I just had my 2000 A4 1.8TQ (ATW engine) chipped with GIAC and am getting the following fault which indicates (perhaps) a faulty N75 valve (wastegate controller):

P1556 17964 Charge Pressure Contr. Negative Deviation
N75 - Wastegate Bypass Regulator Valve

I think the code is thrown when boost is not reached, which is odd, since the car is definitely faster etc. Bentleys has a test procedure that I will try, but has anyone had any reports of this? [/align]
TIA[/align]

 

see if the DV holds vacuum or the vac/pressure line to the top of it is split. also check the lines to the N75 for splits or cracks. I have seen both of these situations throw that code. If not it may be the n75

 

Negative deviation is overboost. My stock N75 valve does it to me everytime I go past 4k rpms at WOT. Though i dont get the n75 bypass fault code, I know its the N75 because this didnt happen when I was using the n75h/race valve. Im going to bypass the n75 altogether and run a mbc to see what happens. Maybe you should try the same. Just my 2 cents.

 

Thanks. I'm going to check the DV and replace it with a 710N I have laying around if the stock one is not an "N". The N75 is the stock C valve, but I may have an F valve around just to check things out.

Negative deviation? I wondered what that meant. But in the past, when I've had an overboost situation, I get an overboost code. The car did not feel that it went into soft limp. Does it always with overboost?

 

No boost gauge? I always go into soft limp (5-10psi until restart)when i get the neg. deviation code.
The stock DV is definately not an N, that might be your problem, especially if youre chipped..

Long time no see btw... Feel free to stop by even when you DONT have a problem!

 

LOL. Don't take it personally. It's been a while since I've posted on other forums too.

This is my wife's car and I couldn't stand driving it anymore stock. So, I got it chipped, but haven't installed a boost gauge because it would block her view of the temperature, etc. I may have to hook one up temporarily. I fiddled around with an MBC which sent it immediately into limp and I had to clear the codes with VAG-COM to get it out of limp (restarting did not help). This did not feel the same, but after I stopped for gas and restart the car, it felt quicker than before.

I may have to swap out the 710 for the 710N.

Thanks

 

Sorry dude, I am and have been wayyyyyyyyyyy off on the whole negative deviation thing... Wow, I am so pissed at myself.. I dont even know how I could have gotten confused... Negative deviation is lower than expected boost.. So, underboost, not overboost. This WHOLE time I have been misdiagnosing mine and other people problems with negative deviation fault codes.. Damn... Luckily, the solution probably still remains the same for most, which is a faulty N75 valve. But it definately points to a problem which physicalystems fromvery close to the turbo, in general. Wether it be the N75 valve, DV, wastegate, leaky wastegate hose, etc. Its probably one of the most difficult fault codes to diagnose. Good luck. Keep us posted.

 

Well, I did find an "F" valve laying around from a VW. It's behavior is close to the stock "C" valve, so I'll give that a try and let you know.

 

I checked the N75 valve according to the instructions in the
Bentley manual and found it within spec. But, what the
heck, I had an N75 "F" valve from a VW and decided to swap out the
stock Audi N75 "C". While I was at it, I swapped out the stock 710
diverter valve with a 710N (used in the TT and VWs with higher boost).

Well, the car is simply awesome (compared to the stock 150 hp). It's
fast and seems really strong throughout the rpms. I'm happy.

So, I went for a test drive and stopped for ice cream. With the car
running, I plugged in my laptop to an DC/AC adapter in the power
outlet to see if any codes popped up. There was a small spark and
then I noticed the EPC light on. I got the following codes:

Wednesday,20,June,2007,20:44:41:38444
VAG-COM Version: Release 704.1

Address 01: Engine
Control Module Part Number: 4B0 906 018 Q
Component and/or Version: 1.8L R4/5VT G 0005
Software Coding: 07251
Work Shop Code: WSC 06335
2 Faults Found:
18084 - EPC Warning Lamp (K132) Circuit: Electrical Malfunction
P1676 - 35-00 - -
18098 - Malfunction Indication Light (K83): Electrical Malfunction
P1690 - 35-10 - - - Intermittent

Readiness: 0010 1001

Now, the thing is, I can clear the codes and they will stay off--
sometimes. If I shut the car off and restart it with the laptop
plugged in, I get the first code again (and the EPC light). If I do
a throttle body adaptation (TBA), shut the car off and start it
again, I get the code. If I clear the codes, unplug the laptop, and
do NOT do a TBA, the EPC light stays off.

I've check the fuses. The car drives great, but the light comes on
(sometimes).

Any ideas what's going on? Could it be the N75F? Did the spark
screw something up? Etc.?

Any help is appreciated.

 

The EPC has to do with the drive by wire sysem. When you say spark where do you mean