PD Underboost limp mode VCDS diagnostics tips n' tricks

Zeitgeist

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Total VCDS noob here. For the last four or five months I've been struggling to figure out why our recently acquired '04 Passat BHW will go into limp mode nearly every time we drive it around, especially on hills. I've checked all the hardware (turbo-removed and cleaned, intercooler pipes--smoked and non-leaking, EGR-deleted, MAP, MAF, N75, tandem pump all replaced), so I'm on to figuring how to run some real time diagnostics using my ridiculously expensive VCDS wireless dongle. As is Bentley's MO, there's no manual for the last few years of a chassis, just like with my Eurovan, so I'm flying blind.

Is there some kind of VCDS for Dummies primer on how one explores the diagnostic tools? I don't even know enough to run a basic search query that can tell me something about measuring blocks, etc. Lil help?
 

Tdijarhead

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Type in Ross-Tech in the search bar on YouTube. Happy video watching
 

Zeitgeist

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I know I deserve that kind of response, but I require a little more hand holding and gentle pats on the head. The VCDS website isn't very noob friendly, though I am trying to navigate to find my own information. Perpetual limp mode is a sad metaphor for my entire life.
 

DivineChaos

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Listing what codes vcds comes up with might help a little more. You could be pointed in the correct direction of where to look and what to look at.
 

Zeitgeist

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It's the ubiquitous P0299 underboost code that seems to afflict every BHW, which can always be resolved by fixing boost leaks except when it can't.
 

Zeitgeist

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Thank you, that's very helpful. Knowing what to look for is half the battle.

Now I'll slink away and read the wiki.
 

Zeitgeist

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'04 Variant and Vanagon mTDI
I'm having intermittent limp issues that seem to be trans related, but my generic code reader doesn't show any trans codes. When I attempt to use my VCDS Mobile reader and run an autoscan, it hangs up at 5% and then never goes beyond. If I try to just run a trans module scan it indicates that CAN failed. It won't even run a basic OBD2 scan for me any longer. It's utterly useless to me.
 

TurboABA

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I'm not familiar with VCDS mobile, so have you tried contacting RossTech for help on the issue?
Sounds like you have some OBD port issues or wiring things to sort out since you can't communicate with the modules.
 

TurboABA

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Sounds like they've seen this issue before with that interface
 

Zeitgeist

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Ugh...but mine is 9% worse. I guess I'll need to register on their site and figure out how to upload files and such.

Thanks
 

TurboABA

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I didn't spend a lot of time looking into it, but it seems fairly "common" and could be as simple as needing to update your firmware or something... don't be such a snowflake :D
 

Zeitgeist

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Ha! I just plugged it into our Eurovan and autoscan and access to the TCU were just fine. Apparently this is an issue with the Passat's wiring system. Not sure what to do about that.
 

TurboABA

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Actually, I just thought about this... your PORT should be ok if I understand what you've said above correctly.
Check this by going into autoscan and clicking gateway installation list.... it should provide clues of the potential wiring\module issues that can't be reached.


That way, you can start your wiring checks at those respective connectors\modules.
If my theory is correct, you should be able to run diagnostics\communicate with some of the individual modules, but not the ones that you're having issues with.

Since the slushbox is near the top of the list, it would make sense that it hangs\crashes early in the process (5% or whatever).
 

Zeitgeist

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Ah ok, it makes sense that it's hanging at the transition from the engine to the trans. It hasn't always done this, but it's been pretty consistent lately. I should say that back when it wasn't doing this, the car was going into limp mode nearly every time we drove it. Now it only does it randomly and far less often.
 

TurboABA

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I'd say he needs to examine\focus on the TCM (J217) 88pin connector and harness since he can't seem to reach it with VCDS (supposedly... since we haven't had confirmation of failed communication from the VCDS tool as requested in #17 above..... but it seems feasible).

Once we know what the TCM is reporting, we can suggest what else needs digging into.
 

Mozambiquer

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I'd say he needs to examine\focus on the TCM (J217) 88pin connector and harness since he can't seem to reach it with VCDS (supposedly... since we haven't had confirmation of failed communication from the VCDS tool as requested in #17 above..... but it seems feasible).

Once we know what the TCM is reporting, we can suggest what else needs digging into.
Which means that he should pull the TCM out and check for water damage.
It's under the passenger side floorboard, you can get to it without removing the front seat by pulling the side trim out and lifting the carpet up and then there's a black box in there that you pull up to release from the clips and pull it out. Open that box and the TCM is in there.
I will link my thread from when I had TCM connection issues. I learned a ton through that.
 

Mozambiquer

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Zeitgeist

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Thanks for that link ^^^

I'll try and run that VCDS procedure in #17 this weekend and report back. I checked my TCM a few weeks back, and there was no sign of ever having water intrusion into the plastic case. As per the aforementioned link, I might need to pull the module out and have a peek inside the metal case, as well as run some resistance tests.
 
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