Diagnosing a malfunctioning N75

[Sirocco20348]

Hi,

I just thought I would put this out to the masses to see if anyone else has experienced a similar problem. Btw credit is to be given to Gavin who has been working through this problem with me as much as he can.

Questions:
- Does an N75 fail gradually? Or is it a part that either works or doesn’t work?
- Does anyone know what is acceptable voltage for the N75 connector from the ecu loom?
- Can a faulty N75 still return an acceptable omhs value?
- After reading either the details below, does it sound like the N75 is buggered and I should replace it? Or is there something else that could be causing problems....

Cliffs Notes:
- New K03s Turbo installed with appropriate custom code tune.
- Original N75 still being used in car, 190,000kms, never been changed.
- N75 duty cycle logged at a continual 99.6% throughout Rev range.
- N75 Connector from ecu loom has power of 5.25v with Ancillary power, 7.25v with motor running.
- N75 solenoid returns acceptable ohms value of 30.5ohms.
- 1 fault found in engine module relating to N75 (01262 - Solenoid valve for boost pressure (n75) 26-10 - Output open - Intermittent.)

More information:

New K03s turbo with Custom Code K03s stage 2 tune. Gav logged some data by doing some 3rd gear acceleration tests. Straight away without looking at the data, It was clear that the car was not operating at its full potential.

One piece of data that was standing out was the N75 Duty Cycle that stayed at a constant 99.6% throughout the rev range. From what I have read this either means the n75 is faulty and stuck in failsafe mode, or there is some other problem that is limiting full boost to be obtained and therefore the N75 stays at 99.6% trying to reach the requested boost pressure.

By calculating the boost pressure with MAF and RPM values, the car reaches a max boost of 10psi by 3700rpm and falls away to 5psi by 6000rpm. These values have not yet been quantified with a boost gauge. Car had been smoke tested upon reinstallation by Jmac, and I replaced the diverter valve back to a 710N for the meanwhile as I think the seals are buggered in the forge DV copy I had installed previously.

I was initially suspicious about the N75 so I did some further testing.

I tested the N75 connector from the loom and it had a reading of 5.25v with Ancillaries on, 7.25v reading with motor running. Bentleys does not specify a valid range. I assume this is acceptable as it is a solenoid that receives variable voltage, but I hate assuming and not having definitive facts.

I also checked the ohms rating of the N75. Bentleys specified it should be between 25 and 35 ohms and mine was 30.5. So it seems the N75 is within the specified tolerances.

Vagcom was used to check for errors in the engine module. 1 error relating to the N75 which I cleared last night, and haven't had a chance to check whether it has reappeared. From reading Ross-Tech website, they say possible causes are 'Wiring from/to Solenoid Valve for Boost Pressure Control (N75) faulty (Open Circuit)' or 'Solenoid Valve for Boost Pressure Control (N75) faulty'.

I just want to rule out that the power supplied to the N75 is satisfactory at the moment, before I go and replace it.

Any thoughts are welcome

[h100vw]

Hi,

I just thought I would put this out to the masses to see if anyone else has experienced a similar problem. Btw credit is to be given to Gavin who has been working through this problem with me as much as he can.

Questions:
- Does an N75 fail gradually? Or is it a part that either works or doesn’t work?
- Does anyone know what is acceptable voltage for the N75 connector from the ecu loom?
- Can a faulty N75 still return an acceptable omhs value?
- After reading either the details below, does it sound like the N75 is buggered and I should replace it? Or is there something else that could be causing problems....

Cliffs Notes:
- New K03s Turbo installed with appropriate custom code tune.
- Original N75 still being used in car, 190,000kms, never been changed.
- N75 duty cycle logged at a continual 99.6% throughout Rev range.
- N75 Connector from ecu loom has power of 5.25v with Ancillary power, 7.25v with motor running.
- N75 solenoid returns acceptable ohms value of 30.5ohms.
- 1 fault found in engine module relating to N75 (01262 - Solenoid valve for boost pressure (n75) 26-10 - Output open - Intermittent.)

More information:

New K03s turbo with Custom Code K03s stage 2 tune. Gav logged some data by doing some 3rd gear acceleration tests. Straight away without looking at the data, It was clear that the car was not operating at its full potential.

One piece of data that was standing out was the N75 Duty Cycle that stayed at a constant 99.6% throughout the rev range. From what I have read this either means the n75 is faulty and stuck in failsafe mode, or there is some other problem that is limiting full boost to be obtained and therefore the N75 stays at 99.6% trying to reach the requested boost pressure.

By calculating the boost pressure with MAF and RPM values, the car reaches a max boost of 10psi by 3700rpm and falls away to 5psi by 6000rpm. These values have not yet been quantified with a boost gauge. Car had been smoke tested upon reinstallation by Jmac, and I replaced the diverter valve back to a 710N for the meanwhile as I think the seals are buggered in the forge DV copy I had installed previously.

I was initially suspicious about the N75 so I did some further testing.

I tested the N75 connector from the loom and it had a reading of 5.25v with Ancillaries on, 7.25v reading with motor running. Bentleys does not specify a valid range. I assume this is acceptable as it is a solenoid that receives variable voltage, but I hate assuming and not having definitive facts.

I also checked the ohms rating of the N75. Bentleys specified it should be between 25 and 35 ohms and mine was 30.5. So it seems the N75 is within the specified tolerances.

Vagcom was used to check for errors in the engine module. 1 error relating to the N75 which I cleared last night, and haven't had a chance to check whether it has reappeared. From reading Ross-Tech website, they say possible causes are 'Wiring from/to Solenoid Valve for Boost Pressure Control (N75) faulty (Open Circuit)' or 'Solenoid Valve for Boost Pressure Control (N75) faulty'.

I just want to rule out that the power supplied to the N75 is satisfactory at the moment, before I go and replace it.

Any thoughts are welcome

If you can see any voltage on the wires to the N75 they must be ok.

I'd say that it's possible to check the resistance of the solenoid in the N75 and get a good number but have it mechanically fail. Substitution would show that up. I have one here somewhere I'll dig it out and send it with the boost gauge.

Gavin

[Lito]

The n75 is basically a bleed valve that is controlled with a solenoid from the ecu.



So even if the electronics are functioning correctly the little plunger inside could be broken and bleeding all your boost away. I have bleed type mbc if anyone wants to bypass the n75 for fault finding reasons. Free to borrow, you just need to pay postage. New n75s are pretty cheap at the dealer (~$70). I think they are up to revision e now.

[Sirocco20348]

Thanks Gavin! That will be icing on the cake to cover off on the mechanical side of the N75.

I tend to agree with the both of you, Electronics are fine, but mechanically it has failed.

I have found a tutorial showing that I can connect 12 volts to the N75 and I will hear it open and close quite clearly. Only problem is I don't have spare electrical cable to try it out.

I will await the parcel from Gav!

[h100vw]

Thanks Gavin! That will be icing on the cake to cover off on the mechanical side of the N75.

I tend to agree with the both of you, Electronics are fine, but mechanically it has failed.

I have found a tutorial showing that I can connect 12 volts to the N75 and I will hear it open and close quite clearly. Only problem is I don't have spare electrical cable to try it out.

I will await the parcel from Gav!

Not sure how good your VCDS a like cable is but there is an N75 test in the output tests of the ENGINE. You can hear it click

Parcel is en route now. Be there tomorrow.

Gavin

[Sirocco20348]

Not sure how good your VCDS a like cable is but there is an N75 test in the output tests of the ENGINE. You can hear it click

Woah I never knew of this vagcom functionality! And yes I have access to it! It's not as fancy as the newer version of output tests in the latest vagcom, but it still works.

I just ran the test and the N75 clicks fairly well... on-off-on-off..and so forth....Sounds mechanically fine.

Ill replace with the loaner N75 and see what happens.

[Sirocco20348]

Hey guys, just to close off on this thread.

My N75 was operating correctly.

All the tests I ran above suggested that it was fully operational. But I'm 100% confident on this info now.

My problem was a boost leak with a vacuum hose.