understanding the n75, actuator rod length, problems, tunes - VNT and wastegate

burpod

teh stallionz!!1
Joined
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cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
BASICS:

the VNT turbo boost is controlled by opening or closing the vanes on the turbo - vanes fully open means least possible boost, fully closed maximum possible boost. the actual amount of boost it will make depends on the turbo characteristics (size, flow etc), rpm and amount of fuel being burned. on a wastegate turbo the wastegate is fully open for least boost and closed for maximum boost.

the n75 controls the vacuum to the turbo VNT actuator. it's a dumb device - a simple valve that is controlled by the ECU (and the "tune") to open/close vanes a certain amount to achieve desired boost. open the vanes fully for least possible boost by passing no vacuum (valve 100% closed) or close the vanes for maximum possible boost by passing 100% of the vacuum (valve 100% open). it doesn't really know how much vacuum it's passing, it's simply opening and closing a valve to let 0 - 100% vacuum through it.

HOWEVER, it does seem that the N75 valve does have a regulator in it - which is set to 25HG according to a brand new pierburg which i just tested. which makes sense for fault tolerance. if your vac supply is 25+ HG your vac supply should probably be good enough. if it's lower than that there will be issues. the % duty cycle reported in vcds should reflect the exact amount of vacuum multiplied by 25hg. so 20% would mean 80% n75 valve open, so a healthy system would show 20HG to the actuator with 20% duty cycle (alh).

in VCDS for EDC15 ALH cars the n75 value shows the percent the VALVE is CLOSED. so if you see 100% that means 0% vacuum is being passed -valve is fully closed, not passing any vacuum through, so vanes wide open, least boost. 0% means valve is fully open, full vacuum, for max boost. for MAS15 (wastegate cars - AHU) and EDC16 cars (BEW PD cars in the US) and EDC17 (common rails), the n75 values are reversed, the n75 value shown is the percent the VALVE is OPEN. so 100% means 100% vacuum being passed through n75 and vanes fully closed (or for wastegate, the wastegate is 100% closed) for max boost.

AFAIK, a healthy 1.9tdi should put out 29+ HG. after fixing the common wiggly vac pump nipple on an alh, the few i've tested are all show 29.5 HG vac supply. both my AHUs also show 29.5ish. the leaky nipple can easily cause 1-4HG lower vacuum from what i've seen.

the amount of vane closure needed to make XXX amount of boost depends on the turbo characteristics, RPM and fueling. start of injection (SOI) will have a large affect on that as well, not just the quantity. and optimal SOI will vary depending on the pump (eg 10 or 11mm) and the nozzle size as well nozzle/injector wear. generally, as rpm increases, less vane closure is needed to make desired boost.

ROD LENGTH:

AFAIK, factory "rod length" for vnt15 and vnt17 cars is such that when applying 18HG the vane lever will hit the set screw stop for maximum vane closure. with 0HG vacuum the lever is hitting the other INTERNAL stop in the turbo for vanes wide open. if rod length has been set too short, the lever may not hit the vanes-fully-open position. you can't see the stop but you can feel it if you disconnect actuator from the lever. the actual HG setting of when it hits the set screw is a little arbitrary - but the important thing is that it is in a range so that it will hit both stops for full vane movement. too short - may not allow vanes to fully open. too long, may not fully close vanes. if the set screw has been altered from factory, it will throw off the reading of how "long" the rod is. so if set screw has backed out a couple threads, if you set the rod length to 18HG, it will now effectively a couple turns shorter than that. where rod length is set is calibrated for the tune.

the set screw is a physical safety stop - regardless of the tune, vac supply, hardware malfunctions, it will prevent the vanes from going beyond the specified amount of closure. you could remove the set screw completely and be perfectly fine --- IFF your N75 valve is 100% perfectly functioning, VAC supply is perfect and never goes above the max HG, and the tune is also done right.

it depends on the tune of course, but the vanes should rarely ever need full vane closure. only at very low rpm should the lever ever come close to or hit the set screw. but this really depends on the characteristics of the tune - namely how much boost it's wanting. if your fueling is correct, at 3000 rpm for example, and you are requesting 38mg fuel (stock max alh fuel), and you close the vanes 100% bad things will happen :)

TUNE:

so... in summary - the N75 is a "dumb device" - simply passing 0-100% vacuum through (but BEGINNING from 25HG or whatever the max is, say you have a leak and its 24HG), where the rod length is set and the amount of vacuum supply is critical to well performing boost control.

in the "tune", factory or otherwise, there is a pre-control map that is used to calculate how much to close the vanes to achieve desired boost. when you first press on the go pedal it makes this calculation and uses this map along with the PID controller and current conditions to determine where to go from there. so if you are at 2500 rpm and press the pedal to command 30mg fuel and 1500mbar of boost, say the tune makes a calculation of wanting n75 to be 40% initially. if the rod length and vac supply are all at factory spec, the tune is perfect, and all other hardware good - you should get perfect boost control. it generally will close vanes quickly to a certain degree and open them up as RPM and boost increases and target boost is reached. but if your vac supply is lower than 25HG, the n75 being a dumb device, 40% isn't going to close the vanes as much as with full factory spec vac supply. if your vac supply is 23Hg, 40% now is more like 42%. so the "first guess" to vane position by the ECU will be a little "short", so it will lag a bit and the pid controller adjusts. vanes will hang closed a bit longer. because this all happens so fast, it will then often cause an overshoot in boost and pid controller now ends up opening vanes, often overcompensating depending on exact conditions, so it may dip below target and then recover.

if rod length is longer than spec, it will have the same affect as low vacuum supply. pretty sure all this stuff is linear. if your vac supply is a bit low, shortening the rod a bit (as long as it still hits the full-open internal stop) would likely cure any boost issues.

in an aftermarket tune, the problem of n75 being off (vac supply low, rod length off etc) is often exaggerated. this is partly due to the pid controller maps relying on fuel consumption for the calculations. pretty much all ALH tunes people have (malone, kerma, rocketchip etc) all using the factory alh fuel quantity value range of 0-51mg. but if you add nozzles or 11mm pump, your fuel quantity range goes up quite a bit. 11mm and .230s can flow 80mg no problem. so in your tune, 45mg may actually mean 58mg fuel and 51mg means 80mg. so any maps the tune uses that rely on accurate fuel consumption are now not going to work right. your MFA cluster won't read fuel consumption correct, MPG readings will be off. you can calibrate it so that it's generally correct, but of course it will never be 100% over the full range of rpm and fuel. but for average driving over a whole tank, fudging the calibration likely will give a decent consistent result.

fuel consumption values now being off because the fueling is squished into the "0-51mg' range means boost control won't work as well as it should/could. different turbos also have different pid controller maps matched to their physical characteristics. larger turbos spool a little slower, but have more inertia. the calculations needed for good boost control are pretty different. lots of tuners use the same vnt15 pid controller maps regardless if you have a vnt15, 17 or a 17/22. so if you have a 17/22, boost control is going to be even worse. sometimes it's possible, errors in calibrations or current conditions can cancel each other out and it may perform well enough. very tricky to sort that out.

so you should now see that the n75 values in the tune are directly tied to your vac supply and rod length. a tune is calibrated for a certain amount of vacuum and rod length. up until a couple years ago, i didn't really fully grasp it all myself, especially the simplicity of it. if vac is a bit low, it may be good enough for a factory vnt15 tune. factory tunes run way more boost than necessary, so if you have some boost "lag" or dont' always meet target, it may not even smoke depending on how much fuel/rpm etc is happening and still drive decent. and the vnt15 is a very responsive turbo so pid control works well enough.

the same thing goes for other tunes. it may generally feel good if you have only 23HG vacuum or rod is a bit long. you would probably get the feeling it has some lag and overboost. depending how much it's off, you may get huge overboosts, spikes followed by big drops in boost. totally depends on the tune, hardware, and how much things are off. if you were to look at boost logs, you would certainly see big problems. if you managed to get vac/rod length perfectly calibrated to the tune, it would definitely perform better. but it's really dependent on the quality of tune you have and how well it's matched to your hardware. there are a LOT of tunes out there that people think are running great, but in fact, doing quite poorly but it's got more fuel than stock so it feels good and maybe MPG are decent enough... and the boost map is probably pretty lean, so the big dips may not cause too much smoke. results vary quite a bit....

-injection timing also plays a HUGE role in how well the turbo boosts as far as how much vane closure is needed to make boost...
-deleting EGR affects timing as well as boost control/N75 (from what i've seen so far, bigname PRO tunes don't account for this whatsoever)
-where the QA is positioned (hammer mod) plays a big role in how well fuel is calibrated.
-lots of bigname PRO tunes aren't nearly as well matched as one is led to believe esepcially with timing/fueling
-between fuel quantity, injection timing, vacuum/rod length issues, little boost leaks - the amount of error compounding going on can be great. sometimes, a comedy of errors produces something that actually runs OK or even decent!

hopefully i haven't made an errors in this, if so, please point them out :)
 
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burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
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82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
i would urge anyone, tuned or not, to take some boost logs (011 only for starters) to see how well their car is actually performing and post it in my boost log thread. i have yet to see a boost log of a tuned tdi that has good boost control (near perfect boost control is very possible, btw, and you should never need one of those crappy boost control valves). the worst of the issues are probably caused by vacuum/rod length followed by fueling/timing not being matched well for the hardware.

i plan to write-up a detailed post on how to take good boost logs that will give a better idea of how good boost control really is. unfortunately the sample rate for our edc15 alh ecus is very low - only about 3 samples per second. so granularity is poor given how quickly boost control happens. if you log more than one measuring block at once, granularity becomes even worse.

edit- i will have to ammend this based on the huge amount of logs i've seen. i would say the WORST of the problems are usually (and this comes from seeing tons of logs of various tuned cars and getting involved with them) caused by tune being completely mismatched for the hardware. there's no single culprit. fueling, timing, n75 map, boost/fuel relation, etc. too much to discuss here. of course, simple hardware issue can be the culprit too. but do NOT underestimate how problems in hardware + problems in tune can compound and react together. you really have to understand how to read the log data... things can feel good, and be bad. you can have hardware problems on a bad tune, and the tune will run "fine". load a "good" tune, and it will run like ****.. vice versa.. etc
 
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burpod

teh stallionz!!1
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Nov 27, 2004
Location
cape cod, ma
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82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
after performing another test, i need to edit my post before I spread MISINFORMATION myself!!! :oops:
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
when testing my BEW PD golf, vac supply showed 27HG. when doing the vnt basic settings test with VCDS, when reported duty cycle was 90%, vacuum to the actuator was showing ~24.5HG. it's a brand new pierburg n75.

so this is a little confusing.. as i would expect 22.5HG for 90% closed. 27HG * .90 == about 24.5HG however, which made me think that the output HG is straight from the vac supply without any sort of internal regulator. i may redo the test. and swap valves around from my other test car.
 

burpod

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82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
if i've made an mistakes or am flat-out wrong on anything, please point them out. i can handle being wrong :)

based on the test with my BEW PD, i am still a little unsure about whether the n75 is regulated internally to to 25. or whether the % opening applies to the input vacuum, and not a controlled "cut" of 25hg. the test i did on my PD would indicate the % duty cycle is straight from the vac supply. but on my other test, it would indicate the duty cycle % is a % of 25Hg (or lower if vac supply is low). i want to repeat my tests by swapping valves just to rule out that one of mine is malfunctioning, despite both being brand new pierburg.
 

burpod

teh stallionz!!1
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82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
i repeated my tests on my msa15 AHU vnt...

vac supply 29hg measured with brake booster included. so healthy vac. both my new N75s measured 24.5hg with 99% duty cycle and 21hg with 20% duty cycle.

i also tested a random old n75 valve which read poorly: only 21hg @ 99% and 18hg @ 20%.

didn't repeat any tests on my BEW, so not sure whats going on there. i would like to repeat the tests by introducing a small vac leak such that supply is 26hg to confirm that at 20% duty cycle they would still read 21hg

another odd thing i noticed, was if you hold the n75 "upside down, vacuum will drop about .5HG. it also responds to vibration and shaking it. if you shake it, the vacuum readings bounce all over within a few HG. so that makes me realize that it's important to mount it so that it has a buffer - as it does from the factory. i've mounted mine right on one of the studs on my windshield cowling area. so i think i will change that so that it's on a buffered bracket again to dampen vibrations...
 
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ghohouston

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Apr 2, 2013
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Lewisville, Texas
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2001 Jetta Sedan TDI 5 Speed
Since I'm not familiar with vcds, is it actually measuring vacuum, or are you tee'ing a gauge in? I checked my vac pump output yesterday, and the pump pulled right at 28 in hg. I checked every vacuum component to make sure I had no leaks, including the booster itself and both check valves and found no leaks or issues. The n75 does not hold vacuum on any of the ports though with it disconnected from power, which I found mixed answers on if it should or not. Tee'ing a gauge into the vac out nipple of the n75 to vnt actuator, I had right at 18 in hg at idle. Sound normal?
 

burpod

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82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
so, i guess i should have included this in initial post... to check basic n75 operation:

basic:s:
0) first check with mityac vac source to n75. take the VAC hose off n75 and plug into mityvac, this would be the available vacuum of the system. should really read 29+ HG, if not you probably have a mild leak somewhere, usually the vac pump nipple if it's wiggly will leak a couple HG out, but if it's 25+ it should be fine and not cause major issues ( i plan to do some more testing on this to confirm). brake boosters can sometimes get leaky
1) plug mityvac directly into turbo actuator and pump it up. it should hit the stop screw somewhere between 18-22hg. this is the "length" of the rod. for factory tunes and most other tunes for vnt15 and vnt17, i think 18" is probably where the tune is calibrated to. the lever moves SMOOTHLY from the fully open stop to fully closed (hitting set screw). now if the set screw has somehow gotten out of factory spec, this will throw off the "rod length"
2) when pumped up (1) the turbo actuator should hold vacuum for at least 15 seconds, else your actuator is leaky. it should ideally hold vacuum longer

vcds vnt test:
a) put a tee in bewteen turbo actuator and n75 output barb and plug that into mityac
b) start car and go into basic settings and run the vnt test under documented settings - charge pressure control test (channel 04?)
c) for an alh, it believe it should cycle between 0% and 100%
d) note the mityvac readings as it does this - 0% should read 25hg and 100% should read 0HG if all is good
e) you should also confirm that at 25hg the vnt lever has hit the stop screw and at 0HG it has reach the other end of movement

if (d) doesn't happen and vacuum source is good, there's an issue with the n75 valve itself, or possibly the wiring to the n75, maybe even ecu
if (e) doesn't happen vanes might be jammed

if your set screw has somehow come out of alignment, my best guess from experience is that for a vnt15, vnt17 and 17/22 you should see around 3 or 4 threads max showing. i think 3 would probably be fine to set it to, i've done it myself on several turbos. but remember, if it's a little taller than factory spec, then the rod "length" measured with mityvac will now read a little low. if you take a factory calibrated turbo, and rod length is 18HG, and turn the set screw up a turn, your rod length would now measure maybe 17HG - even though it's still calibrated the same. it would be unlikely to affect how the car boosts with the same tune, except perhaps at very low rpm (say below 1800 rpm) with lots of fuel might show a hair of lag. for a tune that runs a lower AFR, lower boost, it's unlikely to have any affect.

you can also check with mityvac tee'd in on your current tune and sanity check - while looking at 011 at idle, note the duty cycle (i think stock vnt15 tune will be 25% or so). so that's 75% open vacuum to actuator, so ~18.5HG makes sense (25HG * .75 = 1.875HG).
 

pyromancer

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Seattle, WA
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2001 Jetta TDI
On an ALH to measure vac to the turbo:
While idling in VCDs go to Engine>Measure blocks> select 11> click switch to basic settings in the bottom left corner. It will cycle the N75 from 0 to 99.6% ish. If you have your Mighty vac plugged into the line that goes to the turbo actuator should go from 0 to what ever your n75 is out putting mine was 24, maybe a smidge higher. Might vac on the line to the n75 I was at 29.

If your vac to the N75 supply is less than 25 first check your system for vacuum leaks. Isolate areas of hoses and use a mighty vac to make sure things will hold vacuum.
If you are sure you have no leaks find the nipple from the vacuum pump and gently wiggle it to see if it is loose. Its a very common issue. This thread gives a method of fixing the issue https://forums.tdiclub.com/index.ph...with-loose-nipples-vacuum-pump-repair.457763/ I personally prefer to find a deep socket just bigger than the nipple and give it a light tap (its aluminum so I stress light) to re-crimp the housing and then coat the face of the nipple with super grey RTV. I'm sure the JB weld works but having something flexable and more forgiving always seemed a better idea to me.
 

burpod

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82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
so being the genius i am ;) i wrote a little test tune to map out the duty cycle -> HG. using the go pedal, i made it so it evenly steps the duty cycle from 100%, 90%, 80%, 70%, etc.. with only very tiny fuel changes. appears to be 10% corrresponds to about 3hg, this is with a new pierburg n75. not that it has any bearing on this test, my rod length is 20hg. this test is simply measuring out the duty cycle % to actual vacuum. based on this, it appears i would reach my set screw around 20hg @ 20%. i'll probably redo my test "tune" so i can get down to 0% and try it with a couple other n75s, including a known "bad" one that maxes out at 22hg with the vcds n75 test. needless to say, it's not a drivable tune :)

100% - 0"
90% - 0"
80% - 3"
70% - 5"
60% - 8"
50% - 11"
40% - 14"
30% - 16.5"

the goal of this test would be to test how evenly duty% coresponds to actual vane movement. i would think it should be perfectly linear, and if it's not, the n75 perhaps isn't working correctly in all ranges
 

Dieselgeek

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Golden, CO
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2016 Golf TDI
The minimum flow limit matters more than the actuator rod length / preload. To measure this properly you need to fully warm up the car and high idle it at 1400rpm while looking at Engine Basic Settings Group 011 with VCDS and record the MAP when the vanes are full open and full close. PD and CR will hold the idle automatically; while ALH requires the operator to regulate the idle. Adjust the stop screw to limit actuator travel and you'll get less spread and more exhaust flow, allow more travel and you'll get more spread, less exhaust flow. If the minimum flow is too low you'll have poor fuel economy and a hitch in the power band on tip-in; most noticeable at part throttle. Minimum flow too high and you have part-throttle underboost limp mode and sluggish response. Keep in mind this assumes good vac supply, working N75 that'll deliver 100% of available vacuum when in BS011, no boost leaks, fully warmed up, turbo not FOD'ed, etc.

For an old non-DPF car you want to generate a pressure ratio of 1.1 with the vanes fully closed with the above pre-reqs met. PR = MAP/ambient. As an example 800mbar atmospheric pressure, vanes full close = 880 mbar, vanes open = 800mbar. If you're at sea level the numbers are roughly 1000mbar and 1100mbar. For a DPF car a spread correlating to a PR of 1.2 is about right.

Actuator rod length doesn't matter a ton, but does have an effect on transient response and boost spikes. For most cases a little preload is about right. Preload meaning that you have to apply some vacuum before the rod starts moving. More preload is slower, smoother, and more controlled; less is the opposite.

To set the minimum travel back the stop off all the way and allow the lever to contract the vanes fully to the internal stop. Then force the vanes open by the desired number of turns of the stop screw, then set the nut.

To set the pre-load adjust the rod length such that it requires a certain amount of vacuum to get the rod moving. Keep in mind that most of the hand-held vacuum pumps use a 'grade B' vacuum gauge that isn't worth much in absolute terms.

One of our staff installed a lot of turbos in a previous life and this is where most of the meat of this post comes from. Since setup varies dramatically within manufacturers run to run he set every single one before install. This is what he found worked well over about 10 years of experimentation. Data and measurements were made with a lab grade vacuum gauge. YMMV.

CJAA
Preload 2inHg
Full travel - internal stop limited by 1 turn of stop screw.

KP39, BW ALH
Preload 3inHg
Full travel internal stop limited by 2 turns of stop screw.

VNT17
Preload 3inHg
Full travel - internal stop limited by 2.5 turns.

BV39 (BRM stock)
Preload 4inHg
Full travel - external stop limited by 2 turns.

PD140
Preload 4inHg
Full travel - internal stop limited by 2 turns.

As far as vac to N75 - We've found this varies so much car to car and tune to tune it's not useful for setup or diagnostics at large. Nor is it necessary to derive this curve to achieve a good turbo setup and torque curve. ECU tuning is outta my wheel house anyway, so I'll defer to those folks.
 
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03TDICommuter

Veteran Member
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Dec 8, 2016
Location
So. Cal
TDI
01' NB, 5spd
CJAA
Preload 2inHg
Full travel - internal stop limited by 1 turn of stop screw.

KP39, BW ALH
Preload 3inHg
Full travel internal stop limited by 2 turns of stop screw.

VNT17
Preload 3inHg
Full travel - internal stop limited by 2.5 turns.

BV39 (BRM stock)
Preload 4inHg
Full travel - external stop limited by 2 turns.

PD140
Preload 4inHg
Full travel - internal stop limited by 2 turns.

As far as vac to N75 - We've found this varies so much car to car and tune to tune it's not useful for setup or diagnostics at large. Nor is it necessary to derive this curve to achieve a good turbo setup and torque curve. ECU tuning is outta my wheel house anyway, so I'll defer to those folks.
I don't see a VNT15 in your list. Do you know what was recommended for that turbo? I took my turbo apart and cleaned it, did not disturb the stop screw or rod length, but I don't know is anyone did before I bought the car.
 

dhangejr

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Mar 1, 2017
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PNW is my home
TDI
mk4 Jetta
So I’m working on friends car, it’s a BHW Passat trouble shooting p0299.
When I hook up my Mityvac to the hose coming from the and 75 to the turbo actuator, I get 20 HG. so I swap it for an extra one I have on my shelf. It reads 22HG. So I go test my BEW, it also reach 22HG.

So I start researching part numbers and it appears that the BEW and the BHW use a different part number. According to ID parts.

So, I have two questions:
Firat Does my anybody know how different than 75’s are between the two models?
Secondly, what are the odds that my gauges just reading a little bit low and both are putting out 25 HG?

Thanks this thread is very useful and I’ll likely be reading it several times again.
 

dhangejr

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Mar 1, 2017
Location
PNW is my home
TDI
mk4 Jetta
This one is for to be for ALH and BHW



MEnwhile this is for BEW BRM and CJAA

https://www.idparts.com/pressure-co...-brmmk6-cjaa-1k0906627a-700868020-p-1251.html

Is it just a physical differences with regards to the direction which the nipples are facing?

Maybe I should message Pete and ask if he knows?
 

Tripl3b

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Apr 28, 2023
Location
Michigan
TDI
Mk5 Jetta BRM
Hello, I am lost with my current situation. I hope I can learn something with your help.

I have a 2005.5 Jetta with the BRM motor and 5 speed transmission. I purchased it at the beginning of this year. I was told that it was a corporate vehicle most of its life. The young man I purchased it from owned it for a year and the actuator failed.

This Jetta has 209,xxx miles, stage 2 clutch, stage 2 tune with flash tool from Malone. Egr cooler has been removed, coolant lines looped and exhaust and egr port blocked. Also has a battery kill switch and immobilizer bypass switch that was installed by the corporate company according to pervious owner.

After I purchased the car, I replaced the whole turbo with another off a 2006 BRM. I went through adjusting the vnt. Starts travel with the mityvac 3-4hg and max out on stop screw at 18hg.
Upon completion, I was getting an underboost code. I found that there was little pin holes in the intercooler. I replaced it. Now I am getting an overboost code.

I tried to test the actuator with vcds. When the n75 is supposed to release vacuum. It shows that it does on vcds, yet the actuator never moves. When I pull the vacuum line off the actuator. The rod immediately returns. Mityvac shows constant vacuum during same test when connected to it.

So far i have replaced the map, maf and the n75 with no changes. I have tried the stock tune as well as stage 2 with no changes
I will get pictures of the logs and try to post them.
 

krook

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Jan 13, 2021
Location
Hungary
TDI
A4 B5 AWX, A4 B5 AFN
this is a great thread and i think everyone trying to understand tdi boost control should start here. i have a question though. how would one go about setting the stop screw for a turbo that was electric and converted to vacuum? i have a gtd2060vz mounted on my awx and right now i set it up so that if i do a basic setting on channel 011 which raises the idle to 1500rpm there's a 90 mbar difference closed vs open. how do i know if i went too far and the turbo is surging? i currently have no means to monitor emap, i can only monitor egt.
 

burpod

teh stallionz!!1
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Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
this is a great thread and i think everyone trying to understand tdi boost control should start here. i have a question though. how would one go about setting the stop screw for a turbo that was electric and converted to vacuum? i have a gtd2060vz mounted on my awx and right now i set it up so that if i do a basic setting on channel 011 which raises the idle to 1500rpm there's a 90 mbar difference closed vs open. how do i know if i went too far and the turbo is surging? i currently have no means to monitor emap, i can only monitor egt.
i think you would just leave the set screw in the same place? IMO, it just needs to be as close as you can get it to where the manufacturer/turbo-builder set it. any slight difference there is going to pale in comparison (as far as safety is concerned) to all the other factors involved with the tuning. getting fueling correct, timing, boost control etc...
 

krook

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Jan 13, 2021
Location
Hungary
TDI
A4 B5 AWX, A4 B5 AFN
when the turbo was electric it had no stop screw and i have no way of knowing where it would sit in the fully closed position. i was thinking of converting it back to electric which wouldnt be too big of a job since i know where the switches and constants are in the software, but the same problem arises, i wouldnt know if im closing too much and therefore choking the poor turbo.
 

burpod

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Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
ah ok, guess i don't know enough (or anything lol) about those. i figured there was still a stop screw there. so don't really have any idea. do you have to drill and tap for one? would guess you'd have to just see where the electric one stops at full closed, measure it and do your best...
 

P2B

Top Post Dawg
Joined
Jan 11, 2006
Location
Toronto & Muskoka, Canada
TDI
2002 Jetta, 2003 Jetta, 2003 Jetta Wagon
Hello, I am lost with my current situation. I hope I can learn something with your help.

I have a 2005.5 Jetta with the BRM motor and 5 speed transmission. I purchased it at the beginning of this year. I was told that it was a corporate vehicle most of its life. The young man I purchased it from owned it for a year and the actuator failed.

This Jetta has 209,xxx miles, stage 2 clutch, stage 2 tune with flash tool from Malone. Egr cooler has been removed, coolant lines looped and exhaust and egr port blocked. Also has a battery kill switch and immobilizer bypass switch that was installed by the corporate company according to pervious owner.

After I purchased the car, I replaced the whole turbo with another off a 2006 BRM. I went through adjusting the vnt. Starts travel with the mityvac 3-4hg and max out on stop screw at 18hg.
Upon completion, I was getting an underboost code. I found that there was little pin holes in the intercooler. I replaced it. Now I am getting an overboost code.

I tried to test the actuator with vcds. When the n75 is supposed to release vacuum. It shows that it does on vcds, yet the actuator never moves. When I pull the vacuum line off the actuator. The rod immediately returns. Mityvac shows constant vacuum during same test when connected to it.

So far i have replaced the map, maf and the n75 with no changes. I have tried the stock tune as well as stage 2 with no changes
I will get pictures of the logs and try to post them.
If the gauge shows constant vacuum when connected directly to the N75 output while running the test, either the new N75 is defective or the control signal is not reaching the N75 due to a wiring issue.
 

Fahrvegnugen

Veteran Member
Joined
Jan 21, 2017
Location
Burlington Vt
TDI
01 golf 1.9 alh gls silver
If the gauge shows constant vacuum when connected directly to the N75 output while running the test, either the new N75 is defective or the control signal is not reaching the N75 due to a wiring issue.
I got 15 in Hg on mityvac gauge directly out of the top nipple of n75, before the check valve, whether boost was on or off in vagcom. It only increased in Hg if I increased rpm. It didn't matter that the duty cycle was 0 or 99.6 %. The duty cycle never varies with rpm changes.
I'm getting overboost/limp mode but no codes. It appears I'm in for a new n75.
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
are you sure you were checking the line that is the OUT barb? the line going to the turbo? that should go between 0 - 25hg.

your vac supply to the n75 should be 29-30hg on a healthy alh. if its <26hg there may be some boost problems with not getting enough vac. if your vac supply is 26-28hg, there's a good chance its just got a bit of a leak from a loose vac pump nipple
 

Fahrvegnugen

Veteran Member
Joined
Jan 21, 2017
Location
Burlington Vt
TDI
01 golf 1.9 alh gls silver
I did have the wrong barb, that out barb goes between 0-10hg. Hg supply taken off booster hose nipple from vacuum pump is 30 hg.
 

Nevada_TDI

Top Post Dawg
Joined
Aug 17, 2008
Location
Reno, sort of...
TDI
2001 Jetta TDI
Maybe this would explain some of the "smooth" boost issues I used to have...

I called the Canadian guy and the tune he sent me was constantly over boosting, and he kept telling me to lengthen the actuator arm. Why? it didn't over boost before with the actuator arm "too short"

I do not remember what the vacuum number is as I have not checked in a while, but it appears burpod's tune isn't super sensitive about "Hg" as some tunes are. When today is over I will move a couple of vacuum lines to see what is actually going on.

I now see why vacuum is somewhat critical; if the ECU thinks you have 25"Hg of vacuum it will apply enough vacuum to create boost for 25" Hg, so if you have less than 25"Hg available it seems the ECU would start hunting to apply the proper amount of vacuum.
 

Fahrvegnugen

Veteran Member
Joined
Jan 21, 2017
Location
Burlington Vt
TDI
01 golf 1.9 alh gls silver
In reviewing my selection of vacuum hoses I found that I have McMaster 5041 k521 high temp silicon tubing 3mm ID, and 5054 k531 4mm ID. The bigger diameter is supposed to be 5mm tubing, I wonder if that is reducing vacuum. I probably did that because they don't sell that kind of tube in 5mm ID.
Been driving it with these tubes for 6 years.
 
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