Possible N75 problems

pkhoury

That guy with the goats
Joined
Nov 30, 2010
Location
Medina, TX
TDI
2013 JSW, 2003 Jetta Ute, 2 x 2002 Golf, 2000 Golf
I'm trying to chase down some N75 issues. My tuner has complained of low N75 values.
Finally got some time to do a quick check before I head out to the junkyard (so I can get my other 2002 Golf back on the ground).
The first picture is before the check valve. The second one is after the checkvalve. The first photo took about 3-4 seconds to build up vacuum. When I connected the mitivac to the hose, before it goes into N75 for the second photo, vacuum built up almost instantly.





When I run basic settings 11 in VCDS, it shows 22inHg with N75 charge pressure at 0%. It feels like that's a little on the low side. Should I be looking at possibly replacing N75?
 

P2B

Top Post Dawg
Joined
Jan 11, 2006
Location
Toronto & Muskoka, Canada
TDI
2002 Jetta, 2003 Jetta, 2003 Jetta Wagon
When I run basic settings 11 in VCDS, it shows 22inHg with N75 charge pressure at 0%. It feels like that's a little on the low side. Should I be looking at possibly replacing N75?
Sounds normal to me. Turbo should hit full boost at 18inHg, you've got headroom.
 

pkhoury

That guy with the goats
Joined
Nov 30, 2010
Location
Medina, TX
TDI
2013 JSW, 2003 Jetta Ute, 2 x 2002 Golf, 2000 Golf
If that's the case, then why does one car hit 25inHg, but this car hits 22inHg? Both are 2002 Golfs, same color even.

I measured voltage on the N75 plug - on the one with 22inHg, I'm getting about 10.7V. On the 25inHG, it measures 11V.
 

P2B

Top Post Dawg
Joined
Jan 11, 2006
Location
Toronto & Muskoka, Canada
TDI
2002 Jetta, 2003 Jetta, 2003 Jetta Wagon
The difference in meter readings probably explains it. The ECU sends a PWM (pulse width modulated) signal to the N75, and you have one car where 0% duty cycle translates to 10.7v, but on the other 0% reads 11v. Different tunes maybe?
 

pkhoury

That guy with the goats
Joined
Nov 30, 2010
Location
Medina, TX
TDI
2013 JSW, 2003 Jetta Ute, 2 x 2002 Golf, 2000 Golf
The difference in meter readings probably explains it. The ECU sends a PWM (pulse width modulated) signal to the N75, and you have one car where 0% duty cycle translates to 10.7v, but on the other 0% reads 11v. Different tunes maybe?
Same exact tune actually. Got that 11v value with both stock and the tune I'm working with.
Talked to another friend, who suggested checking to see how ground is.
 

tdi_my live

Veteran Member
Joined
Jul 24, 2011
Location
Germany
TDI
Skoda Fabia
If same Tuning with same PID values/maps than the Hardware is different and the reaction is different.
 

pkhoury

That guy with the goats
Joined
Nov 30, 2010
Location
Medina, TX
TDI
2013 JSW, 2003 Jetta Ute, 2 x 2002 Golf, 2000 Golf
If same Tuning with same PID values/maps than the Hardware is different and the reaction is different.
I'm not sure how this comes into play. Tuning is literally the same exact file loaded via MPPS. Both cars are the same year, except the one with 25inHg is a 5 speed swap. Same ECU.

For what it's worth, I was checking resistance to ground on the N75 connector. The reddish wire was some kilohms (I forgot how much and it's nearly 3am here). I do know that the yellow/black wire returned about 424 ohms to ground.

On my daily driver, which is the one that's outputting 22inHg, that showed as -70k ohms (not sure why the negative on my Fluke multimeter, but whatever).

I guess I now need to figure where the hell each wire goes without pulling an entire wiring harness, though it's tempting, since I'm hoping to raid another 2002 Golf at the junkyard (same color even as my three).
 

tdi_my live

Veteran Member
Joined
Jul 24, 2011
Location
Germany
TDI
Skoda Fabia
Because different PID parameters would result in different responses from the N75 control. Which isn't the case with exactly the same file. So this theory doesn't apply to you.
 

ghohouston

Veteran Member
Joined
Apr 2, 2013
Location
Lewisville, Texas
TDI
2001 Jetta Sedan TDI 5 Speed
Shouldn't the wires go to the e.c.m.? I don't have schematics, but surely the n75 wires come from the e.c.m. I'd find the two wires and check resistance end to end. Also check them against any shield wires in the engine harness too. I've dealt with some engine harnesses (not on tdi's, but other diesels) where wiring had shorted into a shield wire. Usually a c.a.n. wire. You could load test each wire individually. The k ohms and 424 ohm reading to ground you're finding, will go away if you disconnect the e.c.m. as well as the n75, and re check continuity to ground. If they don't, you have shorts. Not sure if power or ground is p.w.m., but I'd guess ground. That's in theory with the key off. If the key is on, ground resistance will go up.
 

P2B

Top Post Dawg
Joined
Jan 11, 2006
Location
Toronto & Muskoka, Canada
TDI
2002 Jetta, 2003 Jetta, 2003 Jetta Wagon
You might try swapping N75s between vehicles. If one of them is not passing 100% of the available vacuum at 0% duty cycle in both cars, that N75 probably has internal contamination.
 

pkhoury

That guy with the goats
Joined
Nov 30, 2010
Location
Medina, TX
TDI
2013 JSW, 2003 Jetta Ute, 2 x 2002 Golf, 2000 Golf
You might try swapping N75s between vehicles. If one of them is not passing 100% of the available vacuum at 0% duty cycle in both cars, that N75 probably has internal contamination.
Already done that on two cars. So a known good N75, on my daily driver, still outputs only 22inHg. So I'm like 99.99% sure it's still electrical in nature.
 

P2B

Top Post Dawg
Joined
Jan 11, 2006
Location
Toronto & Muskoka, Canada
TDI
2002 Jetta, 2003 Jetta, 2003 Jetta Wagon
Already done that on two cars. So a known good N75, on my daily driver, still outputs only 22inHg. So I'm like 99.99% sure it's still electrical in nature.
If the N75 output test is smoothly cycling the turbo between no boost and full boost (against the stop), and there is no leak down from the full boost position, then the actual vacuum number doesn't really matter.
 

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.. :/
If the N75 output test is smoothly cycling the turbo between no boost and full boost (against the stop), and there is no leak down from the full boost position, then the actual vacuum number doesn't really matter.
except that it will throw the tune off. but if it's linear, the rod length could be shortened (as long as it still hits the wide-open internal stop) or the tune could be tweaked. else it lags and most often followed by overboost, underboost, recover. conditions vary of course...
 

pkhoury

That guy with the goats
Joined
Nov 30, 2010
Location
Medina, TX
TDI
2013 JSW, 2003 Jetta Ute, 2 x 2002 Golf, 2000 Golf
This issue is really pissing me off. Tried swapping my vacuum pump from my known good working car. Nope. Ran a wire direct to the ECU for N75. That actually gave me about 23.5inHg instead of 22, but a few times it'd get to 24 and then back down to 23, or it would be jumpy. Checked all my vacuum lines, which are still relatively new. Tried a different check valve. It's pretty exhausting and the only thing I can think of that I haven't done yet is to disconnect the battery and clean all the ground contact points in the wiper cowl, and then see if that helps.
 

jmodge

Veteran Member
Joined
Jun 18, 2015
Location
Greenville, MI
TDI
2001 alh Jetta, RC2 w/.205's 5speed daily summer commuter and 2000 alh Jetta 5spd swap, 2" lift, hitch, stage 3 TDtuning w/.216's winter cruiser, 1996 Tacoma ALh
Are your ecu's deleted so you can swap them? That might narrow your search down
 
Top