n75 valve operation

The Big wheel

New member
Joined
Nov 6, 2014
Location
Cincinnati ohio
TDI
1998 jetta tdi
Hello, I am new to this site and to the tdi world......but not to the diesel world. I own a 1998 vw jetta tdi with the ahu engine. I suspect that I have a bad n75 valve stuck in the gimme less boost condition. While in a hard pull at random times the engine will go into limp mode and sometimes give a p1550 code. I have checked for boost leaks vacuum leaks and found none. I have even checked the line inside the ecu. I have checked the intake for clogging and it is clean. I have checked power to the n75 valve and the resistance reading of the n75 valve and both are OK. With the n75 removed and on the bench if I apply shop air to the charge air port on the n75 valve (red house port) I have same amount of shop air comming out of the waste gate port or (blue hose port). These ports should not be connected to each other until there is power sent to the n75 valve correct? I think this is causing me too have low boost pressure during a hard pull causing the code and the limp mode. Is this possible? If anyone could give me some help it would be great. I do have a stage 1 malone tune with egr delete and a south bend clutch. Thanks
 

Digital Corpus

Top Post Dawg
Joined
Mar 14, 2008
Location
Ontario, California
TDI
'97 B4 w/ 236K mi body, 46K mi soul
The N75 operates off of a PWM signal. Thus, it rapidly opens and closes the valve to allow variable flow rates of air to pass through it. For troubleshooting low power, scroll up a little or click this link.
 

Windex

Top Post Dawg
Joined
Apr 1, 2006
Location
Cambridge
TDI
05 B5V 01E FRF
verify that the WG actuator and arm are working by applying pressure to the red hose. Easiest way is with a cheap bicycle pump.

Verify that red and blue lines are free of any oil.

You have already verified that the hose in the ECU is fine - good there.

If all that checks out, replace the N75.
 

Digital Corpus

Top Post Dawg
Joined
Mar 14, 2008
Location
Ontario, California
TDI
'97 B4 w/ 236K mi body, 46K mi soul
It doesn't toggle with a pulse. It uses a digital signal in a manner to produce an analog output.

Honestly, I cannot recall it's resting state, but I believe it would be open to avoid grenading the turbo.
 

The Big wheel

New member
Joined
Nov 6, 2014
Location
Cincinnati ohio
TDI
1998 jetta tdi
What is the the black line in the bottom of the valve for? I have read that the n18 valve for the egr valve is the same as the n75 valve but I didn't see a 3rd line going to the n18 valve
 

Windex

Top Post Dawg
Joined
Apr 1, 2006
Location
Cambridge
TDI
05 B5V 01E FRF
The black line exhausts pressure into the airbox. as above, the N75 is always switching on/off - the PWM signal determines how much time is on vs off.
 

The Big wheel

New member
Joined
Nov 6, 2014
Location
Cincinnati ohio
TDI
1998 jetta tdi
OK that makes sense now about the 3rd black line. ......I have applied pressure to the wastegate line. .....hose has no leaks and arm moves freely and does not hang. It almost has to be the n75 valve intermittently sticking. I just drove the car 990 miles from Ohio to Florida with no issue's and half way back from Florida to ohio on the biggest hill on the route back it threw the p1550 code and had no power. Pulled off the road cycled the key and had great power the rest of the trip home.
 
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