My  car is a 2016 150 TDI with 6-speed DSG.

 

If I switch off the Stop-Start mechanism and pull up at lights with the car still in Drive, what happens within the gearbox/clutch assembly?

I notice that the engine note drops below normal tickover, although the rev counter reads about normal (750 rpm or thereby) and I am concerned that

the clutch is dragging (and wearing) all the time I'm stopped.

My previous BMW torque-converter auto was said to have a device which completely disengaged the drive while stationary and prevented any drag.

 

Can anyone who understands the mysterious inner workings of the DSG say for sure that this box does the same trick?

 

 

If you foot is fully on the brake. Or the Auto-brake system (A) is engaged, and you have come to a complete stop, then the clutch is fully disengaged.  

Edited 10 June, 20197 yr by digifish

To confirm it, move the gear lever from D/S to N while observing the RPM while stopped under the conditions above.

I don’t think the clutch is fully disengaged, but it is nearly so. If you look at the VCDS parameter indicating the clutch pressure, it zero in N but non-zero in D when stationary with brake applied. I think it effectively “takes up the slack” when in D. The amount of drag is minimal and with a wet clutch, probably isn’t wearing the plates at all, the slight drag coming from the viscosity of the thin oil layer between the plates.

Thanks for the info, folks.

 

I will try Digifish's idea of moving from D to N to see if revs rise a bit.

Curiously, today for the first time, I noticed the rev counter sitting at 1000 with the engine running freely whilst in D with auto-brake on.

Normally, there is a distinct feeling of the revs being dragged down as if by friction.

The normal experience seems to conform to NickNorman's description of there being some slight clutch pressure being applied in these circumstances.

 

More experimentation required, methinks.

10 minutes ago, cardo said:

Thanks for the info, folks.

 

I will try Digifish's idea of moving from D to N to see if revs rise a bit.

Curiously, today for the first time, I noticed the rev counter sitting at 1000 with the engine running freely whilst in D with auto-brake on.

Normally, there is a distinct feeling of the revs being dragged down as if by friction.

The normal experience seems to conform to NickNorman's description of there being some slight clutch pressure being applied in these circumstances.

 

More experimentation required, methinks.

The revs should drop below 1000 with the brake applied firmly. I think it is possible to bring the car to a stop with very light pressure on the brake, which keeps the revs at 1000. But the revs drop isn’t due to clutch drag, it is due to the engine/gearbox management system taking the engine back to idle by closing the throttle. So maybe try pressing the brake a bit more firmly to get the revs to drop fully to idle.

Yes, the 1000 revs thing was unusual, and may have come about as you describe, but what seemed odd was that there appeared to be no drag.

The engine was spinning freely (as if in neutral) and there was no sensation of the car "straining at the leash".

 

The more complexity they build in, the more difficult it is to say WHY things happen!

I notice if I come to a stop, with too light a foot on the brake (Auto Brake on). And I have 'just' stopped. The car will start moving again when I release the brake, as it does with Auto-brake off.

 

I observe this has to do with stop-start traffic when you are 'almost stopping' but not quite, and the car anticipates you want to keep moving so does not fully disengage the clutch you are definitely stopping with a firm pedal.

 

I tend to have a light touch on controls, and so notice this can sometimes cause confusion with the system. You need to give firm inputs when you stop I see. So if the car thinks you are still on the move or about to move, it's not going to drop into full idle mode.

Edited 11 June, 20197 yr by digifish

Also note that a TDI engine revving higher when idling may be the engine trying to regen the DPF under certain conditions - the revs are higher because the engine needs the heat to burn off soot in the DPF.  I have been caught out by this on numerous occasions, so I just take it for a drive and it'll hold onto lower gears for longer to get the revs higher.

 

A

BTW I tested shifting to Neutral  while stopped (with Auto-brake engaged) today. No change in RPM. Mine was around 750 RPM I think

Edited 11 June, 20197 yr by digifish

8 hours ago, digifish said:

BTW I tested shifting to Neutral  while stopped (with Auto-brake engaged) today. No change in RPM. Mine was around 750 RPM I think

Yes, I tend to agree.

 

Tried it today with an app connected to the OBD port which shows the actual RPM figure rather than the somewhat vague rev counter needle.

It sat rock steady at 750 (well, between 749 and 751) whether in neutral or drive.

Incidentally, the way that it switches the Auto-brake off when you select neutral is a bit disconcerting at first, and I can't see any logical reason why it's programmed to do so.

 

Having said all that, I still have the nagging feeling that, if you stop in drive on Auto-brake the car is trying to pull.

From today's tests it appears to me that the car disengages the drive when in Neutral (Duh!) and does not engage the clutch when you re-select drive until you give it a wee dab on the throttle.

In that sense the drive/neutral test is not quite the same as stopping at light and keeping it in drive until the green shows.

 

Maybe another test would be to stop on an up gradient, with auto-brake off, and see if it holds itself on a slope by clutch drag.

(Or is there a "hill-hold" mechanism that would defeat that idea?)  

 

 

 

There are two systems. Auto-brake is the main hill-hold mechanism. However, if that's off  (and when in drive) on a slope, AND you release the brake, I think you have ~ 1 second to depress the accelerator before the brakes release, if not you start rolling back. If you are in Neutral this mechanism won't be in play and you should start rolling back immediately? (this is from memory, I always drive around with Auto-brake on, UNLESS I am doing maneuvering in awkward places where I need to roll forward/backward when I release the brake).

Edited 12 June, 20197 yr by digifish

'Autohold' requires you to press the brake pedal hard enough to actual have 'autohold' on, you can brake and manoeuvre / park without 'autohold' coming on.

I have never found the need to 'disable / switch off' autohold on any vehicle once selected to function, be that a petrol or diesel and ever when towing and reversing with trailers.

3 hours ago, Skoffski said:

'Autohold' requires you to press the brake pedal hard enough to actual have 'autohold' on, you can brake and manoeuvre / park without 'autohold' coming on.

I have never found the need to 'disable / switch off' autohold on any vehicle once selected to function, be that a petrol or diesel and ever when towing and reversing with trailers.

 

This does not make sense. You will at some point need to fully depress the brake (if you are on a steep enough hill) and if you do, its engaged.

 

 

We are talking slow speed parking,  manoeuvring. 

Not driving and braking.

'Autohold' will not come on as you drive along.

(and the E-Brake will not come on unless you choose to pull up the switch or do it by mistake or you die at the wheel and the passenger does.

Or you open the door when stopped etc)

Have your seat belt on if you ever pull on the E-Brake while driving or someone else does....

Edited 12 June, 20197 yr by Skoffski

45 minutes ago, Skoffski said:

We are talking slow speed parking,  manoeuvring. 

 

 

Exactly. And if you are maneuvering on a slope. You will need to press the brake at some point. Then Auto-hold kicks in.

 

How do you change direction for example from forward to reverse without braking?

Edited 12 June, 20197 yr by digifish

@digifish, only if you want it to, and i have only 1 foot and reverse plenty on slopes.

I must be more gentle than you on the brake pedal.

So I managed a swift test on an up-slope today.

 

Auto-brake OFF, driving up an incline and stopping by foot brake. Gear still in D/S and handbrake not engaged.

 

Got a result I diidn't expect -

 

The car stopped on the foot brake, which I then released.

The car stayed stopped on the hill for about 2-3 seconds.

Then it moved off forward, i.e. up the hill, without any throttle input from me.

 

Like Digifish suggests, I would have thought that it would roll back once hill-hold disengaged but I never expected it to head off uphill under its own steam.

I repeated it three times and the behavior was identical each time.

I will try it on a much steeper hill and see what it does with that.

 

I don't think I've got any nearer to answering my original question, but I'm having fun trying!

Even without autohold on, the car will still hold itself on a hill for 2-3 seconds before releasing the brake - it is called hill hold assist.

 

As I like the creep function of an auto I never use autohold and find that the hill hold assist is sufficient.

 

On the RPM question with the foot on the brake and going into and out of drive, the revs don't change but you can feel the release of the slight drag when you put it into neutral. Strangely I have noticed that if the car is doing a regen and idling at about 1000rpm, I can't feel the change when going from drive to neutral.

 

 

Still on the subject of holding on hills, I managed to find a steeper hill to try out.

I got the same result - steady hold for 2-3 seconds then off it went uphill without any throttle input from me.

Quite a cool system, and no danger of a roll-back apparently.

 

To Speedman - I, too, appreciate the creep function and was quite happy to use it with a conventional Torque Convertor automatic where only fluid is being stirred up (and heated up, mind you) whilst holding on a hill.

However with a DSG, creep can only come from a partially engaged mechanical clutch, which must be causing some additional wear, especially if holding the weight of the car on a hill.

I think of it as the auto box holding the clutch at biting point as you can, but would never sensibly do, in a manual car.

 

I also like your phrase "release of the slight drag" which describes very well the feeling that leads me to think that the clutch is never FULLY disengaged whilst stopped in drive.

 

This is why I use auto handbrake. A firm push on the brake engages the handbrake and the clutches disengage.

They stopped being a 'hand brake' when they became a 'finger brake' / e-brake / parking / emergency brake.

 

A firm push on the foot brake has 'autohold engage the brakes', but that is different from the 'parking / emergency brake.

In 'Autohold' if there is movement the accuator will clamp the brakes harder.

 

With the 'E-Brake' set and the car parked on a slope if the DSG should not be in park, some have found the brake discs cool off and the car runs away.

VW Group say this must be driver / operator error, nothing to do with them.

Edited 14 June, 20197 yr by Skoffski

7 hours ago, facet edge said:

......and the clutches disengage.

Ah but do they?

 

That's what worries me.

On 14/06/2019 at 23:56, cardo said:

Ah but do they?

 

That's what worries me.

I think the systems cleverer that we think. I sat in the garage in drive and when my foot is on the brake the tickover is around 800rpm, the car may have been cold so the rpm was slightly raised. This is without autobrake on. As soon as I took my foot off the accelerator the revs rose and I crept forward so it seems to me that the gearbox knows that if you have your foot on the brake it can disengage the clutch.

On 16/06/2019 at 12:16, facet edge said:

I think the systems cleverer that we think. I sat in the garage in drive and when my foot is on the brake the tickover is around 800rpm, the car may have been cold so the rpm was slightly raised. This is without autobrake on. As soon as I took my foot off the accelerator the revs rose and I crept forward so it seems to me that the gearbox knows that if you have your foot on the brake it can disengage the clutch.

The gearbox does know that if you have your foot on the brake it can disengage the clutch - partially! But not to quite the same extent as if you had it in N. As far as I can tell it holds the clutch just very slightly biting, so when you want to move off it can do so pretty much instantly. Whereas in N it fully disengages the clutch.

 

Being a wet clutch, having it very slightly biting is probably not wearing the clutch plates - the slight transmission of power is via the viscosity of the oil. There will be no actual contact between the clutch plates.