Diode Trick Not Working? Troubleshooting Guide

Hi there,

"This weekend, I tried replacing my mass airflow sensor (MAF) with a diode because it wasn't reaching the target values anymore. Unfortunately, that didn't work. I tried it with two different diodes, one with a 450 mV voltage drop and another with approximately 700 mV. The voltage at the engine control unit (ECU) input was around 4.5 V, depending on the diode."

The engine didn't run properly with that setup at all. Even when starting, it lacked power and wanted to stall. When trying to engage the clutch with a lot of throttle, the engine sounded like a truck and started shaking violently. I decided to leave it alone.

What surprises me is that several people in the forum have already reported that the fake [product/software] is working successfully and that there are only problems with newer engines.

Has anyone here ever tried doing that on an ALH engine?

Do you still have your EGR system active? If so, the engine control unit (ECU) will try to reduce the air mass by continuing to open the EGR valve. However, since the diode changes the mass airflow sensor (MAF) values, the EGR valve should be fully open.

Try disconnecting the EGR valve or plugging the hose and testing again.

Perhaps the AGR (Abgasrückführung) note should be included in the article about LMM (Luftmassenmesser) diodes...

Regards,
Private.

PrivatBereich wrote:

Perhaps the AGR note should be included in the article about LMM diodes.

We actually already know:

LMM-Fachartikel wrote:

However, even in engines with "simple" engine control units, a problem can occur because the mass airflow sensor (MAF) value no longer reacts as a feedback signal for the exhaust gas recirculation (EGR) system. Therefore, in the partial load range, the EGR valve opens further than normal, which can lead to increased soot formation and higher fuel consumption.

I could also add things like clumsy jumps, eerie sounds, etc.
But before that, I'm curious to know why such extreme reactions can even occur in the first place.

Has anyone of you ever tried it on an ALH engine?

'Yeah, mine has been running for a few thousand kilometers like that. The fuel consumption has dropped dramatically.'

However, for reasons unknown to me, my EGR valve has been defective for quite some time and resists all attempts at repair.

@ulf

Logically, this will cause extremely drastic reactions. You're completely throwing the feedback loop off with that diode. The system then tries to elicit some kind of reaction by applying extreme conditions, and it initially cranks up the EGR (Exhaust Gas Recirculation) significantly. Then the machine gets overwhelmed, and you end up with a beautiful resonance chain...

CU Gremlin.

Hi there,

First of all, thank you for the "reading aid." I somehow thought I belonged to the "older" generation.

But the explanation is absolutely plausible. I'm going to try again today, this time with the EGR system disconnected.

I've already diagnosed a faulty mass airflow sensor (MAF). Currently, I'm experiencing a loss of power that is related to problems with the turbocharger boost pressure regulation. The defective MAF sensor is also contributing to the issue. Initially, the boost pressure is above the target value, then it's reduced. Afterward, it falls below the target value. Due to the faulty MAF sensor, the soot limiter is now active, and it can reduce fuel injection by 20-30%, especially under full load. I believe this significantly exacerbates the lack of acceleration.
I still haven't managed to fully understand the boost pressure control issues, despite reading a technical article. When acceleration begins, the actual pressure increases from 1300 to 2300 mbar within a single data sample! (It doesn't seem to be stuck). To then reduce the pressure again, it takes 3-4 samples (with 3 logged groups, each sample is approximately 1.2 seconds long).

Holger247 wrote:

As acceleration begins, the actual pressure within a sample increases from 1300 to 2300 mbar! (It doesn't seem to be stuck). To then reduce the pressure again, it takes 3-4 samples (with 3 logged groups, one sample is approximately 1.2 seconds long).

Hmm, I'd say...
VTG rod set too short.
-> Defective solenoid valve.
-> Problems related to vacuum control hoses (e.g., blockage or kinks in the hose connecting the valve to the air filter).

Hello.

Before posting my first contribution here, I used the search function, which is how I came across this thread that at least somewhat relates to my problem. However, the most appropriate title for me would be "Diode trick works poorly".
Background: I have a VW Caddy TDI with engine code ALH, first registered in December 2000, approximately 170,000 km mileage, and it has been equipped with SPORTSYSTEMS chip tuning (from a VW dealer) since the beginning, increasing power to around 85 kW. The EGR valve has been disabled using a blocking plate. Air and diesel filters have been replaced.
For some time, I had noticed a decreasing maximum speed, which I attributed to a failing mass airflow sensor (MAF). Therefore, I tried the "diode trick" described here to confirm that the MAF was indeed the cause.
Something is wrong, either something went wrong or I misunderstood something.
So, I disconnected the MAF sensor connector and inserted the diode (1N4107) into it (into pins 4 and 5), with the cathode connected to pin 5. The MAF sensor connector has 5 pins, but only 4 wires are connected. Before doing this, I checked with a multimeter to make sure I was actually connecting to +5V and the MAF signal wire. After inserting the diode, I left the connector disconnected because it could no longer be plugged back in.

The measure had the following effect on driving: Below 2200 RPM, no change. Between 2200 RPM and approximately 3050 RPM, there was a terrible loss of power, as if someone were holding the car back. Above 3050 RPM, however, there was a massive surge in power, willing high-revving performance, and finally, like before, a top speed exceeding 200 km/h was possible again. Beforehand, it usually ended around 175-180 km/h.
However, driving with the power dip between 2200 and 3050 RPM is more than just annoying, and it's nothing like typical TDI behavior. That's why I've now reconnected the MAF sensor connector. The MAF sensor is likely the culprit for the lack of power, as indicated by the performance results with the diode. But how does this power dip explain itself with the diode? Did I do something wrong? Perhaps the connector needs to be plugged back in so that +12V and ground continue to reach the MAF sensor? Should the diode then be installed in parallel with the MAF sensor? Supposedly, it's possible to use the diode permanently as a replacement for the MAF sensor... but surely not with this power dip?

I would appreciate it if you could provide some clarification.

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

"Sam"

Have you ever logged the sensor data blocks for the mass airflow sensor (MAF) with the diode installed? You should always see the maximum value when the circuit is working correctly. Or, could it be that the diode had a loose connection and randomly made contact at higher engine speeds?

Hello.

No, the attempt at logging unfortunately failed miserably because 1. my interface is not being recognized and 2. my VAG-COM seems to be some kind of demo version. Double bingo!
I believe a loose connection is unlikely. The diode was very firmly seated in the connector, and I checked that multiple times. Also, I drove with it like this for an extended period, so if it were a loose connection, it would have had to be incredibly reproducible. The power output was always precisely the same at the same RPM.
Was the procedure of unplugging the connector and leaving it plugged in with the diode fundamentally correct?

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

"Sam"

SamHotte wrote:

Was the procedure fundamentally correct, to unplug the connector and leave it unplugged even with the diode still inserted?

Yes.

To further analyze the engine's behavior, we really need logs – especially since it has been modified with a chip, which means anything (im)possible could have been programmed into it.

Thank you for the answers!
It seems to me that the simplest way now is to renew the LMM (reluctantly).
Up to approximately 3000 RPM, the engine with the connected mass airflow sensor (MAF) delivers full power (at least subjectively). Above that speed, it requires a diode to be installed. So, everything else seems to be in good working order.
I just hope that VW can sell me the right part this time; that would be a welcome change. Getting parts for this vehicle is a real hassle.

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

"Sam"