AHU: Charge Pressure Difference - Troubleshooting Guide

Hello Foriker,

I'm starting to get really frustrated with my AHU turbocharger, installed in my VW Bus T3. For some time now, I've been having a problem where it goes into limp mode, and the error code displayed is "575: Boost pressure control differential." After restarting the engine, the problem is resolved until the next occurrence.

Background:
While driving in 4th gear, the turbocharger pressure drops out starting around 110 km/h (full throttle according to the accelerator pedal). After various suggestions here on the forum, I have replaced the following:

1) The turbocharger is new.
2) N75 valve.
3) All connecting and pressure hoses for the wastegate and EGR control valves.
4) LMM
5) The control unit no longer has a pressure connection. After the low-level kit (LLK), a pressure sensor is installed in the pressure line.

The wastegate functions mechanically very well and is easy to operate. A functional test of the pressure sensor using a 100 ml syringe revealed that full deflection with the syringe is approximately 1 cm.

According to the actuator diagnosis, the N75 valve is functioning correctly, and the wastegate linkage is also moving as it should during the test.
I temporarily disconnected the AGR (Abgasrückführung) to rule out a fault.

The pressure pipes from the turbocharger to the intercooler and then to the intake manifold are sealed (with the compressor pressurized to approximately 2.5 bar).

Here's the translation:

"The engine runs wonderfully, but as soon as this error occurs, the boost pressure collapses. It's almost as if the pressure sensor isn't being activated at all, and it's opening at 0.6 bar instead of the expected value!"

I suspect the pressure sensor is the problem. The previous model had its turbocharger fail, which made that area quite oily. It's possible that some oil has seeped into the pressure sensor, causing it to malfunction.

What else can be done? Does anyone have any suggestions?

Best regards,

Dirk.

Hi.

Could it be that the pressure sensor in the expansion tank is not identical to the original one in the control unit, and therefore provides different values?

How do you perform an actuator diagnosis of the N75 valve?
You can only adjust it "under boost," and you don't have boost either when the engine is idling or at a higher idle speed – as long as the car is stationary.


Here's a more fundamental point:

Incidentally, a test cannot tell you whether the rod is moving (because there are no sensors for detecting movement). It can only control it, and *you*, as a human, can observe the result, namely the movement, and compare it to your expectations.
And you can draw incorrect conclusions from that, even if you don't intend to correct yourself.

m;

Hello there.

Thank you for your tips. So, my engine control unit doesn't have any input for the pressure hose. According to information from VW, it seems that the latest AHUs didn't have an engine control unit with a boost pressure sensor, but rather an external boost pressure sensor was installed. At least, that's what the AKTE shows me .

I have a suspicion that has arisen from observing the numerous "turbo" images on eBay:
"In my setup, the wastegate control solenoid is mounted parallel to the engine block. In the eBay turbo kits (which are supposedly original), the wastegate solenoid is mounted at a 90-degree angle to the block. Perhaps the issue lies here: the wastegate itself might be functioning, but the control signal might not be sufficient?"
When the threaded collar is removed, the wastegate valve opens at approximately a 45-degree angle. However, the cap itself could certainly be opened to 90 degrees!

I once adjusted the wastegate linkage so far that the turbo now has significantly less power, which results in a bit more smoke. But even in this situation, I'm still experiencing the shutdown. Unfortunately, I haven't installed a boost pressure gauge yet, and my laptop can't display data while the vehicle is moving (due to a defective battery ).

Therefore, I believe that, as mentioned, the issue is either related to the control system, assuming that the 45-degree opening angle of the wastegate valve is not sufficient, or it's related to the pressure sensor in the pressure pipe after the intercooler.

"Does anyone happen to have photos of a turbocharger installed on an AHU/1Z engine from a Passat, or perhaps a photo of such an installation in a T3 van?"

@Dieselmartin:

Following the procedure described in the forum using VAGCOM and the actuator diagnosis function, the linkage moves approximately 7-9 mm (according to the feeler gauge), which corresponds to a wastegate opening angle of approximately 30-40 degrees.
As I said, I suspect there are two errors here, but before I spend any more money...

Here are some pictures of the turbocharger (1Z in the Golf 3).

http://www.luxus-wg.de/~martin/auto_bilder/Kruemmer/


What is the part number of your engine control unit (ECU), and from which Passat model is the AHU engine?

If it's from a 3B model, it's possible that VW created a confusing mix of components, similar to what happened with the AFN engines in the 3B. This could involve things like different 028 and 038 engine control units, etc.

m;

Hi Martin,

Okay, I think it's probably from a 3B model. I'll compare the control unit number tonight. But I'm already relieved to see that the pressure sensor on your 1Z also runs in parallel with the engine. So, that's probably not the problem. I'll adjust it again tonight and then test it again! Next, I'll check the pressure sensor. And then we'll see!

Do you have any quick tips you can offer right now? Unfortunately, I can't provide you with the log files at the moment, as my "measurement laptop" is slowly but surely failing, and it has to be connected to the power supply (EKG = electrical grid) all the time...

Best regards,

Dirk.

Do you not have a DC-AC converter on the bus?

You have to have something like that on the bus anyway . And now you would also have the possibility to log data .

If you're experiencing boost pressure issues, it's also a good idea to use a traditional, mechanical boost pressure gauge (even a simple pressure gauge will work, although it will vibrate quite a bit).

Here's how you can quickly determine if there's a problem with the turbocharger's boost pressure or the pressure sensor.

m;

Yep, I'll try it out tonight. Let's see where I can find something like that quickly!
Just quickly pop into Conrad if you have a chance!
I'll get back to you when I know more.

Oh, so you're from the Palatinate region? I used to live in Gimmeldingen and went to school there too!

best regards

Dirk.

Okay, I checked, and it's a 038 engine control unit. So, it's that mishmash of parts that was advertised!

Best regards,

Dirk.