n75 wgfv.

[Human Joist]

Where is the ground that the ecu uses so I can check that one too.

Also is relay 31 the fuel pump. I had one left in the door pocket when I got the car. Don't know if there was an issue before I got it

[Monkeymagic]

what is interesting is that while testing the wires to the n75 i shorted it and blew the 10 amp fuse for the n71 and the n18. I believe the n75 is on the same fuse as these two. Both the n18 and n71 threw a fault code saying short ground to earth as expected it does not throw a fault even with the n75 connected. its like it doesnt know its there. Does that make any sense to anyone?

[UrS4boy]

what is interesting is that while testing the wires to the n75 i shorted it and blew the 10 amp fuse for the n71 and the n18. I believe the n75 is on the same fuse as these two.

Not sure what an "N18" is but the N71 ISV and the *N80* Evaporative Emissions Control Valve are on the same fuse as the N75, as shown in the Motronic wiring diagram I posted previously in this thread. I also noted that power to these three devices would only be provided IF the engine was running, i.e. the J17 fuel pump relay was powered up by the ECU (after getting good signals from the G4 crank sensor and the G28 engine speed sensor (particulary the G28) for the J17). ECU ground is through the harness to the cam cover and/or the intake manifold and then through the ground strap from the engine to the frame rail by the exhaust side motor mount.

[Monkeymagic]

Brilliant the engine was running and I meant n80 as per diagram
i'll clean the earthy many many thanks

[Mikes2]

pin 23 and pin 37 are the ecu contacts for the wgfv

http://s2-audi.co.uk/workshop/aby_motronic/aby_m.pdf

p24-41 for the full diagnostic test procedure

[Monkeymagic]

We've checked the grounds all is well when I unplug the n75 it does not register an open ccircuit which is odd
I drove the car and it's cuts boost about 90 mph in 6th
I see no reason to suspect anything other than the n75 as it is clearly not responding to output tests
Jim has checked the continuity on terminal 33 and it responded
why is the n75 apparently not responding? Answers on a post card please

[Greg_S]

Could it be the mfts miss reporting temps?

[Human Joist]

Cheers for suggestions. But brand new genuine mfts replaced last week as it had failed. Did not sort it

[Monkeymagic]

However when I disconnected the Mfts the n75 did click like mad!

[twoqu]

I wonder if the MFTS is malfunctioning, in particular the overheat signal. That would in theory disable the WGFV and reduce boost to mechanical level. Disconnect the MFTS and then try the output test. If it works then it either a faulty MFTS or the wiring back to the ecu.

[Monkeymagic]

There is slight corrosion on the wiring pins for the Mfts as the old one was leaking into the boot slightly

[andy10v]

Have you tried taking it for a drive with the mfts unplugged? When my plug/pins were bad my car (3B) would run perfectly with the MFTS unplugged, or spring boost only with it plugged in.

I would have thought the n75 would still click from the output test though?

[Human Joist]

For the sake of thoroughness how do I check the wiring for the mfts. Presume 14v from one etc etc

[twoqu]

That overheat signal is something that does not leave any code.
Pull the connector off and then try the output test. That will prove whether its the issue.
Theory being that the overheat signal is disabling the WGFV via the ecu. So its ignoring you disconnecting it etc.
Just a theory, quick to test. Once you have established the circuit is intact, it would point to the MFTS or indeed the terminals/wiring. Not entirely sure whether it expects a voltage or a closed open circuit but removing the connector usually removes the signal!

[andy10v]

For the sake of thoroughness how do I check the wiring for the mfts. Presume 14v from one etc etc

The problem is normally corrosion on the contacts.