Outsmarted the STG again... LMM replacement

Hello everyone!


I got a little bored during the semester break and ended up experimenting a bit with the LMM connection.
However, I must say that the basic idea behind these results, namely to suggest maximum airflow to the turbocharger, originates from the tdiclub.com forum, specifically from a member named "mickey," who wrote extensively there. That's all there is to say about intellectual property.

Engine: ASV, Ibiza, manufactured in 10/2000.

You can bring the feedback signal of the MAF sensor connector down to approximately 4.5 volts by connecting it to the +5V power supply with a silicon diode in between. The control unit doesn't throw any errors, and at least the glow plug doesn't light up. The driving behavior is (estimated) similar to the standard version, but there's a slight "rushing" sound when accelerating abruptly below 1500 rpm, although I think it's still acceptable.

Connecting the feedback signal directly to the +5V line causes the control unit to enter a limp-home mode. Acceleration is noticeably worse, especially at higher RPMs. Again, the glow plug does not illuminate. The exact same behavior occurs when the plug is completely disconnected.

My personal opinion regarding replacing a Mass Air Flow (MAF) sensor that might fail in the future: Don't replace it, use a diode instead. The driving experience was definitely perfectly acceptable.

How this affects consumption? I have no idea. I'm going to drive it for the next 300 kilometers.


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

"Comments are welcome."

Best regards from the automotive world, Georg.

Hello Georg!

How and where do I need to connect the diode to distort the feedback signal (which terminal)?
I'm asking because I could easily check if my LLM is working correctly!

Sure, no problem!

Regards,

Andreas.

I believe, when looking at the plug from above, they are the two leftmost pins.
There was already a thread about this, discussing how to bypass the STG. Search the forum for "MAF" or "LMM".

It's safer to test it with a multimeter.

There are 4 pins.

Mass, +5V, +12V, and the feedback signal.
When the ignition is turned on, you will find 2/+5V relative to ground. When the ignition is off, there is 0V there.
If you can't clearly identify the pins, I would advise against making this change. "No offense, but I don't know what the STG does when 12V is applied to the feedback signal line."

Hi Georg,

I had already read the American website as well.

Have you ever read the error codes? There are such ugly/treacherous conceivable entries like (in essence) "implausible LMM signal" ...

And with the AU, it might possibly be... Problems cause

Otherwise, your trick with the diode is really clever and will certainly be welcome to Audi drivers whose mass airflow sensors fail every few thousand kilometers, causing oil mist from the crankcase ventilation system to suffocate the engine .

On the other hand, the EGR system could malfunction: in the lower load range, the engine control unit tries to "reduce" the mass airflow sensor (MAF) signal using the EGR system, which it usually achieves successfully.
A diode instead of an MAF sensor, however, will not be particularly impressed by a constantly fully actuated EGR valve, which will cause the engine computer to fully open the EGR . The black smoke you observed under heavy load at low RPM could be (partially) caused by effects like those.

My idea for a solution:
Do not connect the diode directly to the LMM signal wire, but rather connect it in series with a resistor (e.g., 2.2 kOhms).
Connect another diode with approximately 1 kOhm in series to the LMM signal pin, and then connect this to the EGR control wire. This should allow the engine computer to reduce (the apparent) LMM signal via the EGR output (but not raise it all the way to battery voltage!). A 2k potentiometer might be needed for fine-tuning.

To smooth out the resulting rectangular voltage spikes (pulsed EGR valve voltage), add a suitably sized electrolytic capacitor between the MAF sensor signal pin and ground.

To prevent this electrolytic capacitor from retaining the voltage as the last component when the motor is turned off (which could potentially damage the mass airflow sensor's input stage), add a discharge diode from the positive terminal of the capacitor to the 5-volt power supply of the mass airflow sensor.

Hello Ulf,


Good ideas probably never run out for you .

I think your suggestion is quite good. For some of us, it might not be suitable, because these are already equipped with an absolutely verschandeln intake system .
The ABE will expire anyway with this change. Why try to circumvent the AGR control system?

What surprised me was that the TCU (Engine Control Unit) apparently already performs a plausibility check of the LMM (Mass Air Flow) signal. However, I wouldn't have expected it to be so simple. But that probably explains why many TDI engines, whose performance seems inadequate, don't have a corresponding entry in the TCU. It really wouldn't have been much effort to check 3 or 4 air mass setpoints, depending on the load.

According to tdiclub.com, American TDI models trigger an error when the ECU is manipulated in this way, because the faulty EGR control prevents the mass airflow sensor (MAF) signal from dropping. The American versions have complex checks built in to prevent the EGR system from being disabled. What a blessing we have here.

Here's the translation:

"One more question: Based on which parameters is the injection timing adjusted forward? It's certainly load-dependent, but how is this load defined? Perhaps through the mass airflow (MAF) signal?" Then, this would also provide a simple way to always schedule the start of the injection for an earlier time.

Well, good night.

Hi everyone!

Thank you for the description/tips! I'm going to test that diode now!

Regards,

Andreas.

Georg_G wrote:

ABE will anyway be phased out with this change. Why, then, try to circumvent the EGR control system?

Hi Georg,

. . . in order to (hopefully) reduce the amount of soot produced when accelerating from a low RPM

Quote:

One more question: Based on which parameters is the injection start adjusted forward? Certainly, it depends on the load, but how is this load defined? Perhaps through the LMM signal? Then,

one could also find a simple way to always set the start of the injection process to an earlier time.

The basic operating principle is described in the technical article on how to adjust the pump.

The main parameters of the characteristic map are, as far as I know, engine speed and injection quantity. The LMM value therefore only indirectly influences the injection quantity.

In addition, adjustments are made to the engine temperature: a cold engine requires an earlier injection start.
"That is, by adding a series resistor to the engine temperature sensor, it should be possible to advance the injection timing. However, I'm not sure to what extent this correction would be affected by engine speed or..." Loading aspects are built-in .

Progress report...

Okay, I just drove it for about 200 km. It's reaching its top speed, so I assume the mass airflow sensor (MAF) signal is sufficient to deliver full power. I haven't done an Ulf-test. The fuel consumption, according to the MFA (Multi Function Display), is roughly the same as usual. The driving characteristics are not to be criticized.

Then I reconnected the mass airflow sensor. The Speed-Buster box eventually intercepted this signal, which is why it was also deactivated.

Best regards, Georg.

Does anyone know how and where to intercept the accelerator pedal signal? It is supposedly possible to use this as a trigger signal for the LMM power boxes.

How can I get access to this cable? What exactly is it?

Thank you and best regards, Georg.

Sure, here's the translation:

'Small feedback:'
I've now driven approximately 500 km with the diode installed. The throttle response is slightly better (which suggests that my mass airflow sensor might need cleaning!). There's no soot. But: The diesel consumption increases by approximately 1 liter per 100 km...
I also have to mention that my pump is set to a much too early timing, and that the touch ratio is... 'The valve for injection start is set to approximately 45 degrees at idle.'

It's definitely a great challenge when you don't have an LLM at hand .

What surprised me was that the TCU (Engine Control Unit) apparently already performs a plausibility check of the LMM (Mass Air Flow) signal. However, I wouldn't have expected it to be so simple. But that probably explains why many TDI engines, whose performance seems inadequate, don't have a corresponding entry in the TCU. It really wouldn't have been much effort to check 3 or 4 air mass setpoints, depending on the load.


How is that supposed to work with diesel?

Whether you're at full load or idling, it always draws the same amount of air at xyz RPM. Even with forced induction, you won't get any readings that you can compare to other systems. Due to the constant excess of air, you can't accurately determine the load based on the position of the throttle valve or the accelerator pedal.

For gasoline engines, it's simpler because the lambda control system goes into a complete panic if the mass airflow sensor signal is incorrect.

The only way to perform a plausibility check would be through the AGR (presumably a specific system or regulation), but as long as this isn't required by the EU, the software development effort can be avoided.


Here's the translation:

'One more question: Based on which parameters is the injection timing adjusted forward? It's certainly load-dependent, but how is this load defined? Perhaps through the mass airflow (MAF) signal?' Then, this would also provide a simple way to always schedule the start of the injection for an earlier time.


Mainly, it's about the load and the speed. However, the load is also influenced by the amount of air.

You can use the e-GAS system to send a request to the EDC (Engine Control Unit), which will then initially increase the fuel injection amount. It naturally takes into account the current air mass flowing in. The EGR (Exhaust Gas Recirculation) can be closed to increase the air mass. As the increasing exhaust flow accelerates the turbocharger, the increasing boost pressure further increases the air mass, allowing for even more diesel to be injected. The injection timing is also gradually advanced. As you can see, everything is interconnected.

If your LMM (lambda measurement module) signal is completely distorted, the EDC (electronic direct control) system will obviously calculate completely incorrect values, and this will affect things like the injection timing, setting it too early, as well as the amount of fuel injected. This is why you're experiencing excessive smoke and increased fuel consumption.

The occupancy of the LMM is:

1 - temperature sensor (not installed in the diesel engine).
2 - +Ubatt - 12V
3 - Mass
4 - +Uref - 5V
5 - Output (0.02 - 0.96 * Uref)

At an input voltage (Uref) of 5V, the maximum output voltage must be 4.8V.
Normally, there is 1V at the output without airflow. If the mass airflow sensor detects backflow, the voltage drops below 1V.

CU Gremlin.

Gremlin wrote:

If you completely distort your LMM signal now, the EDC will naturally calculate values that are completely wrong, and it will, among other things, adjust the injection start too early, just like the injection amount.

This is why you get excessive smoke and increased fuel consumption.

Hi Gremlin.

I can't quite understand that right now.

1. As far as I know, the start of injection is determined as a function of injection quantity and engine speed, but not directly from the air mass measurement .

"If 'maximum air intake' is constantly reported, the torque curve serves as the (lower = limiting) boundary for the injection quantity at full throttle, even if there is currently insufficient 'actual' air in the combustion chambers. This can lead to soot, poorer exhaust emissions, and reduced engine efficiency."

Since drivers typically don't frequently use full throttle with a TDI engine, I suspect the AGR (Abgasrückführung) system is primarily responsible for the increased fuel consumption. This is because the AGR valve is generally opened to a significant extent under partial load conditions, until the mass airflow sensor (MAF) reading provides feedback that aligns with the current map value.
However, if the Mass Air Flow (MAF) sensor only consists of a diode that is unaffected by the intake air, the EGR valve will remain open longer and wider during partial load conditions. This means that the engine (with a functioning EGR system) must operate with a minimum amount of fresh air, and the power loss caused by a rich fuel mixture will be compensated for by increasing the throttle, approximately until the EGR valve closes due to excessive boost pressure.

Looking at it the other way around: with a diode instead of a mass airflow sensor, without EGR, and with relaxed driving (i.e., without entering the soot range), I don't think there should be a significant increase in fuel consumption, because the engine will still mostly run in "normal mode" = with a generous excess of air .

My AGR has been retired for quite some time (and is sealed with metal).
I only tried using the diode to get an idea of how well my mass airflow sensor is still working. Without AGR (Abgasrückführung), the measurement blocks already show a number that is far too large.
I also have to say that during my 'diode testing phase,' I had many periods of full load operation because:
Town - 100 - Town - 100 Traffic light - 100 etc.
Furthermore, full power is often needed because my car still doesn't accelerate properly.