VW AWD systems (jetta, passat, even phaeton)

Something i'm just curious about... is there much electronics (or any) involved in the AWD systems that VW puts out? For any vehicle really from the golfs and jettas up thru the top of the line phaetons.

Like i'm wondering if they would be "engine swap friendly" because it's just mechanical and hydraulics or if they've got their own dedicated ECU's that have to tie into everything else and systems wont function without them. I'm wondering if special strategies or procedures have to be applied on AWD cars or if it's just bolt and go/at least there's nothing to go wrong there.


The phaeton question is probably a bit of a ringer for here but having heard of the cars and their endless electrical and computer problems i've often wondered if something like a chevy LS swap and trans would fix them. :^)

The Haldex AWD systems (transverse engine) have an electronically controlled clutch in them. There is no center differential. The clutch variably locks together the front and rear drivetrains. https://www.youtube.com/watch?v=tnHPVbSF0cs skip to about 3 minutes in.

I don't know much about the longitudinal systems (which are really Audi drivetrains) but it's rare nowadays to not have electronic control in the system somehow.

Are they all Haldex or are there any other systems in the last two decades of VW/audi powertrains?

Do they have their own ECU that in any way can stand on it's own, or is it hopelessly networked, or is it an unknown/nobody's tried swapping a nonbelonging powertrain before?

The longitudinal car systems are Torsen, and there are a few different versions. But generally they work without any electronic input, and are regarded as superior in power transfer in snow and mud and such. VAG applications would be Audi A4, A6, A8, A5, A7, Q5, Phaeton, as well as the 1998-2005 Passats. These systems however become pretty dependent on an appropriately functioning traction control system to properly vector torque application but will still get much of the power to the ground no matter what. I have a 4motion 2004 Passat wagon, and it is ruthless in the snow. Its only limiting factor really is ground clearance.

Transverse systems are Haldex, with a controller that the engine (and if automatic) transmission, ABS, etc. controllers work with. VAG applications for this would be Audi TTs and A3s, Q3s, Tiguans, Golfs (including R32/R).

The VAG "truck" systems are more arranged like a conventional pickup truck, which for us would be limited to the T'reg, Cayenne, and Q7, and there are a couple different versions of these.

Context to your question would be helpful in giving an appropriate answer.

FWIW, I would pit the longevity a properly maintained belt driven 4.2L VAG V8 up against any "LS" engine any day. They may not be as easy to work on given how they are stuffed into the car, but they are a nearly bulletproof engine. We have LS engines here blown up all the time from low oil pressure. Got one here now.

oilhammer said:

Context to your question would be helpful in giving an appropriate answer.

FWIW, I would pit the longevity a properly maintained belt driven 4.2L VAG V8 up against any "LS" engine any day.

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Wow thats encyclopedic... context is part engineering curiosity and part hot rod part swapping mentality. (whether other parts into VW, or VW into other cars)

The wondering about LS's is i've been a fan of the Phaeton for years and always wanted one (tho it will probably be a few years before I could), I was surprised how cheap the V8's are, but part of it seems to be serious reliability issues incl from too many electronics and repairability issues of a vehicle platform so rare - it's not the fundamental physical reliability of the engine but just what a downed sensor can do to you. Or I hear of things like "the theft immobilizer is malfunctioning but it will take 3 weeks to get a replacement part from germany". Or people saying "the best days of my life were the first day I bought the phaeton and the day I sold it". All that makes me wonder if an LS swap with standalone controllers might somehow give W12 power to the less expensive V8 cars. I start having hot rodder thoughts of thinking I could solve alot of problems shoving an american V8/trans in there and have parts at every autozone - as long as there's not ECU's so tied to one another in the car network/CANBUS stuff that it would just not work.

Yet i'm just as curious about the AWD in other cars like the Passat, I live in the north where snow and ice are a major yearly issue. Again I hear they have bad automatics, and although stick shift swaps are possible this is more curious about a year round incl winter car for the girlfriend likely to become wife. I wonder "could I just swap something" if the automatics are crap, not an LS (too big) but something else with a more reliable auto, and tie that into the AWD. I love a stick but she likes the automatic esp for big city commuting, and the winter.

The Jetta systems sound rare and expensive enough that i'm unlikely to ever seek one - but again just curious.


EDIT: like one of the questions i'm trying to ask is which systems will get you moving with three tires slipping? Concerns raised by this video https://www.youtube.com/watch?v=2OzK-oRPCbs where a VW slips made me wonder more about what was driving, which cars had which systems - it was one of the also linked videos after I watched the previous video.

Quattro with Ultra Technology is very similar to a Haldex system, made by Magna, it can decouple the rear so it can run FWD for fuel economy, but unlike a current Haldex, it can send up to 70% to the rear. Ultra Technology can be found on the A4 and Q5.

Also, the newer Audi Quattro went away from Torsen, in favor for the Crown Gear differential.

midnightoil said:

EDIT: like one of the questions i'm trying to ask is which systems will get you moving with three tires slipping? Concerns raised by this video https://www.youtube.com/watch?v=2OzK-oRPCbs where a VW slips made me wonder more about what was driving, which cars had which systems - it was one of the also linked videos after I watched the previous video.

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The old school way to address this was through locking or mechanical limited slip differentials, front and rear, and with the front and rear drivetrains also locked together, either through the use of a part time (old school pickup truck style) 4wd system or yet another locking differential in the center diff. Locking front differential is not common on road vehicles because it causes steering interference. Most of the traditional 4 wheel drive pickup truck systems have an open front diff, and cannot cope with both rear wheels and one front wheel slipping.

The modern way to do this (and the Trailhawk system in for example the Jeep Cherokee works like this) is to have open differentials all around (center, front, and rear) and have the traction control system apply the brake on wheels that try to spin.

By the way, that video (clearly a Subaru advertisement) essentially claims that transverse drivetrains lead to torque steer and not being "symmetrical". That's a load of bunk. The differential will give the same torque to each side, no matter whether the drivetrain upstream of it is longitudinal or transverse. Unequal length drive shafts can lead to torque steer if steering geometry is wrong ... but it's easily sorted out, and manufacturers of vehicles with transverse engines figured this out a few decades ago.

We have a Volvo XC90 that uses the Haldex system. The driveshaft through the angle gear turns all the time and the Haldex unit at the rear differential engages as is needed. Not very fuel efficient. This is my wife's car and she won't have a non all wheel drive car.

After the buyback I went to VW looking for another new sportwagon (gas) without all wheel drive and they didn't have any. I grew up driving in snow and don't want the extra expense or future maintenance problems associated with it. Since then I found a 2014 TDI FWD sportwagon and am so far happy again.

Almost all AWD systems today rely on the native traction control that is a built in side effect of mandatory ABS and ESP. Because it is pretty easy to just squeeze one of the wheels' brakes a wee bit to induce the torque transfer somewhere else, and this happens pretty quickly.

This is also better for fuel economy, as it takes away some of the mechanical drag associated with some sort of differential or clutching mechanism. Also makes for easier sharp turning: example, the Lexus RX300 ---> 330 (Toyota Harrier). The 300 had a limited slip clutch type center differential, so fore-aft torque application was controlled via that mechanical device. They had traction control VSC, but it was an extra layer. The mechanical passive system alone was enough for most conditions BUT it made the drivetrain "bind" when going around tight turns. Toyota's remedy for that was simply to limit how far the steering could turn. Which resulted in the not-so-nice problem of having the turning radius of a small school bus. If you have ever driven on of these RX300s or the first gen Toyota Highlander (same system), you know exactly what I am talking about. It was awful. But the system was simple, sturdy, and effective.

The RX330, and later generations of Highlander as well as the AWD Sienna, went to a conventional open center diff, and rely completely on the traction control system's ability to more quickly intervene to limit wheel slip. Software changes allowed this, as the ABS/VSC system is essentially the same. They also added in some faster coding for the steering angle sensor. So now, they were able to let the steering go further, and they turn MUCH tighter, and no worries about driveline binding. But if for some reason the ABS is not working, those vehicles are not going to be significantly better than the FWD version of the same vehicle under really slippy conditions, where the older RX300 would slarp right on through, warning lights ablaze and all.

midnightoil said:

EDIT: like one of the questions i'm trying to ask is which systems will get you moving with three tires slipping? Concerns raised by this video https://www.youtube.com/watch?v=2OzK-oRPCbs where a VW slips made me wonder more about what was driving, which cars had which systems - it was one of the also linked videos after I watched the previous video.

Click to expand...

In that video, note, they only tested that scenario with the Legacy GT, which has a rear viscous LSD, so it was able to transfer the power to the rear wheel that has grip.

They didn't test that with the Forester X, as it had a non-traction control/ VSC assisted AWD system. So, it would have failed.

The ability for EDL to assist the AWD system (Audi/VW has been doing this for a number of years), is dependent on how aggressive the ABS brakes the slipping wheel. On some videos, you can see Subaru really holds the slipping wheel longer as you can see the slipping wheel physically pause for a moment.

oilhammer said:

Almost all AWD systems today rely on the native traction control that is a built in side effect of mandatory ABS and ESP. Because it is pretty easy to just squeeze one of the wheels' brakes a wee bit to induce the torque transfer somewhere else, and this happens pretty quickly.

This is also better for fuel economy, as it takes away some of the mechanical drag associated with some sort of differential or clutching mechanism. Also makes for easier sharp turning: example, the Lexus RX300 ---> 330 (Toyota Harrier). The 300 had a limited slip clutch type center differential, so fore-aft torque application was controlled via that mechanical device. They had traction control VSC, but it was an extra layer. The mechanical passive system alone was enough for most conditions BUT it made the drivetrain "bind" when going around tight turns. Toyota's remedy for that was simply to limit how far the steering could turn. Which resulted in the not-so-nice problem of having the turning radius of a small school bus. If you have ever driven on of these RX300s or the first gen Toyota Highlander (same system), you know exactly what I am talking about. It was awful. But the system was simple, sturdy, and effective.

The RX330, and later generations of Highlander as well as the AWD Sienna, went to a conventional open center diff, and rely completely on the traction control system's ability to more quickly intervene to limit wheel slip. Software changes allowed this, as the ABS/VSC system is essentially the same. They also added in some faster coding for the steering angle sensor. So now, they were able to let the steering go further, and they turn MUCH tighter, and no worries about driveline binding. But if for some reason the ABS is not working, those vehicles are not going to be significantly better than the FWD version of the same vehicle under really slippy conditions, where the older RX300 would slarp right on through, warning lights ablaze and all.

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The early RX/highlander did use a variant of the old All-trac system.

Then on the 2nd generation, used 50/50 permanent split, with traction control braking the slipping wheel.

Then when Toyota introduced Active Torque Control, the center differential went away with the electromagnetic clutch at the rear differential, with no center differential, that is only capable of sending up to 40% of the torque to the rear.

Current RX/Highlander/RAV4 have the next generation, Dynamic Torque control, that can send up to 50% to the rear. The Sienna AWD retains the older Active Torque Control.

Yeah I haven't kept up on the newest stuff. We did just have one of those NX things in here (hideous) and its AWD system was clearly a bit different.

And I forgot about the newer Audi changes, which started with the A7. But I think the hardware change was more to allow a greater level of flexibility as well as giving the normally FWD biased big cars more of a RWD feel.

I still like manual controls when the going gets rough. Knobs on the dash you pull, or levers you move. But that is all more for off-road, not keeping you on the road, and there is a difference.

FWIW, my 2004 Passat TDI 4motion wagon with 5 speed manual has no electronic controls (ABS, ESP, ASR) whatsoever. The ABS is not even tied into the CAN system.

The manual transmission AWD cars with longitudinal drivetrain were set up this way through the mid 2000's for sure. Not exactly sure when that ended.

I also agree with Oilhammer about the timing belt 4.2 V8. Always have my eye out for those.

So before I come full circle back to AWD... can VW electronic controls (ABS, trac, stability) work independantly of other powertrain swap/heavy modification conditions or are they tied in? I would think traction control is at least tied into the engine to cut power, and no idea whether it might be possible to have a VW traction control do something like operate an aftermarket EFI system (megasquirt or anything else) in some fashion... but even without that i'm wondering whether stability control could work, or at least the ABS standalone which to me is the most important.

Short answer "no" because it's all part of an integrated system. Longer answer "maybe". I have no idea how much provision has been made to allow continued ABS operation in fault ("I don't have communication with the engine") conditions. Longer answer "maybe" if you have a vehicle that dates back to the old days where ABS was optional and had its own separate controller (late eighties through mid nineties give or take).

Most major engine swap or hot rodding projects (sufficient to not be able to retain the OEM ECU or an ECU that was otherwise intended for use within the same vehicle generation) don't retain ABS, stability control, etc.