VAG-COM: Adjust Fuel Gauge to 5L Reserve?

Hello,

I have a question about calibrating the fuel gauge, if possible using VAG-COM.
"I recently used VAG-COM to read the fuel level, and it showed 10 liters, with a resistance reading of 91 ohms. After driving about 500 meters, the low fuel warning light suddenly came on."

Approximately one year ago, I visited a mechanic who adjusted the fuel reserve setting on my car. The reserve was set to 5 liters for 2WD and 7 liters for 4WD. To achieve this, exactly 5 liters of fuel were added to the tank, and then a VAG diagnostic tool was used to set the reserve level to that amount.
Now I have a question: If I read the fuel level with my VAG-Com and it shows a full 10 liters, why does the low fuel warning light come on shortly after? Is the VAG-Com not accurate, or is the instrument cluster somehow learning and adjusting over time?
While the VAG-COM tool allows you to change values, what exactly do these values refer to? Does a full tank and setting the display to the last bar have any relevance?
"Or can I adjust this value change somehow, even with 5 liters in the tank, so that I can understand it better? If so, can someone tell me how to do it so that the reserve indicator only activates when there are 5 liters left, and the needle shows "full" when the tank is full?"
I'm grateful for any advice. Or does the measurement differ when using VAG-Com? Because when the tank is full, it should theoretically display 55 liters, but I never read anything above 53 liters with VAG-Com.

hi...


Yes, that's definitely possible. My advertisement was also incorrect. There's a channel in the engine control unit (I don't remember the number right now - it's related to adaptation), and the value entered there is 1270 - 135.

This value will cause your fuel gauge to fluctuate. Therefore, you can adjust the display here.

Are you in the MSG? Not in the AI?

Hi, he probably meant in the AI system, because that's where it would normally be evaluated.
What surprised me today was that I completely emptied the fuel tank first, then removed the sender unit along with the pump to measure the values between the minimum and maximum levels.
Fuel tank completely empty: 50-51 ohms (which is also what VAG-COM displays). Fuel tank full: 290 ohms.
Okay, here's the translation:

"Now for the interesting part: I reinstalled all the components and filled the tank with 5 liters. Then I measured the Ohmic value, once with an ohmmeter and once with a VAG-COM. The value was the same with both devices: 51 ohms! This means that with only 5 liters in the tank, VW's system cannot adjust the fuel gauge to this value. Therefore, the diagnostic tool is unable to set it. The tool can only adjust it when there are 6-7 liters or more in the tank, because below 6 liters, the value at the sensor cannot change due to the float arm not being able to rise sufficiently."
(Even when the tank is completely empty, VCDS shows a fuel level of 5 liters and a resistance of 51 ohms.)

Therefore, the fuel gauge can only be calibrated and function correctly with a fuel tank capacity of 7 liters or more.
As VW intends to configure the fuel tank sensor with 5 liters at the empty mark for 2WD and 7 liters at the empty mark for 4WD, it's simply not feasible based on tank volume and sensor readings.
What's also quite interesting is that you can refuel the car with (5 liters are already inside) + 40 liters, and you can immediately see the fuel level using VCDS. However, the display in the instrument cluster remains completely at zero and only shows the correct level after turning the ignition on and off.
Fuel tank capacity: 5 + 20 + 20 liters = 45 liters. The display on the VAG-COM shows 42 liters (within tolerance), but it's not exactly accurate.
That would mean that the instrument cluster requires a kind of reset by turning the ignition on and off before it can perform further calculations and display the information. Based on my initial measurements from two years ago, it seems that 2-3 changes are accepted, and only after the 4th change does the instrument cluster require a certain distance to be driven before it will accept the changes again. Simply turning the ignition on and off doesn't help in this case; the instrument cluster absolutely needs a distance impulse to be able to calculate and process the information.
How nice the display was on the old Golf 3; there wasn't this "calculation mumbo-jumbo" for the fuel gauge yet, but why do things the simple way when it can be much better, albeit complicated?

Based on these measured values, we can conclude that when the fuel level drops below 7 or 6 liters, the instrument cluster (KI) calculates the position of the fuel gauge needle based on the average fuel consumption and distance traveled, and then adjusts the display accordingly.

"However, I'm still unclear about the relationship between the "Please Refuel" message and the position of the fuel gauge. Does this relationship depend on the value of 50 ohms, or on the position of the indicator? Specifically, where is the threshold at which the message appears, and at what ohm value from the sensor? Or does the sensor value play a negligible role, and does it only rely on the position of the needle?"

You're right! I meant the instrument cluster.

That would mean that the instrument cluster first needs a kind of reset by turning the ignition on and off in order to perform further calculations and display the information. According to my initial measurements from 2 years ago, 2-3 changes are accepted, and only after the 4th change must you drive a few meters for the instrument cluster to accept it again. Turning the ignition on and off doesn't help with that; the instrument cluster absolutely needs a distance pulse in order to calculate.

I don't know what you think about today's AI - but did you really think that the value would be calculated so slowly, similar to how you would have to manually push the current from the fuel sensor to the AI?

Simply put, the AI systems we're building are essentially just stepper motors for controlling things like the fuel tank, temperature, speedometer, and other parameters.
To prevent you from having a fluctuating fuel level like in the old Golf 1 days, and something that probably can't be fixed with vortex flow dampeners or gel pads or whatever, the little motors are simply electronically dampened. They only allow for a certain range of fluctuation – of course, the car's computer knows within milliseconds how much fuel is actually in your tank. Try filling your car while the ignition is on and wait – the needle will rise very slowly .

So, my current understanding of this AI is as follows.

If the fuel level drops below 7 liters and reaches around 5-6 liters, there is so little fuel in the tank that the fuel level sensor will no longer be able to rise.

As a result, from that point onwards, the display is only adjusted internally based on the current consumption, because the tank sensor remains fixed at its mechanical and electrical value (empty tank = 50 Ohms). Only when the content in the tank increases to 7 liters does the Ohmic value also increase.
Therefore, for vehicles with a fuel tank capacity of less than 7 liters, only the displayed value is considered valid (virtually) internally. This is, of course, very inaccurate because the actual reference points are completely missing.

Therefore, even with a 5-liter tank capacity, the fuel gauge cannot be properly calibrated. This is not possible due to technical and design limitations of the tank. In other words, the way the tank is constructed and the fuel sensor are integrated simply don't work well together.

"What I wrote before about the sudden drop in the fuel gauge reading when starting the car is simply a software problem with the instrument cluster (IC). The value that drops at the fuel sender when starting is then taken over by the IC, and the software developers have a timing issue with the query interval. Of course, VW doesn't want to admit this."
You wouldn't believe that a huge corporation like Volkswagen would admit that they made a mistake.

I, for one, am not giving up. I may not be able to change anything about the software itself, but I repeatedly report the issues to Volkswagen. If it were as easy to modify other AI systems for testing, I would have done it a long time ago.
That's why I always ask people who don't have problems with the system about their firmware version, so I can find similar, working AI configurations and thus completely validate my hypotheses.