VW T4 Forum - VW T5 Forum banner

1 - 20 of 55 Posts

·
Registered
Joined
·
79 Posts
Discussion Starter #1 (Edited by Moderator)
The N75 valve is like a switch controlling the turbo wastegate actuator. The wastegate itself is normally closed allowing exhaust gases to spin the turbo. When the wastegate actuator opens the wastegate, exhaust gases are diverted away from the turbo. This is necessary as excessive boost would result in engine damage.

The ECU uses the N75 valve to maintain correct boost levels by comparing readings from the MAP sensor (Manifold Absolute Pressure G71) with it's engine map. If the N75 fails to open the wastegate actuator the ECU triggers 'safe mode' limiting fuelling levels resulting in power loss. Always check the wastegate actuator and MAP sensor if you suspect a faulty N75 valve.



This graphic shows how boost pressure from the turbo is used by the N75 to regulate pressure to the wastegate actuator, as found on ACV AJT AYC AYY engines. It is important to appreciate that how it all looks and works will depend on the engine code, as there are different turbos and valves in use.



TESTING

Pull connector (1) off N75 valve (2)
Use a multimeter to measure resistance between valve contacts 2.
Spec ACV AJT AYC AYY = 25 to 45Ω
Spec AHY AXG AXL = 15 to 20Ω

Replace valve If spec not attained

If spec attained...
Switch on ignition and measure supply voltage at connector plug 1 and engine earth
Spec approx 12v. If spec not attained check wiring back to ECU.



ADDITIONAL NOTES

  • The N18 EGR solenoid valve is very similar to the N75 and has the same 2 pin connector plug. In fact, some engines use this type of valve for both functions (see graphic at end for AHY type engines). It is therefore possible to swap over all the pipes and plugs and use it to perform the job of an N75 valve. This is a useful way of checking the performance of N75 valve in addition to the above checks and can save you the cost of buying a replacement if you go for an EGR delete.
  • It is worthwhile replacing all the hoses to the N75 as it is very difficult to 'see' leaks and the rubber does deteriorate. I recommend using performance silicone hoses as they do not cost much more, come in a range of colours and are easier to slip on and off for maintenance.
  • Use a hand pump such as a 'mity vac' or similar to check wastegate actuator is working. Pull off actuator hose and connect pump directly to it. As you apply pressure the actuator rod should at some point begin to move until the wastegate is fully open.
  • Check vagcom for DTCs and check to see that the MAP sensor is working correctly.
  • A 'leaking' wastegate or 'faulty' actuator is not normally repairable and at present requires complete turbo replacement.




 

·
Registered
Joined
·
1,144 Posts
Re: Charge Pressure Solenoid Valve N75 - Testing

Thanks Simon for your gift & putting the time & work into your excellent posts. Really appreciate them, keep them coming.
 

·
Registered
Joined
·
79 Posts
Discussion Starter #6
Re: Charge Pressure Solenoid Valve N75 - Testing

I've got ElsaWin but these guides are soooo much more useful
Big thumbs up!T:T:T:
Thanks - I spent hours on 'photoscape' doing these pics for this and other posts, glad you appreciate them. More to come soon :)T:
 

·
Registered
Joined
·
21 Posts
Re: Charge Pressure Solenoid Valve N75 - Testing

Hi

I would like to say a massive thank you to Simon for this post and excellent diagrams.

Im a new member (first real post i suppose) and have recently bought a 102 t4 and i found it quite sluggish and it really suffered on the longer hills.

There is a biggish hill near me (Bewdley by-pass) that i could only get 40-45 tops out of her without going mad.

I found this post and thought to myself that i have not heard the turbo whoosh since ive had it. I found a few minutes to have a nose under the bonnet and found the N75 valve plumbed up completely wrong.

I followed your diagrams, re-plumbed the pipe and took her for a test drive.

Its a world of difference, i can now hear the turbo and it did the hill at 65mph no problem.

Top Man

Cheers

Rob
 

·
Registered
Joined
·
91 Posts
Hi all,
Just before i bite the bullet and buy a new N75 i had better ask the question.
My 2.5tdi 88bhp 2002 t4 goes along nicely on flat, downhill but when a slight incline is involved it looses all boost and goes into limp mode, which resets back to square one when i switch off the ignition and restart the van (not good at 3am on lonely dark country lanes.

I have checked resistance and am getting 35 ohms across terminals.
I have checked voltage feed and all ok there.
Checked plumbing of N75 and all in correct locations with new hoses

So question is, can this fault still be the N75 or will i be wasting my money buying a new one?

Hope you can help :*:*
Chappers:ILU::ILU:
 

·
Registered
Joined
·
3 Posts
Thanks for this - just managed to get my turbo boosting properly again after a mangled cambelt. The garage had managed to connect the N75 up back to front, so no pressure to the turbo actuator and no boost. 1/2 hour with this guide and it was sorted.
Keep up the good work and cheers!
 

·
Registered
Joined
·
6 Posts
G'day,
I have pretty much a similar problem to Chappers. It happens randomly and resets everytime by turning ignition off and back on again.
Cheers for a great post.
 

·
Registered
Joined
·
183 Posts
A friend of mine is having problems with losing power, so have checked on vagcom - MAF and map seem to be fine. Then just found this guide, having checked the resistance its just over 45ohms, 48ohms to be exact. And when checking the voltage have only got 7v across the plug.....anybody have this issue or know what to try next?

2.5TDI ACV

Thanks

Steve
 

·
Registered
Joined
·
59 Posts
Having same issues with mine, plus VAGCOM says N75 ground/earth intermittent. Good strong 12vsupply across the pins, couldnt reach the ones on back of valve with the votmeter but im pretty confident its the N75 if ive got a good supply...
 

·
Registered
Joined
·
2 Posts
Thanks Guys - Had a T5 for a year now and despite speaking with a local garage and a VW dealer was unable to diagnosis the fault, been told it was either the clutch or the ABS pump (both mega expensive jobs).

Found this last evening described the fault in a linked post (mooing noise a I called it a buzz, maybe that where I went wrong!).

Bought the valve today fitted it (15 minutes test) - driven sorted. £40!!

Thanks again had to join to post my thanks
 

·
Registered
Joined
·
150 Posts
Simliar problems and help welcomed.
I have read every forum post on limp mode and power loss and a lot of help acheived.

2.5tdi 102bhp acv

I have power loss of around 10-15% almost all the time now, it was occasional and presumed this to be limp mode.
Changed all pipes for silicone, replaced weak battery, cleaned ecu cover and replaced, had new cambelt and reassured its "spot-on"!
Tried driving with/without air filter, loads of suction and no difference.
Turbo whistles as it should.
Tried driving with MAF sensor unplugged, no change
Bought the cheapo vagcom cable and free software, plugged in and reads:
05500 Start of Injection Regulation
17-10 Control Difference - Intermittent
Cleared this, but comes straight back

This week i have checked n75 voltage, I have 12v on pin 1 to earth, 7.5v pin 2, and 7.5v pin 1 to 2.
Same reading at point where wires come out of ecu
Ohms reading 33
N18 similar

IMPORTANT:
NOW THEN, drove 4 miles, disconnected/reconnected power to both N75 and N18, and I GOT POWER BACK
Did the vagcom thing again, and no errors
Drove all day on and off with full power, BUT, once engine cooled(4 hours), i was back to power loss.
Did the same steps, and power back again.


I presume changing the N75/N18 are pretty straight forward though i dont wont to damage anything, 1 input and 2 output air pipes on each?

What is the small filter fitted via pipe from N18? This has a tiny whole around (3mm circular puncture wound) in the filter itself inside its housing, might put a blob of glue on it as i cant identify the part

Had this ongoing for 4 months and starting to lose hair!

Please please please, advise and help welcome
(Thanks to Simon and all you others for getting me this far)
 

·
Registered
Joined
·
26 Posts
Hi

I am trying to diagnose why my 1.9td t4 has lost power, ive been looking at your post about the n75, on my van i think this is located next to the expansion tank is this the right thing and do i go about testing it in the same way
 

·
Registered
Joined
·
17 Posts
Hi I am having problems with my t4 2.5 88bhp followed these instructions on how to test . The one plug running to back of n75 is running at 7 volts would this be the problem ,ingnition was turned on no engine running . The problem is the pipe to actuator is doing nothing seems I got no turbo?
 

·
Registered
Joined
·
403 Posts
Great tech article, helped me diagnose my duff N75, replaced it plus all pipes and got power back, but intermittently.. now have N75 open or short to ground fault code, got 12v on pin 1 so it looks like a wiring issue, followed the article but finding the problem is beyond me, time for professional help me thinks !
 
1 - 20 of 55 Posts
Top