How to check n75 valve?

bnmorgan

Active member
Joined
Jul 6, 2010
Location
Ecru MS
TDI
2004 Golf TDI, 2001 Beetle TDI
I have a complete loss of turbo boost. Had a broken nipple on the check valve between the vac pump and brake booster, has been replaced. Replaced a very frayed vac line between the n75 and turbo gate activator. Have good vac level up to the valve, and if i bypass it, connecting the vac line straight to the turbo activator, i get boost just fine. How do I check the n75 valve to make sure it's the problem component instead of some other component causing bad signal.

Also, would a speed sensor cause this problem? One of my speed sensors IS bad (I don't remember the trouble code right off). It's not the one that feeds the speedo, its one of the other two.

Thanks
 

Herm TDI

Vendor
Joined
Nov 21, 2001
Location
Richmond, Maine...The far side of Witsend
TDI
2002 Golf GLS Malone Stage 3, P+520 nozzles, 11MM Inj pump, Sachs VR6 clutch, Stelth Race Pipe, Immo Deleat, EGR Deleat
You're very close.
First this to do is to isolate the turbo and the N-75.
Use a short section of vacumm hose (8") and connect between the "Vac" port on the N-75 and the vacumm nipple on the main checkvalve.
This will eliminate every thing else in the vacumm system.
If you get boost your N-75 is operational.

If you want to verifi your N-75 you can remove your N-18 (EGR solenoid) and connect it to your turbo.
Just a word of caution:
Use some sort of means to mark / ID each of your vacumm lines so you prevent getting things crossed up.

To help make things a little easier you can remove the two 10 MM nuts that secure the solenoid "bracket". This lets you access all of the hoses with much more access than leaving both solenoids bolted in place.
 
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bnmorgan

Active member
Joined
Jul 6, 2010
Location
Ecru MS
TDI
2004 Golf TDI, 2001 Beetle TDI
so are the n75 and n18 physically the same creature just in different places? IE they will respond to the computer signal the same if the wires are changed over?

Can the engine run safely/efficienetly without the n18 if the n75 is the problem (I need the car back on the road) Trying to reach a safe/efficient condition asap, and i will make it 'right' when replacements arrive.
 

LVPAJetta

Veteran Member
Joined
Jun 26, 2003
Location
Northern, NJ
TDI
GSW, 2015, Silver, 6-spd
I had my N75 go bad and I swapped it with the N18 for a couple of days waiting for the part. The car responded normally as soon as they were swapped. I'm not sure of the finer differences between the two valves, however, I have driven 50k miles since swapping them back when the new N75 arrived w/o any issues. I would put the car on ramps and possibly remove the belly pan when you do the swap - unless you have a magnetic socket. I found the location of the nuts holding the valves in place to be a little hard to grab so one dropped through to the ground.
 

bnmorgan

Active member
Joined
Jul 6, 2010
Location
Ecru MS
TDI
2004 Golf TDI, 2001 Beetle TDI
okay I am at my car in the garage and using my phone so please forgive the typing and grammar and lack of Punctuation
swapped to vacuum only on n 75 and nothing else and I get boost as normal
what should be the next thing to check or can I run the car with no vacuum on anything except n75 and they brakes? is this a sign of a vacuum leak some where else since the valve seems to work well when it is the only thing getting vacuum?

also how many total feet of vacuum lines do I need to replace all of it I already did the n75 to turbo line about 5 feet
thanks everyone eagerly awaiting reply at car in shop
 
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Herm TDI

Vendor
Joined
Nov 21, 2001
Location
Richmond, Maine...The far side of Witsend
TDI
2002 Golf GLS Malone Stage 3, P+520 nozzles, 11MM Inj pump, Sachs VR6 clutch, Stelth Race Pipe, Immo Deleat, EGR Deleat
okay I am at my car in the garage and using my phone so please forgive the typing and grammar and lack of Punctuation
swapped to vacuum only on n 75 and nothing else and I get boost as normal
what should be the next thing to check or can I run the car with no vacuum on anything except n75 and they brakes? is this a sign of a vacuum leak some where else since the valve seems to work well when it is the only thing getting vacuum?

also how many total feet of vacuum lines do I need to replace all of it I already did the n75 to turbo line about 5 feet
thanks everyone eagerly awaiting reply at car in shop
With vacumm applied only to the N-75 and the boost is normal under all driving conditions this would indicate a vacuum leak some where else in the system.

The first thing to check is the "Black & White" check valve.
Note the orientation of this check-valve (White side towards the N-75).
You should be able to blow through this check valve in one direction only.
If you can blow through the check valve in both directions you have found your problem.
 

Herm TDI

Vendor
Joined
Nov 21, 2001
Location
Richmond, Maine...The far side of Witsend
TDI
2002 Golf GLS Malone Stage 3, P+520 nozzles, 11MM Inj pump, Sachs VR6 clutch, Stelth Race Pipe, Immo Deleat, EGR Deleat
Sounds like you have isolated your problem to a leak elsewhere in the vacumm system. Most likely a hose worn or one of the plastic unions / "T's" in the system.

One more thing (last thought). Look at the center line (mid car) on the fire wall.
There is a "Y" type fitting. Check this fitting to see if all three (3) hoses are connected.
 

AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
Follow Herm's suggestion, especially the "Y" type fitting! I've seen those come almost loose but appear to still be connected!
 

bnmorgan

Active member
Joined
Jul 6, 2010
Location
Ecru MS
TDI
2004 Golf TDI, 2001 Beetle TDI
did as instructed tees and other fittings no problems found. found a vacuum gauge check every point I could find. 20+ inches mercury and pluged everything back up including e g r. Now I have normal boost after everything checked so maybe something was a little loose and accidentally fixed.

still does not want to shift normally especially into high gear and down shifting problems too I know I have a bad speed sensor which will be checked out soon. this car has been a headache for over a year now. at least I see the light at the end of the tunnel if it's not a train. started with a fuel leak at the inj pump, have now gone through 2 pumps 2 timing belts and had the head fixed from a timing belt thrown by pump lockup >.<
 
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AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
Wow~! Sorry about all the troubles you have had with the NB.

I'm not versed at all own the automatics. But, just guessing, I'd say the auto tranny needs to know the speed of the vehicle for shifting purposes. I know for certain that my Chevy Blazer will not shift past 2nd if the VSS has gone kaput!
 

Doughglass

Active member
Joined
Sep 13, 2017
Location
Shelby Michigan
TDI
14 sportwagen 6speed, 03 wagen 5speed, 00 new beetle 5speed
Just bought an 03 sportwagen and the turbo is not functioning. The actuator moves with a vacuum pump connected to hose that plugs into the n75 valve. Vacuum pumps up to 20 and dwindles back to zero. Engine code stating incorrect CPU due to the transmission has been swapped from an automatic to a 5 speed.
Is the vacuum leak in this portion of the system enough to cause the lack of boost?
 

flee

Veteran Member
Joined
Sep 19, 2011
Location
Chatsworth, CA
TDI
2002 Jetta GLS wagon
Make sure the hose is OK or just replace it along with the rest of them.
If it's not the hose, replace the actuator. They do rust out.
 

Rrusse11

Veteran Member
Joined
Jan 23, 2014
Location
PA Deutsch Country
TDI
2002 Golf, 5spd; 05 Jeep CRD
Just bought an 03 sportwagen and the turbo is not functioning. The actuator moves with a vacuum pump connected to hose that plugs into the n75 valve. Vacuum pumps up to 20 and dwindles back to zero. Engine code stating incorrect CPU due to the transmission has been swapped from an automatic to a 5 speed.
Is the vacuum leak in this portion of the system enough to cause the lack of boost?
You need to change the transmission setting with VCDS. EZ fix.

Sounds like the vacuum actuator/solenoid has a leak, and yes, if
it's leaking, you lose boost. It should hold vacuum when pumped.
20"hg might be high, and when you install a replacement, adjust the
arm to hit the stop around 18", or set it so that the arm starts to move
circa 3"hg. With upgraded nozzles after my auto>manual swap I had
an overboost problem until I reset the arm. I believe the auto turbos
are adjusted high because the tranny shifts well before max rpm.

My $.02.
 

GgOTi

Veteran Member
Joined
Dec 23, 2007
Location
LA, CA
TDI
2003 Golf TDI (Mickey)-RIP | 2011 335d-RIP | 2010 Jetta TDI (Merlin)-Returned to VW | 2001 Jetta TDI (The German)-SOLD
If you want to verifi your N-75 you can remove your N-18 (EGR solenoid) and connect it to your turbo.
Very helpful tip. I was having trouble determining whether it was indeed a failed N75 valve, or perhaps something else causing turbo issues. I switched the N75 valve with the N18 valve and had my turbo come back to life. A successful diagnosis. Thanks again.
 

djmacedin

Member
Joined
Sep 26, 2019
Location
Livingston
TDI
Bora
sometimes my bora 1.9tdi goes into limp mode at around 60mph but once i turn the car off and back on its fine. i got told it was the n75 but i tried the output test in vcds and it keeps poping. does that mean its working or bad?

i will update about what code comes up if someone could help that would be amazing
 

djmacedin

Member
Joined
Sep 26, 2019
Location
Livingston
TDI
Bora
17965/P1557/005463 - Charge Pressure Control: Positive Deviation
Possible Symptoms
  • Reduced power output
  • Limp mode
Possible Causes
  • Boost Pressure too High
  • Hoses/Pipes incorrect connected, disconnected, blocked or leaking
  • Charger Pressure Control defective
    • VNT (variable nozzle turbo): nozzles stuck
  • Solenoid Valve for Boost Pressure Control (N75) defective
Possible Solutions
  • Check Hoses/Pipes to/between Components
  • Check Solenoid Valve for Boost Pressure Control (N75)
  • Check / Clean / Replace Charge Pressure Control
    • Check / Clean mechanism for variable nozzles
thats the code i keep getting when it goes into limp mode - it says intermittent
 

LenoVan

Member
Joined
Apr 26, 2022
Location
Canada
TDI
A4
17965/P1557/005463 - Charge Pressure Control: Positive Deviation
Possible Symptoms
  • Reduced power output
  • Limp mode
Possible Causes
  • Boost Pressure too High
  • Hoses/Pipes incorrect connected, disconnected, blocked or leaking
  • Charger Pressure Control defective
    • VNT (variable nozzle turbo): nozzles stuck
  • Solenoid Valve for Boost Pressure Control (N75) defective
Possible Solutions
  • Check Hoses/Pipes to/between Components
  • Check Solenoid Valve for Boost Pressure Control (N75)
  • Check / Clean / Replace Charge Pressure Control
    • Check / Clean mechanism for variable nozzles
thats the code i keep getting when it goes into limp mode - it says intermittent
Hi, have you solved this problem?
 

AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
Jdmacedin hasn't visited here since August 13, 2021 ...........
Put your cursor over the OPs avatar and you can see the last time he visited..
 
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