Fuel Pump Relay: Buy Online Now!

Hi!
Today, I tried to create a proper wiring diagram legend based on the schematics. Unfortunately, the original one only uses pictograms and is a joke. I also created a wiring diagram for my relay panel. However, one relay, labeled "405," remains unclear.
According to the description in the wiring diagrams, it's called "Engine Control Unit Fuel Shut-Off Relay for Diesel." However, I can't find this relay anywhere in the wiring diagrams themselves, neither near the engine nor the airbag. If I remove it, the engine won't start. The starter motor turns, but the engine doesn't fire up. It's NOT relay 109, which supplies voltage to the EDC (Electronic Diesel Control).
So, what exactly does this relay or control unit do? Wouldn't it be easier to implement fuel shut-off by simply having the EDC cut off power itself via the 109 connection? By the way, I don't have an additional fuel pump in the tank!

Best regards,
Dirk.

Hi!
I'd like to give myself an answer now. I found this relay in the SLPs, after all. I must have really had my head in the clouds back then.
It can be found in the engine diagrams.
The relay is equipped with:
Terminal 31.
Terminal 15.
Terminal 30.
a connection to the airbag control unit.
as well as a signal line to the engine control unit (ECU) that, when the relay is engaged, connects terminal 15 to the ECU via a normally closed contact.

Okay, so: When the airbag control unit detects an accident, it cuts off power supply (terminal 15) to the engine control unit, which causes the engine to shut down. Since the wire leading to the engine control unit is only 0.35mm², it's likely just a signal line.
It's questionable whether a relay malfunction or oxidized contact will result in an error entry in the control unit, or whether the motor simply shuts down... which could be another source of error causing jerky movements.
Nevertheless, the question remains: why not implement this using relay 109?

Best regards,
Dirk.

brezelmann01 wrote:

Despite this, the question remains: why not implement this through relay 109?

Hi Dirk,

Are you sure that completely shuts down the engine? Perhaps it's just sending a signal to the engine control unit (ECU), which then switches to a different program or something like that...

Have you ever activated the relay (if it is normally energized) or bypassed it (if it is normally de-energized)?

Regarding the 109-series system, MSG typically maintains its power supply for a short time even after the engine is shut off. To ensure a clean and gentle engine shutdown, the 109-series system likely requires a very abrupt engine cut-off.

Best regards,
Guste.

Quote:

Are you sure that completely shuts down the engine? Perhaps it's just sending a signal to the engine control unit (ECU), which then switches to a different program or something like that... Have you ever activated the relay (if it is normally energized) or bypassed it (if it is normally de-energized)?

Yes, I did that. Then, when I tried to start the engine, it didn't start. The starter motor was turning (as I mentioned above ).

Quote:

Regarding the 109 model, the MSG typically maintains its power supply for a short time even after the engine is shut off, in order to ensure a clean and smooth engine shutdown.

"You're right. I've experienced the same thing. My thought was that, in the event of a crash, you could signal the engine control unit to "turn off the engine." Whether it then shuts down gently or abruptly is relatively unimportant."
Perhaps this feature simply wasn't possible yet, given the very limited use (thankfully!) of the CAN bus at that time.

Dirk.