Motronic Mysteries!

[twoqu]

In particular, the holding relay within the ecu itself.
I would like to better understand its function, so please add to or correct me if I am mistaken.
The ecu gets fed bat 12V from T30 through fuse S27 into pin 18. This is one side of the holding relay.
Pin 37 feeds out from the ecu to fuse S28, the 87A terminal of the fusebox/fuel pump relay and onto the injectors/MAF and other components. This is the other side of the relay.
I have read that the holding relay's purpose is to provide 12V to the various components with the ign on but before starting has occured. It also controls the MAF burn off with ign off. It also might have some function for the output diagnosis.
Once the car has started the fuel pump relay provides power to those components from 87a.
So my questions are:
What energises the holding relay - Is it the ign 12V signal at pin 27?
Is the relay still energised when the car has started?

[UrS4boy]

Power to Holding Relay

Via the constant on Bus 30, a fuse and pin T55/18
http://forums.quattroworld.com/s4s6/msgs/102009.phtml

[UrS4boy]

I am confused. In another thread, and only a few minutes before you posted this, you had it figured out. What happened?

http://www.s2forum.com/forum/showthr...969#post593969

[twoqu]

I sort of understand now what it does, I was just wondering how!
It might help someone if we know what goes in and what comes out and when.
As its a relay, it must be tripped by some signal. If its the ign feed from S32 into pin 27 that makes sense I guess.
For example if someone has power at pin 18 and 27 but not at pin 37 with ign on, we can suspect that internal relay.
Presumably the reason why this is done this way is some safety related issue in respect of the fuel pump running only during cranking/running. Its interesting that the fuel pump relay sort of boosts the supply but for arguments sake what would happen if that particular contact 87a didn't energise, can the holding relay supply enough juice?
Just wanted to understand it a bit better as potentially considering the age of our vehicles it may well be a cause of troubleshooting headaches down the line.

[UrS4boy]

I sort of understand now what it does, I was just wondering how!....Just wanted to understand it a bit better as potentially considering the age of our vehicles it may well be a cause of troubleshooting headaches down the line.

Gee, with the info in my post, including Paul N's detailed diagram of how the Holding Relay internals work, I didn't think there would be anymore questions. Did you read this:

http://forums.quattroworld.com/s4s6/msgs/102009.phtml

[UrS4boy]

Maybe this additional fuel injector wiring post, and how it relates to the Holding Relay will help (?)

http://forums.quattroworld.com/s4s6/msgs/90802.phtml

[twoqu]

Gee, with the info in my post, including Paul N's detailed diagram of how the Holding Relay internals work, I didn't think there would be anymore questions. Did you read this:

http://forums.quattroworld.com/s4s6/msgs/102009.phtml

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.
What switches the relay so that power flows through it from pin 18 out from pin 37?

I have read through all the info you have kindly supplied but cannot see the answer. Clearly the holding relay is not energised with the ign off, other than when the engine is switched off for MAF burn off etc( I would be interested to know what controls that as well actually!).
I can't see that is a sensor signal as the relay supplies power before the starting condition. So the only difference between ign off/on is that feed into pin 27 which prepares ecu for possible start.

It's not a biggy. Nothing to lose sleep about! For all I know there might be nano Bavarians in their Lederhosen moving the relay contact!

[UrS4boy]

... For all I know there might be nano Bavarians in their Lederhosen moving the relay contact!

Exactly. Now we have to shoot you.

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.

[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.

[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.

[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...

[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!

[Rocco]

Come on,let's speak about motronic secrets

[prj]

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

[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.