'clutch slip' on an auto - possible?

[Pommygranite]

Bit mechanically limited knowledge wise but is it possible to have clutch slip on a modern ZF auto box?

[The Wookie]

Yep, conventional Auto's have an array of multi-plate wet clutches (or brakes if it's stopping something against the casing rather than linking it to another gear).

Any of these slipping (particularly the main ones) causes rev flaring very similar to a knackered clutch on a manual

[McSam]

No, because it does not have a "clutch" in the common sense.

It works by means of a torque converter - imagine you have a sealed container full of viscous oil, and two sets of turbine-shaped blades that don't come into contact. One is engine side, the other is gearbox side. The engine side blades spin, causing the oil to churn and the gearbox side blades to spin, and so drive from the engine reaches the gearbox with no direct mechanical connection. This is why it's possible to stop the car without stalling the engine, because there's no contact between the engine and gearbox.

The only clutches in a traditional automatic gearbox are those that (with various brakes too) engage and disengage the various gears in the epicyclic gearset, and it's quite unusual to see wear in those, particularly in a modern gearbox.

If the engine is revving excessively high for the torque you're getting to the wheels, I'd expect the most likely cause lies with the torque converter.



ETA - Wookie, do you see often those clutches having trouble? Not something I've ever known happen!

Edited by McSam on Monday 21st May 10:47

[kambites]

As I understand it, the only clutches in an automatic box are gear-specific, so if it was clutch slip it would only happen in one gear (I've never heard of a box failing in this way). If it's happening in all gears, it's the torque converter.

[Pommygranite]

Thanks for the dual answer!

I'm not sure if my car is suffering as it is quite powerful and turbo'd so as soon as revs are used turbo spools up and the noise hides the engine noise but there seems to be a greater flare of revs than before.

It's only 2 yrs old so still under warranty so reckon will just have to check it in.

[McSam]

How big a flare of revs are we talking about? When you suddenly come on the power in an automatic it's quite reasonable to see a spike of 500rpm or even more at the same roadspeed, just because of the way the engine is "connected" to the wheels.

Though if you're starting at 2000rpm and it's wrapping itself all the way around the dial, I would say that's not reasonable

[Pommygranite]

No just slight but with a more pronounced level of engine noise and sometimes a slight thud when coming sharply off the accelerator. It's like a millisecond before the power catches.

Ever get the feeling you appear paranoid but are sure something's not right?!

[Dogwatch]

Some 'auto' boxes are what you might call 'smart manuals' rather than conventional automatic boxes as defined above.

Perhaps the OP could give details of the make and model?

[Pommygranite]

The car is a Ford Falcon G6E Turbo (2010) with a ZF 6 spd auto.

[McSam]

Do ZF even make dual-clutch stuff? They're certainly best-known for traditional automatics, which is why I presumed that's what we were dealing with, but I agree that make and model would help us be sure. If it's the Falcon, then it's almost certainly a traditional torque-converter automatic as we've been talking about, isn't it?

Anyway, OP, of course you get more engine noise when you put your foot down engines are meant to do that! As I say, a rise in engine speed before the roadspeed starts to change is perfectly normal - think of that small rise as the engine taking up the slack in the torque converter, ie, accelerating the oil slightly faster than the gearbox input responds to it. Not a problem.

Your thud when backing off sharply could frankly be many things, I'd like to think of something to do with the flywheel and how it's connected to the torque converter, but.. So long as it's not causing any obvious problems I wouldn't be worried.

[kambites]

Don't some (most?) modern torque converter automatics use lock-out clutches to cut down transmission losses? If so, what do they do with the torque converter while the clutch is engaged?

[Pommygranite]

Thanks for the info. In some respects it's no biggy but on ther something has changed and given its a pricey bugger it seems wrong not to try and get it sorted whilst under warranty.

This makes me wish I was into bangernomics so I could care a bit less

[lgw]

When the Lock up clutch is engaged the drive is basically bypassing torque converter and transmits the engine power directly to the gearbox input shaft eliminating any drive slippage through the convertor, on smaller vehicles the lock up only works usually on the top 2 gears but on large diesels(trucks) it will usually operate on all forward gears apart from 1st

[The Wookie]

With a thud coming off the power it could be the torque converter or if the gearbox has a one-way clutch it could be a fault with that.

As Kambites said earlier, if it's gear specific then it's likely to be a clutch pack, although to expand on that in newer boxes the clutches can be handling significant torque in more than one gear. These gears need not be adjacent, so it could be 2nd and 5th for example.

[kambites]

lgw said:

When the Lock up clutch is engaged the drive is basically bypassing torque converter and transmits the engine power directly to the gearbox input shaft eliminating any drive slippage through the convertor, on smaller vehicles the lock up only works usually on the top 2 gears but on large diesels(trucks) it will usually operate on all forward gears apart from 1st

So it doesn't actually disengage the torque converter; if the lock-out clutch slipped, you'd still have the torque converter providing drive?

[The Wookie]

kambites said:

So it doesn't actually disengage the torque converter; if the lock-out clutch slipped, you'd still have the torque converter providing drive?

You would, but the engine speed would increase slightly with the action of the torque converter (i.e. it's no longer 1:1). Not massively mind you, but a few hundred RPM.

[lgw]

kambites said:

lgw said:

When the Lock up clutch is engaged the drive is basically bypassing torque converter and transmits the engine power directly to the gearbox input shaft eliminating any drive slippage through the convertor, on smaller vehicles the lock up only works usually on the top 2 gears but on large diesels(trucks) it will usually operate on all forward gears apart from 1st

So it doesn't actually disengage the torque converter; if the lock-out clutch slipped, you'd still have the torque converter providing drive?

Once the lockup clutch is engaged the torque converter (as in the fluid coupling itself) is no longer providing the drive to the gearbox

The lockup clutch is also inside the torque converter assembly

[kambites]

The Wookie said:

kambites said:

So it doesn't actually disengage the torque converter; if the lock-out clutch slipped, you'd still have the torque converter providing drive?

You would, but the engine speed would increase slightly with the action of the torque converter (i.e. it's no longer 1:1). Not massively mind you, but a few hundred RPM.

I wonder if that might be what's happening in the OP's case then?

[McSam]

lgw said:

Once the lockup clutch is engaged the torque converter (as in the fluid coupling itself) is no longer providing the drive to the gearbox

The lockup clutch is also inside the torque converter assembly

That was my understanding too, though the exact situations in which the gearbox might or might not decide to lock up are varied.

[lgw]

The operation is dependant on road speed and engine load