Motronic Mysteries!

[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.

[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*.

[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.

[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]

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]

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]

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]

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

I guess you got your answer?

[prj]

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.

[prj]

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

[Rocco]

Come on,let's speak about motronic secrets

[twoqu]

Thanks for the offer but its not necessary! Just a bit of Motronic trivia!
Its worthwhile raising the issue of the holding relay as not everyone is aware of it and its function!

[prj]

You are probably correct in that there is some power from the switched pin that energizes the holding relay to grab power from the No. 30 bus wire. However, that answer would only be confirmed *IF* we had Motronics Schematics. And *we* don't (generally). Some people *might* have them but such diagrams are considered gold and would have had to been smuggled out of a Bosch facility on a microdot stuffed up somebody's backside.

Is fuel relay the same as holding relay?
Well anyway, fuel relay is activated by fuel chip (SAB80535 on top board) by pulling a certain pin high if the time between the pulses of G28 goes below a certain number.

I remember finding the code in ASM a while ago.
If I stumble upon it again, I can post it for you if you like. Not sure if it's of any use though?

Nothing to see here, move on

EDIT:
Just read - holding relay is something different.
I can look if you want...

[UrS4boy]

See! Thats why the thread was called Motronic Mysteries! If it hadn't had been for those pesky kids... Fair enough, nobody knows for sure. I can accept that. Considering its design is over 20 years old its very clever.

Well prj (Dmitri) *might* know but he would NOT tell us, and if he did, he would tell us that he told us the answer before.

Paul N. probably knows and would tell us but he hasn't been around much lately.

[twoqu]

See!
Thats why the thread was called Motronic Mysteries! If it hadn't had been for those pesky kids... oh no, thats Scooby Doo!
Fair enough, nobody knows for sure. I can accept that. Considering its design is over 20 years old its very clever.