What gives the N75 it's output signal?

The vacuum output from my N75 to the turbo actuator floats at 12 psi at idle. When I give it throttle, it jumps up to 16psi for a micro-second, but then instantly drops back down to 10 and falls down to 5-6 psi. I've got a full and strong 25 psi vac going into the top of the N75. I'm measuring the VNT output right off the nipple. I bought a brand new one from IDparts and it does the same thing?

What causes this?

What sensors or whatever from the ECM is giving the N75 it's signal for vac output?

Thanks in advance.


Gruni

Nobody??..

While I'm at it, is there a common sensor input that has significant bearing on BOTH the N75 and the other solenoid that controls the EGR?

Thanks

Both units are connected to the vacuum piping and they use that vacuum as a source for output.
The wires to each unit carry the info from the engine controls (ECU) which take into account things like throttle position, air flow, fueling etc.
I don't know exactly which sensors make up this input, but study of wiring diagrams might help. You may also need a "logic tree" for the ECU.
hth

Full vacuum = full boost, and the ECU won't call for boost with no load on the engine.

You would have to "T" your vacuum gauge off the line from the N75 to the turbo and watch it while driving to see what it is doing.

What is the problem you are chasing?

Yeah, what problem do you think you have? The ECU is very good at not only controlling the boost, but monitoring it as well, and thus if there ever is a problem, will alert you with an MIL.

3turboz said:

Full vacuum = full boost, and the ECU won't call for boost with no load on the engine.

You would have to "T" your vacuum gauge off the line from the N75 to the turbo and watch it while driving to see what it is doing.

What is the problem you are chasing?

Click to expand...

Yes it does, ECU will always request full boost until boot rises above its set limit and then pulls back.

This here map in the ECU at hex address 56858





25% is around fully closed vanes. 75% + is fully opened.


So for direct answer, your Injection Quantity request (from Pedal position, moderated by Smoke and Torque limiter maps - with some MAF influence for limiters), at a certain RPM, gives you the value the N75 should be at. It's all prewritten.

And to give a fully rounded answer for better understanding...

Here's what I did myself to remove the potential for Overboosts after tuning my turbo map to go up to 17psi. Basically you open the vanes up a little more, and you reduce high peaks.

I was hitting peaks of 21-22psi before it would go to the sustained 17psi. This would mean I reached the maximum of the 2.5bar sensor. Now I don't peak as high with the settings I flashed in this map. (I do not mind if anyone uses this for inspiration)

Your problem is either the actuator is bad, or the vanes are dirty and need cleaning. See oven cleaner method, works like a charm.

Those graphs are awesome!

Correct me if I am wrong. But, doesn't the N75 activate the actuator at start-up by applying appoximately 18 inches of vacuum to the diaphram? That action pulls the rod down into the diaphram of the actuator to the set screw stop. This places the angle of the 9 vanes in such a manner that there is virtually no boost at idle.

If that be the case, then full vacuum doesn't equal full boost, just the opposite. Also, I suspect the ECU can sense if there is a sudden loss of vacuum. Othewise, the engine and/or turbo could experience casthrophic failure due to extreme over-boost.

And, the other parameters (air flow, pedal position, etc.) come into play as BobnOH stated.

Ok, thanks guys. Well you asked (and I appreciate it), but below is the symptoms and what I've done:

The problem started out of the blue coming back from work it suddenly began bucking and blowing heavy black smoke. At start-up it barely runs and surges wildly (no codes) but a huge plume of smoke that could swollow a house! Trying to accelerate in park (or drive too) ...the engine doesn't want to accelerate and engine chuggs and blows copius amounts of smoke. But if ya floor it, it'll increase in RPM and smooth out (but engulf the whole house in smoke). If I drive it, and more or less floor it, it'll take off and smooth out at high rpm ..and eventually when warm will idle perfect and run out smooth (no codes initially). If I shut it off and instantly restart it, it'll run rough again and if I accelerate hard up to high rpm it'll smooth out again. Probably running in limp mode. Car has 235k Mi, auto tranny (rebuilt). I dont' have the Vagcom. Here's what I've done:

1) New turbo assembly (cause of miles mostly). Made no difference. Turbo vanes on old unit loosened up with some work when removed. After the fact, I'm sure I could have run it longer, but I already had the new turbo and just put it on anyway rather than wait for old one to crap out in the deep of winter.
2) New EGR (no difference)
3) Cleaned intake and all turbo pipes, inner cooler. They weren't bad.
3.5) New fuel and air filters. Cleaned snow screen.
4) Checked cam/lifters (all good)
5) Replaced all vacuum hoses (no I didn't get any crossed or mixed up. I quadruple-checked with a diagram. No leaks that I can find. Check valve good. 25psi vacuum. Didn't see any leaks around brake booster pipe.
6) Replaced N75 with new VW unit (also swapped it around N18). No difference
7) Pressure tested turbo and piping. I only used about 8psi on entire system. Found minor leak on hose coming out of turbo where clamp cut into hose. I trimmed it back and is good now. (this has made start-up smoother. Idles better on start-up but still blows major smoke and chuggs when accelerator is depressed until about 2000 rpm then starts to level off).
8) VNT is new, doesn't leak .
9) If I actuate the VNT manually when accelerating it seems to accelerate properly, run properly. Doesn't chug or hesitate.
10) Did not change MAF. It runs worse when unplugged
11) Temp sensor was replaced 2x yrs ago when Timing belt was replaced (Jason TDI did it).
12) Cleaned all battery and ground connections.
13) Read a ton of crap on the forums and pretty much followed all that I could find.

When warm it'll idle great all day long and accelerate, drives out ok, therefore I don't think it's fuel system related. So as far as I can tell, it comes down to whatever key components are controlling the vacuum of the N75. As the engine accelerates, the vacuum to the VNT goes down. This is testing in the driveway.

I could replace the MAF/MAP, but I don't have any way to measure at the moment and thought I'd get your thoughts what might be behind the N75 signal.

So, what are your thoughts? I don't know that want to start down the VAGcom route ...I have so many other car projects/family/work/Home restoration, etc. I know Jason Daniels and I might ready to take it to him (he's been busy though). He offered some suggestions. I thought I might get it resolved more easily, but don't want to keep going down the new parts scattergun approach, which I don't usually like to do. Most everything I've done already, as I see it, probably needed to be done anyway to keep a reliable vehicle.

Your thoughts and ideas more than welcome. Thanks

Gruni

Read my post above (#11). .....Yes, vacuum to the actuator on the Turbo does drop when accelerating. That is how the actuator is controlled by the N75 via the ECU.

You need access to VAG COM!

I'd check the timing first thing!

AndyBees said:

Correct me if I am wrong. But, doesn't the N75 activate the actuator at start-up by applying appoximately 18 inches of vacuum to the diaphram? That action pulls the rod down into the diaphram of the actuator to the set screw stop. This places the angle of the 9 vanes in such a manner that there is virtually no boost at idle.

If that be the case, then full vacuum doesn't equal full boost, just the opposite. Also, I suspect the ECU can sense if there is a sudden loss of vacuum. Othewise, the engine and/or turbo could experience casthrophic failure due to extreme over-boost.

And, the other parameters (air flow, pedal position, etc.) come into play as BobnOH stated.

Click to expand...

[EDIT] Vacuum to the actuator = CLOSED vanes.**
That is what the 100% spike is at 760rpm/0 air flow. If I were to edit that down to 25%, it would keep the vanes closed. It more a function for testing. The ASV engine from Europe doesn't have that 100% spike programmed in.






Now as for the behavior explained on the first post... Since he is at 0 RPM and at 0 airflow and vanes open fully, it seems the PID Controller (a built-in feedback mechanism) is trying to create boost by overriding the system and closing vanes to bring boost, because he requested it (as well as IQ) from the pedal. If the vanes are open too much, the PID controller is responsible for adjusting to bring the requested boost in. This is a normal behavior.


Can the actual vacuum pump go bad? Or the spherical container have a leak or something?

Thanks guys.

Car holds 25" of constant vacuum. Even under acceleration. 25" to the N75 too. I don't see how it could be the timing since it was good one second/then bad the next. Timing belt is pretty new ...I inspected the idlers and water pump too recently. Once the car warms up to a certain point, it runs and idles fine (albiet in what I believe is limp mode). That was my thought process anyway. Seems like a sensor or something flaked out. But I could definitely be off base in my thinking.

I agree I probably should have the Vagcom software.

AndyBees said:

If that be the case, then full vacuum doesn't equal full boost, just the opposite. .

Click to expand...

Perhaps I am wrong, but why then do you get no boost if you have a broken vacuum hose? I thought this was the fail safe design so that you can't overboost if something in the control system fails.

Also, shortening the actuator rod makes = higher boost pressures. I think behavior at WOT under load and at idle may not be the same, as is the reason for the VNT in the first place.

3turboz said:

Perhaps I am wrong, but why then do you get no boost if you have a broken vacuum hose? I thought this was the fail safe design so that you can't overboost if something in the control system fails.

Also, shortening the actuator rod makes = higher boost pressures. I think behavior at WOT under load and at idle may not be the same, as is the reason for the VNT in the first place.

Click to expand...

I think you may be right about vacuum = closing. I switched the idea around. The N75 graphs are still correct, 25% vanes closed, 75%+ vanes open. I'll edit the earlier comment to not misguide.

Well, my ALH Vanagon is extremely easy to crawl under without being jacked up. So, I had someone to start the engine repeatedly. Each time it was started, the actuator began pulling down on the arm until it was resting on the adjustment stop-screw. And, BTW, the Turbo boosts just fine.

I did the above procedure because I had seen the claim made that the N75 was activated when the engine was started and allowed full vacuum on the actuator.

I do agree that it seems opposite of what would seem logical.

And by the way, the Vanes are never completely closed, otherwise, exhaust would be extremely restricted from exiting the manifold. The Vanes' purpose is to "direct" the angle at which the exhaust blast hits the Turbine blades.

Also, sticky Vanes are more than just accumulated soot and/or rust. I've found a few with distorted Vanes, distorted Vane assembly and the back plate can be pushed out against the Vanes/Vane assembly due to rust behind it.

OK, I am totally WRONG! I have 18" of vacuum to the actuator at idle.

Back to the OP, No MIL/CEL?

Are you saying it runs normal when warm, or just better?

May need to find someone with VCDS to find out what is going on.

Awesome maps, thanks.