causes of machanical boost?

[driftin jo]

it will not through a code.

[driftin jo]

I wasn't sure if it was only the 3b but Paul offers the Valet Mode on the ABY so it must be applicable. I doubt whether this would throw a code either.
Anyway, must be worth investigating as its clearly something that could be preventing boost. Might just be something like a broken wire!

it will not through a code

[twoqu]

According to S2Central pin out, Pin 46 in the ecu has the input from F76 R pin2 bl/ws colour.
So that is a good place to start. I am not sure whether it should have voltage on it or be grounded when activated.
Paul talks of pulling it to 0V. So not sure if he means open/closed circuit or grounding it.
So I would measure that pin to see whats on it at the moment with engine running and then check continuity back through the round connector back to the F76.
It might be that the wire is broken somewhere and the signal from the F76 is not reaching the ecu!

[UrS4boy]

Might just be something like a broken wire!

Or a bad connection that works when they test the ECU to N75 signal via the ECU test procedure BUT then fails to work when the engine is running.

This could be related to the Holding relay that back flows 12V juice to the N75 via the S75 thermal fuse prior to starting OR MAYBE MAYBE it is an S75 issue. Lately there have been issues with the S75 style thermal fuses getting old, tired and dirty. It might have enough integrity to carry voltage during static N75 stimulation tests (when the key is just "ON" (engine isn't running) and 12v juice flows to the N75 from the Holding relay) but it might not have enough integrity when the ECU finally triggers the J17 fuel pump relay and 12v flows to the N75 via the FP relay pin 87A and the S75.

First thing I would do is replace the S75 with a normal 15 amp fuse and see what happens. (I am not sure where the S75 is located in a S2 so these diagrams only relay to the idea (as applied to an AAN in an UrS4/S6):

[s2tommy]

no fault codes at all to be found, with scanner or vag com

[twoqu]

Thats what we are saying. If the F76 signal or lack off shuts down the boost control it doesn't cause a fault code to be logged as it might just be a temporary thing.
Obviously if you another vehicle handy then just see whats on Pin 46 of a car that works. If you have the same then its another thing eliminated. If its different you might be on the right course.
Its seems consistant. Its something related to the vehicle and its not a primary input of the major sensors!
See how you get on and report back.

[s2tommy]

the last two ideas will be checked first thing in the morning, and report back findings!! thanks to everyone for the help it seems some have into detail looking things up and spending time doing it. cheers tom

[prj]

OR the OP is relying on the factory digital boost gauge for the boost reading and has not confirmed the readings with a analogue mechanical gauge.

I am trying to map that car. I would say my SLC DIY with a digital MPX4250GP pressure sensor is good enough to measure boost.

But are you trying you ecu complete with your chips?
If you permentantly ground the wastegate valve by bridging the switched to the ecu ground and it boosts that makes sense. Also if it operates from the output test then that would suggest the wiring down to it is ok. So it would appear it operates at least fully open or fully closed. Question then is what controls the duty cycle.
If your ecu works in other peoples car with their chips but not in your car with your chips, I would have said i was your chips! However it does not explain why other peoples ecus produce the same results!
I can see how it sort of defeats logic.

It's not the mapping in the ECU or the ECU itself.

Right! So your complete ecu works in other cars but their working one does not in yours?
It might be worh investigating how Paul N does the "Valet" mode modification. It's on here somewhere. It's obviously some sort of input to the ecu which restricts boost.

"Valet mode" is just reducing the reading of the altitude sensor, nothing else.
Altitude sensor is perfect on this car.

Anyway, leave the ECU issues to me... there are none with the ECU's themselves, three different ones have been tested on this car, actually this car has had this problem for years... through different specifications, engine rebuilds etc...

Read this regarding how the ECU responds to high boost signals or bad MAF signals in controlling the N75:

Thank you, but I can recite that slide from the top of my head.

Is this a 3B? As dkw_dude says above there is something funny about the MFTS switch disabling boost under certain circumstances. I remember Paul talking about that in another thread.

That is very interesting, best lead I have had so far, thank you.

[UrS4boy]

Prj/Dmitri: Always good to get your patented condescending reponse(s) when all we are trying to do is give the owner some alternative ideas in case something has been overlooked.

Have fun. I'm done on this thread if that is your attitude.

[prj]

It was the MFTS.
No code whatsoever...

Thanks for the tip. I was 100% sure that the MFTS has nothing to do with the ECU.
And I was right - it doesn't in AAN, which is where I looked at the plug and checked which pins are used and present or not.

Please tell me why the hell Audi would do this and not really document it very well (says nothing about it in the ECU handbook)... and not even throw up a code.

Oh well, there is now officially no car that I can't fix.

[UrS4boy]

It was the MFTS.....Oh well, there is now officially no car that I can't fix.

That is, *thanks* to forum members who provided you with the clues that you were missing. Looks like you don't know everything after all.

I presume the F76 MFTS failed in a way that it made the ECU think that the coolant temperature was over 119 C. You really need to do more research Dmitri. I guess you can't recite this "slide" from the top of your head yet:

[prj]

Ha, did not know that indeed.

Well, I was 100% sure that it wasn't connected to the ECU because I learned everything I know about these cars on my car, which is AAN.
Bad excuse really, but I never gave it a second thought *because* of that fact.

And yes, as I said in the previous post - thanks for the tip.
There was tons of other things wrong on the car which we managed to fix though.

[prj]

Btw the stupid thing is - it actually is perfectly showing the signal of G62. It uses that for all of it's inputs as well.
The connection from the MFTS to the ECU is either just a switch or a threshold, which is only responsible for turning off boost control.
There is not even a diagnostic function in the ECU to check the reading of G62 against MFTS, so even though it reports *coolant* as fine and does not throw a code, it drops the boost.

Looks like a bit of a bodge if I may say so.

[newsh]

That is very interesting, best lead I have had so far, thank you.

MFTS has been the bane of many 3B owners!

I should have suggested this earlier but I got stuck thinking TPS...

Have fun. I'm done on this thread if that is your attitude.

Bye

It was the MFTS.
No code whatsoever...

Thanks for the tip.
Oh well, there is now officially no car that I can't fix.

from 3B to AAN you've got them all covered

That is, *thanks* to forum members who provided you with the clues that you were missing. Looks like you don't know everything after all.

Hello back

Ha, did not know that indeed.

And yes, as I said in the previous post - thanks for the tip.
There was tons of other things wrong on the car which we managed to fix though.

Looks like a bit of a bodge if I may say so.

Its a 3B, say no more..

[twoqu]

One of the lesson to learn from this is that both the 3b and the ABY (not sure about the ADU) have this overheating alert from the MFTS which can be caused by a faulty sensor. The AAN does not.
It looks like this will limited boost to the mechanical spring pressure when tripped withoul leaving a fault code. I guess this is because it might be a temporary condition.
I don't think the same applies to the the altitude sensor. Even if this fails completely a substitute figure is used which limits boost to the bottom of the permissable range, not just mechanical. The function of the altitude sensor being to allow different boost levels to take account of air density at elavation.
Anyway, not being an expert, I stand to be corrected.