Speed-Dependent Diode: Explained & Uses

Hi.

Yesterday, I took advantage of the opportunity to warm up my TDI engine by driving it 1500 km and recorded some data.

AB Full throttle with mass airflow sensor and diode - logging data from modules 003, 008, and 011.

And I was about to fall over!

The device indicates a full 849.7 mg/H in a stationary state and during moderate driving.

But the value decreases at high speeds!
At 4000, it was only 814 mg !!

Will the value of the LMM be "adjusted" again, because "850 mg at 4000 [units] definitely can't be right," or what's going on there?
When I stopped accelerating, the value went back up.

Detailed logs will be available tonight.

m;

dieselmartin wrote:

The diode indicates 849.7 mg/H in a stationary state and during moderate driving. But the value decreases at high speed!!! At 4000, it was only 814 mg !! Will the value of the LMM be "rounded" again, because "850 mg at 4000 [units] definitely can't be correct," or what is happening there?

I believe that a healthy LMM (Linear Mode Mixer) provides a few millivolts more voltage than a 5V - Udiode at its maximum power output (Pmax).

-> Try using a Schottky diode, which has a lower voltage drop.

A second diode of the same or a similar type, connected in parallel with the first, could have a similar effect.

That also means that the voltage at the LLM signal output is not the final measured value.

Because otherwise, why would 4.3 volts at 2000 RPM indicate a higher air mass than 4.3 volts at 4000 RPM?

I always thought that the LMM itself was responsible for processing the measurement data.

Unfortunately, I'm not getting the actual voltage reading at the sensor pin during this measurement.

m;

dieselmartin wrote:

This also means that the voltage at the LLM signal output is not the final measured value. Because otherwise, why would 4.3 volts at 2000 RPM indicate more air mass than 4.3 volts at 4000 RPM?

Because the LMM (Laminar Mass Flow Meter) can only measure mass flow, simplified: mass per suction stroke multiplied by speed.

To determine the amount of fuel injected per cycle, the engine control unit must "divide the voltage by the engine speed."

-> Same LMM voltage at increasing RPM = decreasing mass per cycle.

Ah, I thought it measured per intake cycle, which would create a voltage frequency of half the engine speed, or that the EDC (Engine Control Unit) would check it.

But as you explained it, it fits with the concept of a diode that has a "too steep" voltage drop.

Thank you.
m;

at 4000: 814 mg with a diode is still better than 700 mg with LMM .

Yes, you can clearly see the 1/(U/min) for the air mass here.

However, with the 814 mg, there is still 2 mg of "headroom" remaining before reaching the torque limit in the Russfeld.
The original LMM sometimes "caps out" (meaning the Russian border is the minimum value).