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View Full Version : How to Test an N75 valve? And other bonus questions!



WonPointAteTea
03-26-2009, 12:03 AM
Car: 2000 A4 1.8t, quattro, Tip, Speedtuning chip 1.1 bar, forge DV, everything else stock

Heres the three questions i have and hopefully someone can clear this up for me:

1. Is 1st gear really supposed to be super laggy? the car falls on its face from a dead stop

2. Is the boost limited on autos by gear? I have read mentions of this on a few other threads but never really a confirmation one way or the other.
Example my car 1st-3rd is about 10psi 4th 13psi and 5th 15-16psi Does any body else's tip do this?

3. How do i tell if my N75 valve is bad? Maybe it is contributing so some of these other issues i am having. Also if i am at WOT in 4th or 5th my gauge will read back and forth between a 2 psi range so could an N75 be causing this? Also i have noticed that if i ease on the throttle the peak boost will be higher than if it nail the throttle (peak not the spike). I have read you can do a output test with vagcom to make the solenoid click, but i was wondering if there was some kind of pressure test i could do to make sure the valve isnt at fault here (like maybe blow on one of the three ends to see if it hold the pressure or not and whats normal). I pressure tested the car itself and it seemed fine and i have already replaced most of the vacuum lines, check valves, and the infamous L-Hose


Thanks and i would love to hear any input. I have been a long time lurker and the forum has helped me battle a lot of problems but these i was hoping to get some help with.

ModifiedA4
03-26-2009, 11:10 AM
you can check a n75 by testing its resistance. there used to be a good post on Audiworld listing all the resistances of the various N75 valves.

good luck finding the post with the crappy vB search + IB screw over.

ct1.8t4me
03-26-2009, 11:26 AM
I want to say it was 27 ohms or something like that.

WonPointAteTea
03-26-2009, 02:14 PM
ok thanks i did some more searching and 25-35 ohms seems like that considered "normal" so i will test that when i get home

my other question is when i have the N75 out if i blow in the passages, which ones should i not be able to blow through, or which ones should be air tight? It would seem to me that the valve should be air tight in each, especially the one from the compressor housing and the one to the wastegate... does this seem right? I am wondering if the valve might be leaking. Can anyone say one way or another what should be air tight or if it doesnt even matter? lol Thanks alot!

http://pictureposter.audiworld.com/12010/audin75.jpg

ModifiedA4
03-26-2009, 04:52 PM
ok thanks i did some more searching and 25-35 ohms seems like that considered "normal" so i will test that when i get home

my other question is when i have the N75 out if i blow in the passages, which ones should i not be able to blow through, or which ones should be air tight? It would seem to me that the valve should be air tight in each, especially the one from the compressor housing and the one to the wastegate... does this seem right? I am wondering if the valve might be leaking. Can anyone say one way or another what should be air tight or if it doesnt even matter? lol Thanks alot!

http://pictureposter.audiworld.com/12010/audin75.jpg

IIRC you should be able to blow from the compressor to the wastegate side. when the valve is activated, the wastegate to TIP path opens and the compressor path closes.