Steuerproblem PD-Motor: Rätselhafte Ursache

Hello,

The TDI engine control units in the Polo 9N do not have a constant power connection; instead, they switch on their power supply themselves via relay 109, using a control signal from terminal 15 (ignition).

What seems strange to me is that, in addition to the other connections, pin 88 of the engine control unit (ECU), which is one of 121 pins, is connected to a separate fuse called SB 5 on the X terminal (according to the wiring diagram, this is also the case for the Euro 3 ATD, while on the AXR engine, pin 88 is not connected).
This pin draws only about 5 mA of current and is therefore likely a pure input signal.

The X-connector powers various peripheral devices and interrupts their power supply during engine start-up to reduce the load on the battery.
So, the engine control unit detects the voltage drop of the X-contact to 0 volts, indicating that the starter motor is engaged – but what is the subsequent reaction?

Initially, I thought it might be a trigger-related procedure, such as the injection of the starting amount or the activation of the pre-feed tank pump, but when the SB 5 is engaged, the engine starts just as it always does; also, monitoring the pump activation via the LED showed no differences.

My next thought was to suppress error messages that might be caused by a drop in the vehicle's voltage during starting, specifically for monitored solenoid valves for the EGR system, turbocharger boost pressure, and shut-off valve, as well as for the glow plug relay, etc.
However, without SB 5, the corresponding errors (when the valve/relay connector is disconnected) will also be logged, including the error message for a faulty EGR system, which is derived from the mass airflow sensor (MAF) signal.

After that, I drove for a while without the SB 5, but I didn't notice any difference at all (in terms of fuel consumption, performance, handling, etc.).

Does anyone have any insights into what the X-contact achieves in the area of engine management, and why the Euro 4 AXR engine can function without the specific "start" signal?