Volkswagen Passat Forum banner

Quick Help Needed FAST - How to test N75

1 reading
24K views 33 replies 11 participants last post by  deusexaethera  
#1 ·
Hey, I've got Urlik here in ABQ trying to get him home with some power. Need to test N75. Can't find using search, my Bentley is at home.

How do you test it with a voltmeter? Is it supposed to be like 14 Ohms resistance between the contacts?

Problem: No boost. No boost gauge, but you can tell there's no boost when you drive it.
Codes: One intermittent overboost code
Fuel Trims: -1/-3 i.e. perfect, no indication of vac or boost leaks.
MAF: logs 2.5 g/sec at idle, up to 38-50 g/sec max. Seems like MAF is logging lack of boost accurately, so I don't think it's an MAF problem.
Logging Data while driving: Block 114 Wategate duty cycle goes to 99.6%, block 099 fueling goes from closed-to-open loop, so the chip is working and asking for boost (i.e. it's not in limp mode).

Therefore something downstream from ECU is not working, either N75 or Wastegate. Accurate conclusion? Hence need to test N75.

Thanks - Rusty and Urlik
 
#6 ·
I don't know if it helps, but perhaps disconnecting the MAFS temporarily
just to see if it isn't just slowly dying may help to discount it. Any codes?
 
#7 ·
Thanks everyone.

No codes stored related to N75. Only code was the intermittent overboost, but now that I've cleared codes, it hasn't come back:

VAG-COM Version: Release 607.2-UD

Control Module Part Number: 8D0 907 557 P
Component and/or Version: 1.8L R4/5VT MOTR HS D04
Software Coding: 06001
Work Shop Code: WSC 05311
1 Fault Found:
17965 - Charge Pressure Control: Positive Deviation
P1557 - 35-10 - - - Intermittent

Readiness: 0000 0001


There wasn't a convenient way to clamp off the line to the wastegate, so we're just gonna try a new N75 - the local Audi dealer has one (VW dealer didn't).
 
#9 · (Edited)
I put on the new N75 and there was no change at all. While there I threw a vaccum pump on the wastegate, no apparent movement ov the rod on vaccuum. (only moves on pressure anyway right?) I've got no internet access in my hotel this weekend but I'll check back later to see if anyone has another idea.
 
#10 ·
This is really getting frustrating. I just finished driving about 15 miles around town. While driving I got partial boost back. It feels like stock to me and not chipped boost. I just hope it holds up but I'm afriad it won't.
 
#17 ·
Yeah, that's the next step. Would have been nice to try it B4 the N75, but that's just the way things went.

I agree with Paul that the MAF readings seemed a bit low. But if they were low, then wouldn't he be getting positive fuel trims to make up for the low air reading(s)? His trims were -1/-3.

Timing and knock retard look normal.
 
#20 ·
Did you replace the diverter valve yet?

Secondly, my $$ is on a vacuum leak under the manifold. Something is telling the wastegate to cycle like crazy, so there must be a bad signal either to the wastegate (N75), or from the charge pressure sender. The cruise works by actuating the throttle and still takes manifold pressure into account. If it can't get the throttle response it wants, it shuts down. The failing CC is a symptom, not a cause.

I'll try to remember to look into this more tonight at home with the Bentley. Check your grounds. Particularly the ones that the N75 and charge pressure sender go to.
 
#33 ·
...Check your grounds. Particularly the ones that the N75 and charge pressure sender go to.
No go, he has an AEB, no charge pressure sender.

Which is what makes the "charge pressure control - positive deviation" code so weird. For an AEB to get this code means that the ECU saw too high of an MAF reading for a given RPM and throttle opening. Which could point to a bad MAF.

However, I keep coming back to when I logged WG duty cycle. During the periods of "no boost", the ECU is asking for boost. Which kinda shoots down what I just said in the post above! :lol: The ECU is not getting what it's asking for. Aside from a pinched N75 harness, doesn't it simply have to be either the N75 or the WG? And he's replaced the N75, that leaves WG, right? What else could it be? :confused:

I suppose if he got full boost with the WG line removal, then it would indicate bad wiring to the N75?
 
#21 ·
any updates guys and gals? I got a steady 5.X volts going to the N75 and a stable 35 OHmS out of the unit itself. I am thinking wastegate. Have you guys pulled yours off yet? HOw did you get down in there to apply vacum to it or did you just suck on the hose with your mouth?
 
#27 ·
Well, I made it home safe today. I got a check engine light after fueling in Vegas but it cleared itself later on. I'll pop it on the VAG-com this weekend to read it. I'm with AA, I'm checking vacuum lines first. I was pretty rough on a couple of them replacing the check valves and digging the "T" pieces out. It's late, more info later. Suffice it to say, half boost from ABQ to Vegas with cruise working for 300 of the 500 miles. Some boost from Vegas to Great Falls today with the cruise only working about 35 of 990 miles.
 
#28 ·
2 Faults Found:
17965 - Charge Pressure Control: Positive Deviation
P1557 - 35-10 - - - Intermittent
16521 - Oxygen (Lambda) Sensor B1 S2: Signal Too Low
P0137 - 35-10 - - - Intermittent


Friggin still no good clues. I think the O2 sensor set off the check engine light but the light reset after a couple more starts. I'll start replacing some vac lines next weekend.
 
#29 ·
Drove the car to work this morning and everything is working like it should!!!! The only thing I did over the weekend was move some vacuum lines around. I noticed a nice divot worn into the one to the cruise throttle actuator but it didn't appear to co through the whole way. I'll replace vac lines this weekend.
 
#32 ·
Have you tested the wastegate yet? You need to pinch off the line from the N75 to the wastegate. I prefer to simply take it off. Drive carefully, because, if the wastegate is working, you will get full boost all the time, except as you moderate the throttle. I wish we'd had time to but my MBC on your car, that would've told us something in a much safer manner than removing the line to the WG.

There's something that's bugging me about this, though. Seems like there's something we're missing. It's so weird that you have no codes for anything else, and the fuel trims are OK. I'm starting to suspect solder problems with your chip, or something like that.