


Regarding the sensor: It should be the 3-pin connector located at the water inlet terminal block.

1 --> Activating the coolant temperature warning light (indicating overheating).
2 --> Mass
3 --> Controlling the coolant temperature indicator.

cold --> 560
hot --> 58
Is it about Ohms?

He might just lack muscle mass.

Which version do I need?

It is well known that the transmitters in the C4 model frequently fail. If you search for "Multifuzzi," you will find a wealth of information on this topic.
It was the same with mine; I installed a new one and everything was fine.
Best regards,

Quote:

Which version do I need there?

If you want to stay with VAG in the future, you will need KKL+CAN.

Specifically for your vehicle, KKL would likely be sufficient, but it doesn't really make sense.

Rainer (our administrator) will certainly be able to provide you with more expert advice.

It is well known that the feeders in the C4 often break, and by using the search term 'Multifuzzi,' you can find a lot of reading material about it.
It was the same with mine; I installed a new one and everything was fine.
Best regards

Okay, if they continue to fail every 18 years, I don't mind. I have two sensors on the flange above the thermostat valve cover. One is a 3-pin connector from below, and the other is a 2-pin connector from above. Which one is for what?

P.S. I couldn't find anything related to the keyword 'Multifuzzi' in this forum.

Quote:

P.S. Using the search term "Multifuzzi", I can't find anything here in the forum.

Because the part is called a "coolant temperature sender." You can also search for "coolant temperature sensor."

The fact that the temperature reading on the display doesn't change when the connector is unplugged suggests a faulty sensor. This is because a fixed replacement value is then used.

Best regards, Rainer.

This beautiful name is thanks to Audi .
Just try entering it into a search engine; you'll find numerous topics specifically related to the C4 series.

Since a sensor was already used as a replacement, doesn't that also increase the likelihood of a wiring defect along the way...? Or would the insufficient mass at this point explain the problem?

Hi,

update:
1.
Die '5 0' über das Klimabedienteil abgelesen waren richtig. It was 50°C.
Morgens 1 5 = 15°C und nach 20min Fahrt 8 1 = 81°C , passt also. Das misst ein anderer Sensor im Heizungsteil des Kühlsystems. Daher bringt hier das Abziehen des F76 Steckers keine Änderung. Hab ich gelernt. Der 2-pol Sensor über dem F76 ist nur für die Motorelektronik. Noch what I learned.

2. A 200 Ohm resistor is connected to terminal 'T' of the disconnected F76 connector, grounded. This should correspond to approximately 80-90°C on the display. -> No effect, nothing happens.

3. Voltage measured at terminal 'T' of the F76 connector (motor off, Kl-15 connected): 12.3V. This seems a bit high. -> However, there is no broken wire.

4. Resistance measured at terminal 'T' of the F76 relative to ground (at approximately 80°C): 186 ohms. --> Seems to be within the expected range.

5. I cleaned and tightened two copper screws, but it didn't make a difference.

6. Spannungsregler im Kombi kanns nicht sein, denn die Tankanzeige geht ja. Or maybe?

And now what?

Best regards, Kai.

Okay, vehicle with an electronic thermostat switch (F76) -- it's difficult to tell from a distance without complete data.

Connections: Thermostat switch.

+ - Power supply
C - Control of the coolant temperature warning light (indicating overheating).
R - Safety shutdown of the air conditioning system.
T - Control of coolant temperature indicator.

Quote:

And now?

Check the cables. A short circuit to the positive terminal is still a possibility. If the IO module is in the distribution board, continue searching there.

Hello,

Solution to the riddle:

The internal resistance was infinite. I disassembled the meter. It's broken. I looked inside. Nothing. It's a moving-coil meter. The pointer is moved by a bimetallic strip.
This is heated by a heating wire, which is powered by the current flowing through the F76. This constantan wire is welded to the two threaded posts and sealed with a blob of silicone.
Protected against corrosion by a coating. A weld joint has come loose.

After carefully cleaning away the adhesive residue, I reattached the constant wire with a generous blob of solder. It's quite large, so I believe it won't soften much from the thin wire when it gets hot. Perhaps the bare wire will corrode over time, as the insulation was slightly damaged by the soldering temperature. Before, the insulation extended all the way into the adhesive blob, which fortunately was transparent.

The threaded bolt accepted the solder well; it just needed to be hot enough initially. I then embedded the thin wire in the hot solder because the wire wouldn't wet with electronics-grade solder. The contact is good. Now the internal resistance is 51 ohms.

Tataaaaa... The procedure was a complete success. The ad hasn't displayed as well in the last 3 years.
Thank you to everyone who offered tips. It was a tricky thing to do. It's impossible to do without tweezers and at least one child at a summer camp .

I suspect that in many cases, the F76 component is being replaced frequently, even though it's causing damage to the AI system.

Best regards, Kai.

Thank you for the feedback . You did a great job.


PS: In future questions, please provide the relevant vehicle data first.