URs4 engine trouble

[S2central.net]

Well the Bentley diagram from Dave is a big help - the AAN wiring is slightly different with regards fuel pump wiring based on 1st look.

However - assuming the AAN ECU is wired same internally as ABY then I am confident that pin 37 goes to 12V when key is ON. This should see 12V at the black/red wires on the AAN injectors and black/red on the MAF pin 5... Same as ABY except for wire colours.

@prj - go and try it out if you can.

[S2central.net]

FYI - This is the thread that started the whole pin 37 confusion... when I was wiring up an LED onto FPRy so I could see when it was activated by ECU. http://www.s2forum.com/forum/showthr...lay+LED&page=4

Using pin 87a for anode is at 12V when key is ON - whereas pin 87 for anode can only be 12V when relay is energised by ECU.

If you look at Bentley AAN schematics carefully... follow the fusebox connection from 31/87a on the FPRy which emerges on Track 105 - also labelled 87a - as a BLUE wire.

This blue wire feeds into S75 to protect supply for N71, N75 & N80
The blue wire also links to Track 44 where it feeds into S72 that protects supply for injectors and MAF. The fused side of this is black/red and also connects to ECU pin #37 - see track 56.

So when the ECU is powered up that pin 37 goes to 12V - and brings FPRy pin 87a to 12V at the same time as supplying juice to MAF, injectors and such.

When the ECU is happy the engine is turning over - THEN the FPRy is energised such that pin 87 goes to 12V (see track 51) and that is only way to get 12V into the fuel pump and the lamba probe heater (Track 59).

Thats the theory of AAN operation and it looks same as ABY - just need some AAN owners to prove me right - or shoot me down in flames of shame

[UrS4boy]

What I see is 31/87a is NOT powered from line 30 of the bus until the FPRy is energized by the ECU.

Prj, please test that ECU your desk, monitoring the pin 37 voltage while the ECU is powered a) through pin T18/55 (constant power) and then b) when you add power to pin T27/55 (switched power). If you get 12v coming out of pin 37 at any point, I will be shocked (pun intended) and confused. Eventually, I will get over it.

[S2central.net]

My thing is - why / what / would anything inside the ECU need to wait until the FPRy is energised before supplying it with 12V ?

Is 'what you see' about 87a from looking at the schematic or actually what happens on your UrS4 - do I need to annotate the diagram to help you more.

PS - I don't care who is right or wrong on this - absolutely not trying to score points here - I just seek the truth - if its out there !!!

[UrS4boy]

- do I need to annotate the diagram to help you more.

Paul: Without further data from prj (or me, I have a spare ECU and a multimeter), your statement *could* be construed as condescending. I don't *think* you meant it that way, right?

[S2central.net]

@Dave - not condescending at all - is just that not everyone can read these Audi wiring diagrams properly. Electrical stuff is my thing by trade - but you could be a dentist for all I know... Hell I missed the pin 37 thing on ABY until last October - I thought the diagrams were wrong at first.

Here is annotated version of relevant sheets. I have highlighted the current path from Pin 37 on ECU. I hope you will see my logic now - just remains to be seen if the AAN ECU behaves same way as ABY on this one.

Cheers,

Paul

[S2central.net]

Here is a subtle difference though chaps... I see that the AAN drawings show the fuses S72 and S75 as 'circuit breakers' with a time delay. IF these are an active overcurrent protection device - with a diode that prevents reverse current flow - then when ECU #37 (key ON, engine stopped) is at 12V it will only reach the injectors and the MAF - it will not pass 12V up to 87a on the FPRy or N71, N75 & N80.

I know for sure the ABY uses good old mechanical fuses - but if AAN uses these active devices it could explain a difference at the FPRy terminal but my bet is that ECU #37 still goes to 12V via internal relay when key is ON.

[prj]

but my bet is that ECU #37 still goes to 12V via internal relay when key is ON.

It does. Just tested.
This is with the various grounds connected and +12V permanent supply and +12V ignition supply. Measuring 12V between Pin 37 and ground.

From what I have seen, AAN and ABY ECU's are completely identical anyway.

[UrS4boy]

@Dave - not condescending at all - is just that not everyone can read these Audi wiring diagrams properly. Electrical stuff is my thing by trade - but you could be a dentist for all I know... Hell I missed the pin 37 thing on ABY until last October - I thought the diagrams were wrong at first. Here is annotated version of relevant sheets. I have highlighted the current path from Pin 37 on ECU. I hope you will see my logic now - just remains to be seen if the AAN ECU behaves same way as ABY on this one. Cheers, Paul

I for one can follow the little [105] track boxes (Ph.D. in Environmental Engineering - not that that necessarily helps me.). What your "annotated" diagram lacks, to me, is current flow direction. It makes no sense to me that current could flow back *to* 87A, as in what it the point of the FPRy energizing 87A if everything down stream is already energized by mearly having the ignition key on. I see from prj's post that you are right, i.e. 37/55 is hot with the key on. So now I am confused as to Audi's logic, not what you are saying.

[S2central.net]

It's a load balancing thing. With key on the current flows from ecu only via pin 37.

But when engine is running the current to injectors etc *also* comes from FPRy - this must be because the internal ecu relay or ecu wire tracks cannot deliver high amounts of current when the injectors are running at high duty cycle.

I think it also nicely allows the injectors and MAF to be powered up before the engine is actually cranking.

[S2central.net]

The yellow annotation shows all the points that have 12V. There is no current flowing as such - except into MAF while key is just on... All the injectors, ISV etc are activated by switched earth via the ecu - only then will current flow thru an injector.

[UrS4boy]

It's a load balancing thing. With key on the current flows from ecu only via pin 37. But when engine is running the current to injectors etc *also* comes from FPRy - this must be because the internal ecu relay or ecu wire tracks cannot deliver high amounts of current when the injectors are running at high duty cycle. I think it also nicely allows the injectors and MAF to be powered up before the engine is actually cranking.

Okay. I guess I have to "buy" the above (including the current flow thing in your subsequent post). I guess I'm not too old to learn. (or be "schooled")

Not to cross threads but back to Durr McDank's AAN into B3 swap issue, I still think that pin 37 needs to be connected to 87A on the FPRy via the fuse/breaker. The reason why is different than my original contention but I still think that connection needs to be made. May or may not be Mr. McDanks's non-starting issue.

[S2central.net]

www.allaboutcircuits.com is a great place to start on understanding some simple electric circuits. That will help explain the difference between voltage and current and how they are related by resistance and Mr Ohm !

[pasbogdan]

how is your ground conection to coils?what about engine ground under the turbo?

[Gormj]

I tested another ECU in my car today, and it started :-)