Fuel gauge not working - A4 Cabrio 2.5TDI (BDG)

Hi!

I am new here and apologize for any stupid-sounding questions, I have only just started using the VCDS module.
My Problem:
My fuel gauge fluctuates slightly, usually dropping a little when the tank is about half empty. This manifests itself in the way that when the ignition is turned on, the display initially shows the correct value plus approximately 1/5, before gradually decreasing to the correct value within a few minutes.
It all starts, as mentioned before, when the tank is approximately half empty; the display is accurate at that point.

I have already found the value for the fuel level sensor in the engine electronics - after about 10 minutes of the engine being on, it showed 189 Ohms and 19 remaining liters.

Now, my silly question: The Ohm's value will be the measured value of the emitter (I assume). But is the number of liters the one that was calculated accordingly, or the one that the AI displays?

The reason for the question is that, according to {SOURCE}. The workshop either has a faulty donor or an AI issue - I would like to find out which part needs to be replaced.

Okay, so, I just ran a log.
Unfortunately, it is only mildly informative, as I first had to find the correct MWB (I initially searched for the engine electronics before finding the KI).

Therefore, the effect is not so extreme, but I was wondering if that trend is sufficient on its own.

Attached is the log from the fuel sender and display. At the beginning and end, I was in the same, level position. Within the 8-9 minute driving time with very few kilometers (I estimate 3-4), the sender value increased by 5 Ohms (from 144 to 149), which caused the display to drop from 32L to 30L - note that this happened after the ignition had been on for several minutes and the effect was not fully realized.

Would that be sufficient for you to see Tankgeber as the culprit and replace them, or would AI still be an option for you?

The log abruptly stops, just unplug it?

Around 705s, the average of the transmitters changes in the log, previously it remained approximately the same. Was, is that car parked that way?

Since the average had remained roughly the same, nothing should change on the fuel gauge needle either.

Best regards, Rainer

According to my limited understanding, the fuel level sensor is essentially a mechanical device with a potentiometer (pot). If this is correct, and there is no further electronics installed, I would not know how it should reproducibly change its resistance after each ignition switch-on.

For me, the only option that really makes sense is AI. To test, you could connect a simple multimeter to the fuel level sensor and observe that the resistance will not change when the fuel level is at rest / without movement.

Greetings
Guste

Hello Rainer!

Thank you for the response!
It was my first log, so I may not have finished it correctly. I pressed the "Stop" button when I was finished, then closed the program and unplugged it.
Did I miss a step?

Exactly, the car was parked at the end in a double parking space, with the front facing downwards. Therefore, the average value changes.

I will try again this evening and hope that the values will be more distinct if I log the correct cutting value directly after ignition.

Quote:

This was my first log, so I may not have finished it correctly. I pressed the "Stop" button when I was finished, then closed the program and unplugged it. Did I miss a step?

No, that's okay.

The first log was, however, ended in an unclean manner, and another was appended to the same file:

Quote:

, 727.13, 17.0, 197, n.v., n.v.,,,,,,,,, ,727.26,17.0,197,n.v.,n.v.,,,,,,,,,, ,727.39,17.0,197,n.v.,n.v.,,,,,,,,,, ,727.51,1Wednesday, May 11, 2016, 10:46:12:63257-VCID:2D56B604E7E62756E5-5164,VCDS Version: RKS 15.7.4,Data Status: 20160226 8H0 920 930 F,,KOMBI+WAYFARE. RB4 D22, ,Block A:, '004,,,,Block B:, Not working,,,,Block C:, Not working ,,Fuel gauge,Fuel sensor,Fuel gauge,Fuel sensor,,,,,,,,,, ,Time,(Donor 1),(Donor 1),(Donor 2),(Donor 2),Time,,,,,Time,,,, Markierung,BRAND, l, ohms,,,BRAND,,,,,BRAND,,,, ,0.07,17.0,197,n.v.,n.v.,,,,,,,,,, ,0.21,17.0,197,n.v.,n.v.,,,,,,,,,, ,0.32,17.0,197,n.v.,n.v.,,,,,,,,,, ,0.45,17.0,197,n.v.,n.v.,,,,,,,,,,

Best regards, Rainer

Hello Guste,

m.W. is the sender essentially just a potentiometer with a float.
My thought is that the 'mechanism' of the lever has become somewhat stiff over the years due to deposits from diesel.
I can also see the value in Ohms with VCDS, and it's already there.

Is there a way to check the functionality of the fuel gauge in the instrument cluster using VCDS?

I'm guessing that the needle will move further to the right with more tension (because it's stuck at the far left when the ignition is off). If there's something that's difficult to move, why would the needle initially move too far to the right before slowly returning to the left? Or does it happen the other way around, where a high voltage causes the needle to swing to the right?
Or are you aware of any electrical or other errors in AI that could explain such behavior?

Ah, ok, thank you, Rainer.

I think I was a bit impatient at the first stop, wasn't sure if it was the right way, so I started and stopped again...

Possible, that it was difficult in certain positions, but what happened in the log starting at second 705? If the input would be constant at a certain point, then the output would also be constant.

As mentioned before, the average remains approximately the same.

From second 705, I drove into my double parking space.
Just before, I was standing there, briefly checking the value (which was also quite stable), then I drove into the double parking space (with the front facing down) and looked at the value there for a few more seconds.
Therefore, the average is different there.

Florian320 wrote:

My thought is that the 'mechanism' of the lever has become somewhat stiff over the years due to deposits from diesel.

You wrote at the beginning:

Florian320 wrote:

This manifests itself in the way that when the device is turned on, the display initially shows the correct value plus approximately 1/5, before gradually decreasing to the correct value within a few minutes.

I understand that this also happens even without any movement of the fuel level sensor (i.e., when the vehicle is stationary). If this is the case, I cannot explain it with the giver's lack of awareness.

Florian320 wrote:

I can also see the value in Ohms with VCDS, it's there.

No. You are seeing what the AI measures. If the AI is defective and no longer measures correctly, then this measurement is useless to you. Since Rainer has now checked your log and described the values as constant, the issue is already resolved.

Florian320 wrote:

Is it possible to check the functionality of the fuel gauge needle in the instrument cluster using VCDS?

There is a position test. In this test, the needles move from one end to the other. Whether a sluggishness is discernible depends on the type of error (generally sluggish or point-specific).

Florian320 wrote:

Why would the needle then initially swing too far, only to then slowly return to the left?

I am unsure whether this is actually a simple stepper motor display, or rather a stepper motor...

Hello Guste, thank you for the detailed response!

My logic problem is: if the problem lies with the instrument panel and the needle is stiff, why would the needle initially move too far before returning?
Theoretically, if he experiences sluggishness, he should actually move more slowly from left to right towards the target position.
Or have I made a mistake there?

As I have said, in this case, I assume that the movement of the pointer is generated by the tension.

I would prefer a faulty fuel tank sender, of course, because it is cheaper, than a new one...

Florian320 wrote:

My logic problem is: if the issue is with the instrument panel and the needle is stiff, why would the needle initially move too far in one direction before returning?

I'm thinking more of a problem with the evaluation or control electronics, rather than a difficult-to-diagnose fault.

Florian320 wrote:

Of course, I would prefer a faulty fuel tank sender, as it is cheaper, than a new KI...

Finally, take responsibility and identify the root cause yourself :
1) Either you attach a DMM to the fuel sender and then realize you're missing it or
2) You attach a resistor (e.g., 50 Ohm or 150 Ohm) to the cable of the fuel sender and observe what the needle does.

I would do that, however, the last mechanic I spoke to said that it would require removing the tank - which has stopped me from doing it. It's an 8H Cabrio, manufactured in 2004, but not a Quattro model.
Is anyone of you aware of how to get to the donor?

I've also found out that it's actually stepper motors that are moving the hands.
I performed a stadiogram diagnosis: all the needles moved evenly and also stood exactly in the center at the corresponding test.
But: before the fuel gauge diagnosis, the fuel level indicator was close to empty, and then it was approximately 1/4 full. Unfortunately, I only obtained the fuel level reading afterward: it was (although the car hadn't been driven since it was parked, it had only been moved in and out of reverse parking once) 197 Ohms / 17L compared to 170 Ohms / 30L when parked.

That sounds like it's more about the instrument than the performer, or?

Have you ever just left your car running with the ignition on, and noticed unusual behavior with the fuel gauge after a while?

Yes, with the ignition on, it was the first time yesterday. The behavior was the same: the fuel gauge initially rises somewhat before dropping down.
I have now scheduled an appointment for next week with a company that repairs combination instruments.
Then I'll definitely know if it was because of that...