Mysteries of Pin 37 revealed

[UrS4boy]

Thank you for that Paul. Some questions:

1. How much current do you think that relay can handle? Previously you have suggested that once the engine has started the current to the injectors comes to them through the fuel pump relay. At some point while running does power from Pin 18 to Pin 37 via the relay get shut off or is it on all the time? I know that power is supplied to the MAF after the engine is turned off via this pathway (18 to 37).

2. What is SSP 218 (self-study program on what topic)?

Dave F.

[S2central.net]

1 - I could only guesstimate 20A limit - it is slightly smaller than a normal relay you would find in the fusebox. When the FPRy is activated (when ECU is happy that engine is spinning fast enough) then the FPRy *shares* the job of delivering current to injectors etc - its not an either/or thing - unless the ECU holding relay is opened again after engine start... I don't know yet but I will find out soon when I blow some cobwebs off the engine simulator project. I wont embarrass myself with a photo of the state of my garage as evidence that I cannot do any work in there for a well. Life is rather overtaking me at the moment. Going to have a massively satisfying spring clearout this year.

2 - SSP218 is actually an Audi of America publication. One of the first that I found when collecting up Audi documents - it covers the AAN and V8 engines. Another number you can reference it by is WSP-521-218-00. It is essentially the same info as SSP143, but in black n white - with 'murrican engrish spelling. SSP143 is superior with lovely colour pics and easier to read descriptions.

[UrS4boy]

Question about the engine starting with a failed Holding Relay

As promised... the circuit... ECU chip S220 controls a switched path to 0V to energise the relay. I don't yet know if software is involved in that decision - I doubt it - more likely hardware timer driven when key turned on/off.

This is common to 3B, AAN, ABY & ADU.

Paul (and prj if you want to chime in): I compiled a Holding Relay post the other day on my home forum. http://forums.quattroworld.com/s4s6/msgs/102009.phtml

Two questions (first on for Paul, second one for both Paul and prj and anyone else)

1. Should I include a version of your electronic schematic?

2. If the holding relay fails, will the engine still start, i.e. get the power for the injectors from the J17 fuel pump relay (after the ECU counts 23 rpm, etc)?

The second part of question 2 is, if the holding relay fails and the engine starts, the MAF won't get its burn-off power when the engine is turned off. I wonder if this isn't a "hidden" issue and a cause of excessively dirty MAF hot-wires.

[S2central.net]

1 - no probs with the schematic... as long as credit is there

2 - engine will not start if holding relay fails... it provides power to the chip that creates the +5V reference for MAF, CMP etc - so even if the FPRy is energised when RPM is fast enough - there won't be supply voltage for MAF etc and there is no spark if CMP isnt energised

[UrS4boy]

Thanks Paul. Another "theory" shot down in flames.

[S2central.net]

To close this one off... the easiest way to test if your holding relay is working - without removing anything... is turn the key ON and use a voltmeter at the white OBD plug on the 'K' wire. That should be approx 0.7V lower than actual battery voltage - if not then the holding relay is dead.