Audi A4 AFN: Engine Won't Start - Faulty ECU?

Hello,

I'm only here because I drive an Opel (diesel) myself, and I have absolutely no knowledge of VW/Audi.
The following problem:
My uncle's Audi A4 (TDI 81kW / AFN engine, year 1997) won't start anymore. It started with difficulty starting.
However, it was still able to run at that point. But it has deteriorated further, and now it won't start at all.
Workshop #1 (a contact of my uncle) is unable to establish a connection with the control unit.
Workshop #2 (Audi authorized service center) Unable to establish a connection with the control unit (their diagnosis: control unit defective), and also found a faulty pre-lift pump.
According to them, the car should still run even with a faulty fuel pump, and fuel is also reaching the ESP system at the front.
Since I work with electronics professionally (but not in the automotive field), I should now take a look at the control unit.

However, I still want to find out more information here first.
-Are there any typical problems or issues associated with this model (e.g., ESP malfunctions, variable valve timing problems, broken cables, porous fuel return lines, cold solder joints, corroded contacts)?
-Is it possible to clear error codes without a diagnostic plug?
-läuft der tatsächlich mit def. Voil booster pump?
-If different ECUs (Engine Control Units) were installed (if I need to contact xbay), do the ECUs need to be programmed to the car, or is it simply a matter of connecting them and they work?
-Does anyone have any other ideas?

Today, I will be disassembling the SG unit to check for corrosion, cold solder joints, and defective components. I won't be able to perform a system test under load.
I somehow have the feeling that the error might be outside of the SG (System Group).


Andi.

Rekord81 wrote:

Workshop #1 (a friend of my uncle) is unable to establish a connection with the control unit. Workshop #2 (Audi authorized service center) Unable to establish a connection with the control unit (their diagnosis: control unit defective), and also found a faulty pre-lift pump. According to their statement, the car should still run even with a defective pre-injection pump, and fuel is also reaching the ESP at the front.

I only know about electric pre-injection pumps on VAG diesel engines in PD engines (not: AFN). This is not to be confused with the rotary vane pump in the VP37.
"No connection" can also originate from the infamous relay 109, which provides the main power supply to the engine electronics. If the preheating light doesn't even flicker briefly when the ignition is turned ON, that's a strong indication.

Quote:

Are there any typical diseases associated with this model (ESP, variable valve timing issues, broken cables, porous return lines, cold solder joints, corroded contacts...)?

Occasionally, the timing chain gears loosen on the crankshaft.
Porous leak lines are also common, and when playing the organ, many air bubbles pass through the transparent supply line.

Quote:

Is it possible to clear error codes without a diagnostic plug?

No.

Quote:

-Does it actually run with a defective pre-pump?

See above.

Quote:

Since different ECUs were installed (if I need to mention xbay), the ECUs need to be programmed to the car

. Yes, because of the WFS. If this is not done, the (otherwise intact!) engine will start, run normally for about 1 second at idle, and then shut off again.

While a faulty MSG (Message Security Gateway) is not impossible, it's an extremely rare occurrence within this forum.

Is a third-party radio installed? This might be blocking the diagnosis. Try disconnecting it and trying again.

The diagnostic communication path runs from the WFS (Vehicle Communication System) to the MSG (Modular Service Interface). Is it still possible to communicate with the WFS? Depending on the findings, the wiring between the diagnostic connector and the WFS, or between the WFS and the MSG, should be measured and checked for short circuits.

Is the MSG (Main Switching Gear) receiving power? Either measure the voltage or check if the preheating indicator light comes on.

The most common problem with VP engine starting is air in the fuel line. Check if the A4 also has a clear fuel line and watch for air bubbles during startup. Other frequent causes include a weak battery or starter, misaligned timing, and, with some distance, a faulty ESP (Electronic Stability Program). We have a technical article here about starting problems; please take a look.

Error codes in VAG vehicles, depending on the platform, can generally no longer be displayed via LED blinking starting in the early 1990s.

I'm not familiar with a pre-lift pump on the AFN engine, although I haven't had the chance to work on one in an Audi yet. However, the VP37 remains a self-priming pump. If there is indeed a pump in the tank, its purpose, as with the V6 engines, is to pump diesel from the lowest point in the tank to a reservoir within the tank when the tank is empty, from which the engine then draws fuel. Unless the tank is almost empty, the pump is not needed.

Several control units are compatible with the AFN engine. There are essentially two key differences: A) with/without Wastegate Function (WFS) and B) for VW or Audi (the brake/clutch pedal is connected to either ground or positive voltage). Check the Bosch Ecat catalog to find out which control units are compatible and which have been replaced by newer generations.

Regards,
Guste.

EDIT: Okay, I must have fallen asleep on the keyboard, and Ulf was faster again.

Das geht ja flott hier. Thank you very much.

Relay 109 sounds interesting, where is that relay located?
What does the glow plug indicator light look like in an Audi? Does it always light up briefly?
If the WFS (Window Film System) were defective (which I also considered), the motor would still have to run for 1 second, right? That would rule it out, because the motor isn't running at all.

"The timing belt was replaced some time ago. If the valve timing was misaligned after that, it should have caused noticeable problems while driving. I need to investigate this further."
"If the valve timing or ignition timing is incorrect, does white smoke come out of the exhaust? Or could it be due to insufficient fuel injection or a low starter speed? I haven't seen any smoke at the rear, nor have I smelled any diesel fumes."

The pre-lift pump only comes into play when the fuel level is low? Perhaps it has already sucked in air prematurely from the back at some point?

Rekord81 wrote:

Relay 109 sounds interesting, where is that relay located?

Take a look at the fuse box . It might have a different number (use the search function, or first check if the indicator light comes on).

Rekord81 wrote:

What does the glow plug light look like (the filament)? Does it always light up briefly?

Just take a look at the instrument cluster (speedometer). You should be able to see something that indicates it.

Rekord81 wrote:

If the WFS were defective (which I also considered), would the motor still have to run for 1 second?

Yes, the motor would still run for one second, and then the MSG would stop the motor.

Rekord81 wrote:

The timing belt was replaced some time ago, and if the NW timing was misaligned after that,

it should have caused the engine to run worse while driving.

Rekord81 wrote:

Adjusted valve timing often becomes noticeable primarily in the starting behavior, especially for drivers who are not paying close attention.

The pre-charge pump only comes into play when the tank level is low?
If it even has one. We've also had cases here where a workshop claimed it was defective, even though it wasn't installed }.

What about radio and air?

Regards,
Guste.

The car is currently parked outside at workshop #1. Since we're having terrible weather here, I'm waiting until it gets a bit better. I'll decide what to do next by tomorrow morning at the latest.
By the way, the workshop #2 wanted to replace the MSG unit (for €1300) and the pre-injection pump (for €700). That's on hold for now until I've at least had a look at the control unit.

Thank you very much for the answers!

Andi.

For the test, you can check the MSG on any TDI engine with 90 or 110 horsepower, specifically models manufactured from 1994 to 1997. The engine must run for at least 1 second.

If you have access to VAG-COM (shareware plus adapter is sufficient) or the Bosch diagnostic tool used by the workshop, and you want to read the MSG (Engine Control Module) data from your computer:

Rudi in https://community.dieselschrauber.org/en/viewtopic.php?t=15170&highlight=schreibtisch wrote:

If you have the opportunity, build an adapter that allows you to connect to the engine control units. you can operate directly on your desk. ECU: Pin 1: Ground. Pin 23: 12V. Pin 61: K-Line. Diag: Pin 4,5: Ground. Pin 16: 12V. Pin 7: K-Line. "The K-Line needs to be directly connected. Both connectors require 12V and ground connections, and there needs to be an input for a power supply (12V / 1A is sufficient; the MSA15 draws more power)." approximately 300mA when it's running).

Regards,
Guste.

Okay, so...
"The control unit is open, all solder joints and components look perfect visually, and all capacitors are okay. I expected that."
"New radio not installed, fuel line not transparent, ESP receiving diesel."
According to the mechanic, the pre-lift pump located at the rear of the tank is drawing 17 amps or more, which is causing the fuse to blow. Apparently, the pump is needed for full load operation, as I understand it.

"What I noticed earlier in the workshop is that we measured 0V at pins 1 and 23 (though using a strange LED workshop tester). I'll double-check that with my own meter. I haven't located relay 109 yet. Is it in the engine compartment near the large fuses (next to the MSG), or is it inside the cabin near the fuses? So, relay 109 powers the MSG, but where does relay 109 get its power from? From the ignition switch contact?"

What does... actually mean?

Quote:

Diag: Pin 4,5: Ground. Pin 16: 12V. Pin 7 K-Line

Is the diagnostic tool being powered? Should I measure the voltage at pin 4 and 5 (GND) and pin 16 (12V) on the connector in the vehicle, where the control unit (MSG) would normally be connected?
Is the K-line from the MSG connector directly electrically connected to the diagnostic connector? If so, I can check for continuity there.

"I don't have VAG-COM, but I do have an oscilloscope. Can I stimulate the MSG with a simple voltage level to measure a confirmation telegram?"

"I'm asking some of these questions in advance. First, I need to verify whether pin 23 is actually not receiving voltage, or if that was a measurement error. By the way, none of the glow plugs illuminated when the ignition was turned on, but the light directly next to it (presumably for auto start/key=WFS?) lit up for 2 seconds and then went off (which is normal for the cars I'm familiar with, it's just a normal light check)."
Are the two K-lines on MSG pin 7 and pin 61 electrically parallel, or are they two separate K-line channels?

Almost forgot: the MSG is a Bosch part, according to the sticker. Can the local Bosch service center test it for functionality, or are they unable to do anything with it?
Andi.

Hello.
"I looked at the car again tonight, trying to figure out if the control unit receives power at pin 23. I found copies of circuit diagram excerpts in the car from workshop #2 (an Audi workshop). I think I've found the relay that supplies power to the MSG (Main System Controller). According to the circuit diagram, it's labeled J322 (relay socket number 3) and provides 12V to pins 23, 68, and 45 of the MSG."
Here's something strange: The relay is controlled via a wire from pin 42 of the MSG (Module Control Unit). This means that as long as the MSG is not plugged in, the relay cannot be activated, and I cannot measure 12V at pin 23 (connector 23) on the vehicle-side connector strip. If I trace the wire from connection 42 in the MSG, it ends at an unpopulated SMD component. So, it's essentially disconnected. I don't understand this.

Does anyone have a picture of an open AFN-MSG so I can see if the corresponding component is installed?

Andi.

On the A4, the part number is different. I looked for it on my car too.

It is located in the water tank, within the compartment containing the engine control unit, and has the number 219.

Rekord81 wrote:

Now, here's something strange: The relay is activated via a line from the MSG (pin 42), meaning that as long as the MSG is not plugged in, the relay cannot engage, and I cannot measure 12V at the automotive-side connector strip at pin 23 (i.e., connector 23). If I trace the connection 42 in the MSG, the line ends at an unpopulated SMD component. So, it's essentially disconnected. I don't understand this.

You probably missed something along the way.
Here's how the main relay is activated:

At 3:00 PM, a signal also goes to the MSG (pin 38?) and activates the switching stage for the main relay (output = pin 42), which is then connected to ground. This energizes the circuit and supplies power to the entire motor electronics.

Not entirely wrong, but incomplete.
Pin 42 has a hidden connection to an SO20 SMD component. It might be a relay driver IC. The other connection of the unpopulated SMD is directly connected to GND, meaning that if I bridge it, the relay should activate (and likely drain the battery within a maximum of 3-4 weeks).
Okay, here's what I'm going to do: Tomorrow, I'll reassemble the MSG (presumably a module or component) and measure at the relay socket to see if the relay is being activated when the ignition is turned on. If it's not, I'll temporarily solder a 200mA polyfuse onto the empty SMD pad. This should then activate the relay. If that's the problem (meaning the relay driver is likely faulty), I can build my own driver stage controlled by the KL15 (presumably a power supply line).
If the relay is being correctly activated, I have to assume that the MSG is being powered but is defective. Unless the relay itself is faulty, but that was replaced by workshop #2 yesterday.

Does the K-line only become active when the MSG is powered by voltage via the relay (i.e., pin 23), or should it already function when only voltage is applied to KL 15? Does the relay control only work with a running MSG processor?

I'm asking in advance: Does anyone know what type of voltage regulators are installed in the MSG? I won't be able to see them until I remove the heat sinks.


Andi.

guste100 wrote:

Rudi in https://community.dieselschrauber.org/en/viewtopic.php?t=15170&highlight=schreibtisch wrote: If you have the opportunity, build an adapter that allows you to connect to the engine control units. you can operate directly on your desk. ECU: Pin 1: Ground. Pin 23: 12V. Pin 61: K-Line. Diag: Pin 4,5: Ground. Pin 16: 12V. Pin 7: K-Line. "The K-Line needs to be directly connected. Both connectors require 12V and ground connections, and there needs to be an input for a power supply (12V / 1A is sufficient; the MSA15 draws more power)." approximately 300mA when it's running).

Now, connect to pin 23.

Rekord81 wrote:

1) Is the K-Line from the MSG connector directly electrically connected to the diagnostic connector? Can I stimulate the MSG with a simple voltage level to generate a receipt telegram that I can measure? "By the way, none of the preheating lights illuminated when starting the engine, but the light directly next to it (car + key = WFS?) turned on for 2 seconds and then off (which is normal for the cars I know, it's just the normal light check)." 4) Are the two K-Lines connected to MSG pin 7 and pin 61 electrically parallel, or are they two separate K-Line channels? 5) Almost forgotten: the MSG is a Bosch part, according to the sticker. Can the local Bosch service center test it for functionality, or are they unable to do anything with it?

1)

guste100 wrote:

The diagnostic communication path goes from the WFS (Vehicle Communication System) to the MSG (Modular Service Gateway). Is it still possible to communicate with the WFS? Depending on the findings, the wiring between the diagnostic connector and the WFS, or between the WFS and the MSG, should be measured and checked for short circuits.

2) When the supply voltages are switched on (according to the wiring diagram above), the MSG communicates with the immobilizer via the K-Line. You can test the function with a simple LED (of course, with a current-limiting resistor ).
3) Then that might also be your mistake. Locate the relay and check its functionality. The relay is a very common cause of errors . And if the relay doesn't engage, the MSG (Modular Sensor Group) cannot be reached via diagnostics either.
4) Unfortunately, I don't have a wiring diagram available right now, but I'm not aware that the MSG has two K-lines. However, I can only be certain about that with a circuit diagram.
5) I highly doubt that the standard Bosch service can test these things, but it's possible that they might have another 90 or 110PS TDI engine on hand that they could use for testing. But I'll say it again: In over 99% of cases, the MSG is _NOT_ your source of error . So, reassemble the MSG and first check the surrounding components!

However, the relay itself cannot be the sole cause, because if that were the case, the starting problem would have worsened more gradually. Nevertheless, it's important to check that the MSG (Main System Generator) is also receiving power on pin 23.

What is connected to the fuse for the alleged pre-charging pump? For testing, I would disconnect the pump and insert a working fuse. In the worst case, the fuse might also provide voltage for relays or other components (this is just an example, I don't have a SLP available).

guste100 wrote:

Unfortunately, I don't have a wiring diagram available right now, but I'm not aware that the MSG has two K-lines. However, I can only be certain about that with a schematic diagram.

Shouldn't one of the two wires rather be the (archaic) L-line?

Hello.
I think that's probably it with the MSG. Yesterday, I measured the voltages (at the tantalum capacitors) on the open MSG in the car. Everywhere, I got a stable 5V. The preheating indicator light is NOT on, nor is the oil warning light.
At home, on the lab table, I can't detect any activity on the K-line. I can only measure static voltages everywhere (at all connector pins and at other more or less random points on the circuit board). There's no PWM or anything else. Only the two quartz oscillators are oscillating. The MSG draws 320 mA after the voltage is applied, briefly increases to 330 mA after 1 second, and then settles down to approximately 312 mA shortly thereafter. So, it's possible that some kind of initialization process is taking place after power-up.

I'm now assuming that the MSG is likely defective, and I lack the equipment (and, most importantly, the MSG wiring diagram) to conduct further investigations.

If I purchase a used MSG (Modular Scalable Gateway), does it need to be reset in the donor vehicle?
Does a code number need to be included with used MSGs (Motor Sequence Generators) for unlocking/pairing?

Andi.