T4 ACV performance only from 2500 RPM, no MAF signal

Hello!

I have a T4 TDI model year 1997 with an ACV engine (75kW).

For some time now, it hasn't started pulling until around 2500 RPM, and even then, it's not performing as it should.

The diagnostic scan on the VW vehicle showed that while a target value was being sent to the mass airflow sensor (MAF), there was no actual value being received. The MAF sensor was replaced, but the issue persists. Airflow LMM - MSG replaced -> No success.

You can measure the signal at the MSG's input pin using a multimeter (1.6V at idle, 4.5V at the cut-off speed).

He manages to reach 170, but he's tired.

Has anyone ever experienced something like this before? By the way, the control unit has already been replaced.
The turbo (including the wastegate), EGR valve, and vacuum hoses have all been checked.

Thank you for your help!

Jonathan.

Hello,

Is there an error code stored in the error memory?

Based on your description, the analog input of the LMM signal may be defective.
Then, the control will be based on the replacement value of the LMM. Consequently, the turbocharger is no longer activated, and the injection quantity is limited via the smoke reduction system.

The whole thing is essentially an emergency mode and should leave less residual power.

If you have the opportunity, please log data for MWB 008 and 010, and try to log the engine speed at least once, from 1500 rpm to 4000 rpm, while pressing the accelerator pedal.

MWB 008 should show you why you are not getting any output. MWB 010 will demonstrate that the LM will not be measured and that the LD will not occur.

If there is a signal present at the MSG connector, it doesn't look good for the MSG.
Currently, I'm missing a fault code entry to be certain that the MSG is broken.

Best regards, Jochen.

Hello!

Unfortunately, I don't have access to a VAG-COM or similar diagnostic tool.

The error codes were read from the vehicle at Volkswagen.

A fault code was stored: Mass airflow sensor (LMM) mass closure (DTC number unknown). Therefore, the 5-pin cable connecting the MSG (module) to the LMM was replaced, as a new LMM did not resolve the issue. However, according to the VW wiring diagram, the pin assignment is correct, and the MSG configuration is also correct.

Currently, the car is in Scotland.

I don't think the MSG is to blame, because the problem also occurred with the old MSG (the MSG was replaced about 1.5 years/50,000 km ago; the engine wouldn't start at that time either).

Since the top speed is still at 170 (with a maximum possible engine speed of 180), I don't think the car is in limp mode. I've always read that in limp mode, the speed is limited to 120, or at most 130 (and it already has an electronic speedometer).
"When the cable connected to the ESP system, which controls the valve for adjusting the injection, broke in Serbia, the engine was still running at 140 km/h. We simply repaired the cable, and it returned to normal operation. So, if the problem had been resolved by the aforementioned measure, the engine should have functioned correctly without any errors."

Now the question is: Are there any possibilities (specific circumstances, operating conditions, etc.) that could cause the MSG to simply ignore the values from the LMM?

Thank you for your help!

Jonathan.

Hi,


A fault code was stored: Mass airflow sensor (LMM) mass closure (DTC number unknown). Therefore, the 5-pin cable connecting the MSG (module) to the LMM was replaced, as a new LMM did not resolve the issue. The pin assignment is correct according to the VW wiring diagram, and the configuration of the MSG is also correct.
'The error mentioned above indicates that the MSG is not detecting a signal.' Either the data is not relevant to the MSG (Mass Spectrometry Graph), or it cannot be meaningfully interpreted because, for example, the common mass is missing.
Accordingly, I would first check the connector at the MSG; perhaps the pin on the wiring harness side is no longer in good condition. Then, check the mass readings from both the load cell and the vehicle, and if necessary, also measure the resistance between them.



Now the question is: Is there any way (specific circumstances, operating conditions, etc.) that would cause the MSG to simply ignore the values from the LMM?
I'm not aware of that. And even in your case, the signal isn't being ignored. It is 'missing,' which is why the error message appears.

For browsing: http://www.t4-wiki.de/wiki/Luftmassenmesser

Well, the issue of volume has also crossed my mind, considering I've been in this business for almost 7 years now.

I don't believe in something that simple, because I've already tried it.

I'm not saying I'm perfect, but I've definitely tried a lot of things on this car.

And hopefully, one can't compare that to a rover.

I hope someone has encountered a similar problem before and can offer some guidance.

Regards,

Jonathan.

Hi,

Well, the issue with the quantity also occurred to me, considering I've been in this business for almost 7 years.

I don't believe in something that simple, because I've already tried it.
This is often a problem; the simple things are only checked with moderate thoroughness .

Okay, as you mentioned in another post, you have the LMM signal connected to the correct pin of the connector leading to the MSG, and – as noted above – the vehicle and sensor grounds are fine. This means that the LMM signal is being received correctly and is usable at the aforementioned location. The connector is connected to the MSG.
If the pin on the connector and the corresponding contact on the MSG are in good condition, then the only remaining reason for your problem is the engine control unit. There is no other way if Ohm and Kirchhoff are correct.

I'll double-check all the values listed in the circuit diagram as soon as I get the car back (it's currently in Scotland).

Here's a question:

Is the power supply typically provided by the MSG? If so, which pins? That's how accurate the wiring diagram, or rather, the legend on the VW wiring diagram, actually is.

The funny thing is that the error exists with both the old MSG and the new one.

Therefore, I suspect that there is a condition "X" that prevents the MSG from evaluating the LMM value.

Perhaps Udo will come up with something, some kind of error that doesn't trigger a full emergency mode, but rather some kind of "half-emergency" state.

Thank you for all your help!

Regards,

Jonathan.

Hi,

Is the donor mass typically supplied by the MSG? If so, which pins?
Yes, pin 33 (brown-white). Almost all the motor sensors are connected to it. The LM is the only one (?), that is connected to both the sensor ground and the vehicle ground. No idea why.


The strange thing is that the error exists both with the old MSG and with the new one.

Therefore, I suspect that there is a condition 'X' that causes the MSG to not evaluate the LMM value.
To the best of my knowledge, there's nothing like that – except, of course, for a bug. And then the MSG takes on a replacement value.
If the same error occurs with 2 different MSG units, it's less likely that the MSG electronics are the cause. Therefore, I would focus on the pin, which is the contact point between the MSG and the connector. It's possible that the pin is simply broken, bent, or otherwise damaged. Or it could be that moisture has gotten into the connector.