How is boost controlled

xKailx

Member
Joined
Jan 10, 2017
Location
Noblesville
TDI
98 Jetta TDI
I've heard that some vw have diverter valves. Does the 98 Mk3 tdi have a diverter or is it just waste gate on the turbo? Thanks. Still searching but if someone knows that would be very helpful.
 

Mongler98

Top Post Dawg
Joined
Mar 23, 2011
Location
COLORADO (SE of Denver)
TDI
98 Jetta TDI AHU 1.9L (944 TDI swap in progress) I moved so now i got nothing but an AHU in a garage on a pallet.
Boost comes from the turbo to a slitter that takes it to the ECU and to the n75 valve, the n75 is controlled by boost from the intake manifold and gets its supply from the air filter box and sends it to the waste gate. thats it. the VTN turbos on the ALH engines use vacuum in a similar fashion. Yours being a 98 means its a AHU and this boost controlled as i stated above.
Why what you trying to do, you can build your own boost controller to up the boost manually for a few bucks at the hardware store.
 

xKailx

Member
Joined
Jan 10, 2017
Location
Noblesville
TDI
98 Jetta TDI
That's what I am attempting to find. Wouldn't mind upping the boost a bit once I order my tune from Malone. Just couldn't find what was actually controlling it. I've read that the more recent VW use diverter valves when boost gets to high however I couldn't find one on mine, nor a blow off valve. But from what I just looked up on the N75 valve is that the excess boost just goes out the vacuum tube to the air box. Is that correct?
 

KLXD

Top Post Dawg
Joined
Aug 22, 2009
Location
Lompoc, CA
TDI
'98, '2 Jettas
Like he said, the '98 has a wastegate.

N75 is a three way valve. The common port goes to the actuator, the normally open port to the intake and the normally closed to the airbox.

(Edited again to remove bad info as CoolAir pointed out)

It's a continuous process.

Edit: Had it bass-ackwards. Corrected thanks to compu 85's post below.

Too early.
 
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compu_85

Gadget Guy
Joined
Sep 29, 2003
Location
La Conner, WA
TDI
... None :S
Yup, the wastegate is set to open at ~8 PSI. To control boost, the ECU uses the N75 valve to block boost from reaching the wastegate (this is the failsafe - with no computer control the mechanical wastegate takes over). With the N75 being PWM'd, the ECU can call for more than ~8psi.

-J
 

CoolAirVw

Vendor
Joined
Nov 9, 2005
Location
Kansas City Missouri
TDI
Jetta
I'm dealing low boost problem on a 98 jetta tdi right now. I might start a thread of my own but need clarification on what you two are saying.



the wastegate is set to open at ~8 PSI.
What function makes the WG open at 8 psi? The spring inside the WG actuator? What pressure makes the WG open at 8ps? Boost pressure? Exhaust pressure on the WG itself?

Intake pressure is used through the N75 to hold the wastegate closed.
To control boost, the ECU uses the N75 valve to block boost from reaching the wastegate
Here's two things I know for a fact. From the car we're working on right now.

1. Spring inside the WG actuator holds the wg closed. Adjustment of the actuator preloads the spring about 5mm. (checked on two turbos)
2. Pressure on the WG diaphragm overcomes the spring and OPENS the WG. (checked on same two turbos)

If I'm understanding the descriptions above by compu_85 and KLXD, they are both saying that pressure passing through the n75 help to hold the WG closed. But pressure on the diaphragm OPENS the WG.

This is not an attack on these guys, and I am not even saying that these guys are wrong, but seems like the car I'm working on must be the other way around.

This may be a problem with my understanding of the 1z/ahu boost control system.

Based on #1 and #2 above my the theory I had been operating on was that the n75 opened and allowed boost pressure to pass through in order to open the WG. Theory being that would control over boosting. Just a theory though.
 
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KLXD

Top Post Dawg
Joined
Aug 22, 2009
Location
Lompoc, CA
TDI
'98, '2 Jettas
Demit. I splained it wrong again. You're right. Compu is right.

It's plumbed like I described but...

In the unpowered position N75 applies manifold pressure to the wastegate like you're saying. Electrically fail-safe at about 8 psi.

Power to N75 from the ECU vents opening pressure away from the wastgate actuator to the airbox so the WG stays closed allowing greater intake pressure than the WG setting as compu 85 explained.
 
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compu_85

Gadget Guy
Joined
Sep 29, 2003
Location
La Conner, WA
TDI
... None :S
What function makes the WG open at 8 psi? The spring inside the WG actuator? What pressure makes the WG open at 8ps? Boost pressure? Exhaust pressure on the WG itself?
The AHU / 1Z use a "conventional" wategate, spring pressure holds the wastegate closed. Preload on the spring sets the PSI the wastegate will open at. When 8 PSI of intake pressure air acts on the actuator, the wastegate starts to open.

On a non-computer controlled turbo the "feed" line for the wastegate is hooked up right at the turbo. My 91 Mercedes 350SDL is like this.

(image stolen from the web)


Some cars with a computer controlled wastegate work the opposite way: Spring pressure holds the wastegate open, and vacuum is used to draw it closed. A 91 Mercedes 2.5 turbo is like this. So when one of those goes into limp mode, the wastegate is never closed = no boost. On my SDL, the safety function is a pressure switch on the intake. If there's too much boost it blocks off the boost signal to the injection pump, cutting fuel. With less fuel, the turbo can't make as much boost.

-J
 

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis, MO
TDI
There are just too many to list....
The wastegate on the AHU is closed at rest, meaning full boost (gate closed, ALL the exhaust passes over the compressor drive wheel).

So any overboost condition on these gets the fueling cut by the ECU. Those of us old enough or been around here enough (as well as those of us who have owned and modded the AHU) remember the dreaded Warp Field Collapse that happened with early PROM chips. :D WFC could happen so abruptly and with such absolute authority that the engine could completely stall, and you'd need to cycle the key and restart the engine. I did this a lot doing burnouts in my '98. :cool:

The "fix" for this was to install a manual boost control valve, in parallel to the N75, that would act quicker than the relatively slow ECU could think, and push the wastegate open faster under these extreme conditions.
 

compu_85

Gadget Guy
Joined
Sep 29, 2003
Location
La Conner, WA
TDI
... None :S
So it seems to me if hose to waste gate was plugged the vehicle should over boost. Would this be consistent with your understanding?
Yup. That's why limp mode cuts so much fuel. Without the fuel "power", there's not enough exhaust energy to overboost the turbo. Same idea on my SDL.

When I picked up my SDL the boost enrichment line going to the IP was plugged. It had so little power that at WOT it wouldn't ever do the 2-3 gearshift!
 
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oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis, MO
TDI
There are just too many to list....
In theory, yes, but remember that the DLC only allows for a sampling of actual data... and on those older (slower) ECUs like the AHU had, the sample rate may be low enough that the overboost spike you witness on a VCDS graph could perhaps not be exact (but certainly pretty close).

I cannot recall ever graphing the boost on my '98 to see when WFC happened if it was lock step with an overboost spike or not. I did learn to drive with it and manage it though. :p
 
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