causes of machanical boost?

[s2tommy]

We have done all the obvious 3 or more times now, I hav changed the wastgate just to rule out a good looking one too!

[s2tommy]

Wat can cause the ecu to stop pulseing the earth wire to the n75???

[twoqu]

So you have swopped known good ecu's into yours and have the same sympoms. That would indicate a sensor/wiring problem on your car.
You have then put your ecu on to other peoples cars with their chips and the vehicles boost correctly? If you have a 4 bar map sensor (Motorola?) are the other peoples chips mapped to that?
Just wondering if there is some incompability between the chips and the map sensor as unless the other peoples chips were compatible with your map sensor, then surely they wouldn't exhibit the right boost characteristics. Has the boost chip actually got data written on it?
Not sure if anyone would know if the output check is a seperate function, i.e. doesen't use the chips to run. I guess it might be just purely an electrical check if you see what I mean by grounding.

[s2tommy]

Ya the others are mapped up on the 4bar sensor.I don't know wat make the sensor is but no problem with no one elses car! That's wat we are thing, a electic fault but don't know where or wat to look at! If there was just a fault code then we wud know where to look. Everything looks fine an doin wat it should be on vag com????

[prj]

What about the obvious the wastegate is knackered, split diaphram or is stuck open

Uhm, how about... I wrote that if I ground the N75 manually the car has full boost all the time? Needle flies to about 34 psi before you can even let off?

Yeah, there are no boost leaks, stuck wastegates etc on this car.
Something is making the ECU request zero boost, but all the usual sensors are fine and no faults are coming up.

[UrS4boy]

What about the obvious the wastegate is knackered, split diaphram or is stuck open

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.

[twoqu]

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.

[twoqu]

Does the car have aircon or has had it removed?
It's ABY right?

[Ross]

Tom's ABY coupe did have AC, now removed. They need to put a "Why am I not boosting?" button in vagcom (next to a "why am i not starting" button...)

[s2tommy]

Ya using my chips well I am trying to get me car remapped!! But just can't as it won't make boost!! Been chaseing this fault now for 3 day an just at dead end with no other opp but for it 2 be mapped on machanical boost!!

[twoqu]

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.

[UrS4boy]

Uhm, how about... I wrote that if I ground the N75 manually the car has full boost all the time? Needle flies to about 34 psi before you can even let off?
Something is making the ECU request zero boost, but all the usual sensors are fine and no faults are coming up.

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

[twoqu]

This is what I was thinking of:

http://www.s2forum.com/forum/showthread.php?t=24254&highlight=valet+mode

If I read that correctly then clearly the R terminal signal can limit boost. I would quickly check the wiring from the switch to the round connector for continuity. Unfortunately he hasn't said how the ecu gets the signal which would help. So either PM him for an answer of spend the evening trying to work it out from the schematics!

[mcandmar]

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.

As twoqu says it sounds like either wiring/sensor issue or the chips not driving the N75 circuitry.

[twoqu]

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!