No boost and low voltage to N75

[gmac78]

Hi guys,

I've just bought a 2002 2.5 v6 tdi quattro and a problem has occured which has cut boost. Through a process of elimination I have discovered low voltage at my new N75 valve which in turn is not allowing the valve to operate and control turbo actuator (http://www.shopzilla.co.uk/rd2?t=http%3A%2F%2Ftracking.searchmarketing.com%2F click.asp%3Faid%3D1491768072&mid=261693&catId=100001009&atom=100001010&prodId=&oid=4468442712&pos=1&bId=18&bidType=0&bAmt=f4eb46813f5d4611&cobrand=2&rf=af1&af_assettype_id=10&af_creative_id=6&af_id=11444).

With ignition on or car running i am seeing around 9.5v. If i disconnect the battery for a few minutes and reconnect, then measure voltage between N75 plug ground wire and positive of battery i am seeing around 11.5- 12v (http://www.ebay.co.uk/itm/ws/eBayISAPI.dll?ViewItem&catId=131090&item=390525130427) which is what i would expect. As soon as i turn the ignition the voltage drops back to 9.5v.

I think this is the cause of my no boosting issue but not sure what would cause the voltage to drop back to 9.5v on turning ignition on or starting the engine. I've scanned the ecu for fault codes with an ELM327 bluetooth reader and TORQUE PRO APP but not showing anything, which i find strange. Not sure how good these diagnostic tools are? Any one used them for this type/year of Audi?

I have tried removing the battery cables and touching together for 30 seconds in the hope to clear ECU of any faults but this still does not allow sufficient vacuum through the N75 to operate the actuator.

I hope there are some VAG gurus out there who can help.... Someones bound to have had a similiar problem over the years,

Thanks in advance
Gordon

[Doctle Odd]

You need to scan the car with VCDS as I recall the N75 doesn't operate at 12v

[gmac78]

Hi, Thanks for the quick reply. How sure are you about that? As i connected a couple of jumper wires direct from the battery on to the N75 whilst the car was running and the actuator stroked smoothly with no problems then snapped shut or is it open? when i removed the wires. This leads me to think that it does need the 12v to operate. It's got 12v stamped on the side of it so not sure if thats the minimum voltage needed to operate?.... Your right though a proper scan is needed. Any more thoughts?

[Doctle Odd]

I knew I read this somewhere, questioning n75 resistance and voltage from ecu, maf log in the toilet - TDIClub Forums (http://forums.tdiclub.com/showthread.php?t=200451)

[gmac78]

Hmm interesting! Well i guess the best thing is to get it checked with a proper VAGCOM and take it from there. Thats my mission first thing tomorrow. I'll post my findings once it's done. Cheers.

[Alan02]

The N75 is `Duty cycled` and operates at a much lower voltage when engine running.

[gmac78]

Hi Alan, Didn't get round to getting the ECU scanned for codes today, spent most of the day out in the snow changing suspension components. Couldn't have picked a worse day for it! So you reckon the N75 works at a lower voltage when the engine is running. I did a quick voltage check today at the N75 when the car was running and it was showing 11v but there still isn't much of a VAC coming out of it towards actuator. Certainly not enough to stroke the actuator. However, when i connected jumper wires from the battery to the N75 when the car was running the valve operated and allowed sufficient vacuum through it to stroke actuator. As far as i can see so far the N75 needs 12v at least for it to operate. Any ideas why this might be? Cheers

[zollaf]

what sort of turbo do you have, a vnt or wastegated one ? can't for the life of me think what they have .

[gmac78]

It's a VNT mate.

[zollaf]

cool, just checking.

[gmac78]

Quick update guys, turns out the connection on the manifold pressure sensor had worked itself slightly loose and was no longer making a contact, so a nice and simple fix!! Now here is the next question, when i rev the engine when idling and let off the throttle i am definitely making boost once again but sounds like it's getting dumped back via the airbox. Is this correct for these engines? Sounds like compressor stall but not sure if this is normal or not? Still learning the ways of these engines so any input much appreciated.
Cheers.