05 passat ECU wiring harness check

Can anyone tell me how to check the wiring harness from the ECU to the N-75 valve on a 2005 Passat TDI?

Also does the Mass air flow sensor control the N-75 valve?

Back up a bit. What problem are you having that you are trying to correct?

The MAF and the N75 are both wired to the ECU, the MAF is an input, the N75 is an output. If either one had a wire problem, the ECU would have a DTC to that effect. Many times a low boost problem will also be accompanied by a 'MAF implausible' type DTC, along with some sort of 'boost deviation control range not reached' type of DTC. A boost leak somewhere in the charge air tract is the likely cause there.

oilhammer said:

Back up a bit. What problem are you having that you are trying to correct?

The MAF and the N75 are both wired to the ECU, the MAF is an input, the N75 is an output. If either one had a wire problem, the ECU would have a DTC to that effect. Many times a low boost problem will also be accompanied by a 'MAF implausible' type DTC, along with some sort of 'boost deviation control range not reached' type of DTC. A boost leak somewhere in the charge air tract is the likely cause there.

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I am having a under boost issue but with no fault codes. Car is blowing black smoke and in limp mode. I have checkes all vacuum hoses. From vacuum pump to the N-75 valve, I have 28ibs. coming out of the N-75 valve to actuactor I have 20ibs.

Actuator arm does not move unless the RPM is around 3500 - 4000.

You cannot have low boost without a DTC. Have you mapped actual and requested boost as you drive the car?

oilhammer said:

You cannot have low boost without a DTC. Have you mapped actual and requested boost as you drive the car?

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this is my original post on this ongoing isssue. hopefully reading this will help you better nunderstand my issue.

Thanks for your help thus far...

I'm having an issue with Under Boost on 2005 Passat TDI.

Car is in Limp mode with no codes and blowing black smoke. when Idling car has a ton of power but when in drive it wont even pull it self.

these are the things that I have checked so far.

1) Actuators to VNT. It moves at start about 4 - 16Lbs. of mercury.

2) Actuator is ok and holds it's pressure.

3) Ck turbo boost control valve N-75 with vag com. It has 10 - 19Lbs pressure - brake vacuum pump has vacuum leaking at hose connectionloose in vacuun oump.

4) Ck fuel injectors quantity #1). injector 0.21 #2). 0.00 #3). 0.00 #4). 0.31.

5) Ck for pluged exhaust with vacuum gage. Ehaust checks to be free.

6) Ck intake EGR. No corrosion build up. ck with vag com has 100% duty cycle.

VNT (verible nozzle turbo), actuators, oxygen sensors, & intercooler.

7) used a smoke machine on Intercooler. No leaks at Intercooler and no leaks
on hoses.

Please any help will be greatly appreciated. Thanks in advance!

oilhammer said:

You cannot have low boost without a DTC. Have you mapped actual and requested boost as you drive the car?

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Car blows black smoke and is in limp mode. I unplugged the vacuum line from turbo actuator. Car quits smoking and will accerate but it still has no boost.

Once again (and please understand, I really am trying to help), have you monitored actual and requested boost with a scan tool as the car is driven?

I ask because you seem to not know (or maybe not understand?) what "boost" actually is. You may be confusing "low boost" with the more generic albeit often more applicable "low power".

When the ECU detects low boost, via the data sent from the MAP sensor, it will set a DTC, typically a P0299, and place the engine in limp mode. The EDC16 system in your car is VERY good at monitoring and detecting this condition. So as is usually the case, if you do indeed have "low boost" as you describe, you will have a DTC set.

By "no codes" do you mean that the CEL is not illuminated, or have you actually checked it with VCDS or a generic code reader? Codes are set even if the CEL is not on.

QuickTD said:

I'll second the need for a scan and monitoring manifold pressure, its hard to work blind. You also can't have "limp mode" without a code set.

Have you checked the cam for wear?

Given the symptoms, I would also check for a stuck open EGR valve flooding the intake with exhaust.

MAF can fail but not set a code. Unplug it and restart the car. If it runs better it may be the problem. Unusual on a BHW though.

Could also be a very badly adjusted VNT stop screw, usually this problem develops slowly as the VNT linkage wears. If the VNT vanes are allowed to close too far, exhaust manifold pressure will spike and there will be too little exhaust flow to spool up the turbo. You'll get smoke and misfiring but usually you can push though it and the engine will eventually rev up.

What is the history on the problem, did it develop suddenly or slowly?

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Yes I have checked cam for wear, it looked good and I also have unplugged the MAF and plugged it back in and no change. I checked the VNT screw and it was adjusted properly but I lenghthened rod so it wouldnt pull up so far to see if it would help. took the car for drive and at first it had a lag then it started boosting up but continued to blow black smoke. Does this mean that my turbo is bad?

QuickTD said:

Again, any history?

I have my doubts about turbo failure, it's rare on a BHW. Pull the intake hose and give it a spin, if the shaft turns freely and does not rub the housings it's probably ok.

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Vacuum to the N-75 I have 28lbs. mercury to the N-75 valve. Coming out of the N-75valve I have 20lbs. of mercury to the actuator. I have tested the actuator. It holds vacuum. I hooked up an air vacuum gage to the N-75 valve and it holds a steady vacuum of 28lbs. I understand that the vacuum above 1000 rpm should drop to 0 lbs. Vacuum does not drop until it is above 2000 rpm and then its only breifly the drops back to 28 lbs. I have not checked the volts coming from the ECU to the N-75 valve. What volts should I have to the N-75 valve?

Hehe, ignoring the obvious still, eh?

THIRD TIME (and then I am done trying to help you):

HAVE YOU MONITORED THE ACTUAL AND REQUESTED BOOST IN THE DATA BLOCKS AS YOU DRIVE THE CAR?

Please, do not post again in this thread until you have done this and have this information.

ok I was able to use a VCDS tool on this car while driving it. here is what was observed.

at idle:
Intake manifold pressure - 1111.6
Intake manifold actual pressure - 958.8
Charge pressure - 80.5%

While Driving (turbo boosted)

Engine speed - 19 32
Intake manifold pressure - 1356.4
Intake manifold actuel pressure - 1030.4
charge pressure N75 - 79.9%
car dropped to 9.9.9 and lost boost

No Boost:
2268
1550
1132
9.9%

these two codes are now showing Mass air flow P0101
and Under Boost P0299

Yes, you have a boost leak, and the DTCs show this. See, wasn't that easy?

Charge air hoses are the most common... there are four of them. Turbo-to-crosspipe (most common), crosspipe-to-intercooler (never seen one of those fail), intercooler-to-firewall tube (rarely fail), and the little elbow from firewall tube-to-throttle flap (second most common).

Look for oily trails, and/or have someone brake torque the car briefly while you listen for the hissing. It should be pretty obvious.

oilhammer said:

Yes, you have a boost leak, and the DTCs show this. See, wasn't that easy?

Charge air hoses are the most common... there are four of them. Turbo-to-crosspipe (most common), crosspipe-to-intercooler (never seen one of those fail), intercooler-to-firewall tube (rarely fail), and the little elbow from firewall tube-to-throttle flap (second most common).

Look for oily trails, and/or have someone brake torque the car briefly while you listen for the hissing. It should be pretty obvious.

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Well back to the drawing board. pulled hoses and inspected each one. found no holes or spots that are questionable so this is what I did.

I made a pressure hose tool kit. I pressured up the turbo hoses and found a leak at the O-ring so I replaced all three O-rings around the turbo hoses. It now holds pressure. I cleared the codes and test drove car and it still blows black smoke around 3000rpm then it clears up and will boost up after driving car but will throw a P0299 boost pressure regulations not reached code and P0101, mass air flow sensor implausible signal then goes back to limp mode.
I also checked turbo actuator. I used a hand vacuum pump. Actuator rod starts moving around 4 mercury. Actuator stops at 18 mercury. I again plugged in my vagcom and went to basic settings. group 11. hit go reading is, hit on and motor goes to 1400rpm and holds there. the N-75 is fluctuating from 9.9% to 90.3% Millibar (actual).

This car is driving me nuts!

oilhammer said:

Yes, you have a boost leak, and the DTCs show this. See, wasn't that easy?

Charge air hoses are the most common... there are four of them. Turbo-to-crosspipe (most common), crosspipe-to-intercooler (never seen one of those fail), intercooler-to-firewall tube (rarely fail), and the little elbow from firewall tube-to-throttle flap (second most common).

Look for oily trails, and/or have someone brake torque the car briefly while you listen for the hissing. It should be pretty obvious.

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Have you done this?

oilhammer said:

Have you done this?

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Yes I have checked for oil trails and have found none so far. Power braked car and cant hear any hissing sounds. I have also watched the actuator and it is working properly. Turbo is boosting and hoses are holding pressure but car is still setting codes and going into limp mode and blowing heavy black smoke after a couple of minutes of this.
could their be a problem with the vaines in the turbo?

Well it still sounds like a boost leak issue to me. I could probably find it within a few seconds if the car was here in front of me.

Unless the camshaft is tooefed, but that is usually a different situation, and it will run poorly all the time and have a dead cylinder.

And you've checked it sounds like control side vacuum, which thankfully on the BHW is brutally simple. Just that the N75 is in a bit of a fiddly location. But the actual vacuum system is easy to check.

The small elbow to the throttle flap is my bet. It's very easy to miss a small split in this hose. You must remove it and inspect it.

oilhammer said:

Well it still sounds like a boost leak issue to me. I could probably find it within a few seconds if the car was here in front of me.

Unless the camshaft is tooefed, but that is usually a different situation, and it will run poorly all the time and have a dead cylinder.

And you've checked it sounds like control side vacuum, which thankfully on the BHW is brutally simple. Just that the N75 is in a bit of a fiddly location. But the actual vacuum system is easy to check.

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You say that it's still a leak but I have pulled all hoses and inspeacted them. I also made a pressure testor with a schreader valve. I capped both ends and aired system to 15 psi. System held pressure for over 5 minutes until I released it.

I also brake stalled the car putting turbo under load as you said to do but I can't hear any hissing noises as you described.

I did go ahead and pulled the turbo just to inspect it and I did see that the turbo impellor on the intake side was badly damaged. Looks like something has gone through turbo so I bit the bullet and replaced it. Car still has the same issue.

While driving the car it has a low speed lag under 3000 rpm. Over 3000 rpm turbo will boost up and has plenty of power but blows heavy black smoke. It remains that way until I come to a stop. It then goes into limp mode and will not boost up again until codes are cleared.

I have checked the back pressure for clogged exhaust with a vacuum guage and it is clear. I also removed the oxygen sensor and checked it but it remains the same. Exhaust is clear. I also checked the EGR to see if it was stuck in the open possition. It was not.

I ohmed the booost pressure wiring harness on terminal 1 and 4 and it has 5.6 volts.

This is what I've replaced so far:
Air mass meter, N-75 valve, Turbo, Brake vacuum pump, 02 sensor and all turbo hose O-rings. So far nothing has worked.

QuickTD said:

Whatever went through the turbo may be in the bottom of the intercooler, mouse nest, rag, whatever.

And for the fourth time, what is the history? Did you buy it like this? Did it develop over time? Did it come on suddenly?

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Only history I have of the car is that it has 300k on odometor and that motor was rebuilt about 45 thousand miles ago. Thats all I know.

I was thinking about a cam, too, but I rarely see one that has ANY DTCs set, even misfire DTCs, which puzzles me. However, from his description of the way it acts it sounds like the problem is not consistent (in other words, it runs fine until more power is asked of it).

But yes, with a bad cam/holed lifter, there is a very obvious "bump-bump-bump" sound to the engine. I do not even have to do anything to diagnose one other than starting the engine.

Well I got this one licked. Jesus, what a mess.

Can't say I have seen this many things wrong with one car.... and I never even lifted into the air.

oilhammer said:

Well I got this one licked. Jesus, what a mess.
Can't say I have seen this many things wrong with one car.... and I never even lifted into the air.

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Do you mean that the OP drove the car to you from Oklahoma City?

1854sailor said:

Do you mean that the OP drove the car to you from Oklahoma City?

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Oh no, this thing couldn't make it across the street under its own power before, LOL. It was towed here.

It starts and runs and moves well now under its own power though. Still wouldn't drive it very far in its present state though.

Battery was flat, (and wrong) so I had to stick a battery in it just to get it in the building.

Vacuum system was a mess, missing parts, connected wrong, loose, etc. Check valve was bad.

Then found the throttle flap was full of oil (inside the electronics).

At the same time found the EGR valve stuck open.

EGT sensor wire was left wrapped around the catalyst, not plugged in (the entire engine harness routing is a mess).

MAF sensor connector mangled, but it stays on at least.

Air filter element full of crud and folded in on itself, 1 missing air cleaner lid bale, 1 intact and snapped shut, 2 flopping loose.

All that done, and I got it to start and idle and run OK at very light loads, turbo working, EGR and MAF now working correctly, still way down on power....

And then the final part? Someone had the wrong MAP sensor in it (from a BEW, "B" suffix.... BHW uses the "C" suffix MAP).

Still has:

timing belt clearly installed wrong, and tensioned wrong.

alt pulley locked up, belt flopping around

steering assist not right (may be due to belt)

firewall grommet for brake booster tube replaced with section of heater hose that sucks itself shut, so the brake assist is bad.

leaves little puddles of oil everywhere you park it.

front end crashed (lock carrier broken, bumper cover brace GONE, fender liners mangled/half gone)

Wow, wow, wow! I hope that "you'd have to pay me to take that car away from here" happened on the front end of this deal.