Motronic Mysteries!

[prj]

Yea, found the ASM code.
It's all in the fuel chip.

I guess you got your answer?

[UrS4boy]

EDIT:
Relay is turned on by providing +12V switched to it.

Yes, we *think* we have that figured. However, do you know for sure that the power comes in via Pin 27 and then to the holding relay?

[prj]

Yes, we *think* we have that figured. However, do you know for sure that the power comes in via Pin 27 and then to the holding relay?

Do a continuity test. Pretty much the only thing Pin 27 does is power up that relay and that relay powers up the whole unit.
The unit then can decide when to shut itself down.

[UrS4boy]

Considering it says that "engine speed must have exceeded 2000 rpm and temperature 30C", only software in the fuel chip can make that sort of decision.

But that is about the decision to continue to provide power to the MAF to burn off the residue on the hot-wire anemometer when the ignition is turned off.

The original question related to when exactly is the holding relay is energized during an ignition on sequence.

[prj]

It's energized instantly, because it's directly connected to Pin 27.
The fuel chip turns it off again, it has a few control lines going into a messy circuit near it, and it has ASM code in place which makes sense, I don't want to reverse engineer that further really.

[UrS4boy]

Fuses that feed the ECU and the ignition coils

It's energized instantly, because it's directly connected to Pin 27.
The fuel chip turns it off again, it has a few control lines going into a messy circuit near it, and it has ASM code in place which makes sense, I don't want to reverse engineer that further really.

Apologies. I see what my problem is. The AAN Motronic diagram that I was using is wrong. It shows the S64 as a 5A fuse that is powering the Holding relay. And then it does not show any fuse between switched power and the ignition coils. This is wrong. The 5A fuse is the S26 and its purpose is to supply constant power for the ECU memory via pin T55/18. On that basis I *think* Paul N. has it wrong regarding Pin 18 feeding power to the Holding Relay. It is Pin 27 via the S64 fuse that feeds the holding relay.

Incorrect AAN Motronic diagram:



Above ECU fuse call out:





Now if Paul N. would just comment.

[UrS4boy]

It's a very pretty diagram but unless I am being very thick (quite possible) shows the relay in its unswitched state. I guess the diagram shows the electronic gubbins, resistors, diode etc, which is interesting but doesen't answer my fundamental question.

Apologies. That "pretty" diagram is wrong. *I think*.

[prj]

The reason that relay is there is so the unit can shut down when it deems it to be safe.
For example to perform some housekeeping on memory, burn off the MAF wire, and whatever else it needs to do, before it decides that it is "done".

If you were to just abruptly remove power, it could be in the middle of doing something and could for example corrupt it's fault memory.