4 wheel drive '“ does it work?

Mark Elder · Dec 16, 2008

ORIGINAL: Porker993

So, I'm thinking, as you do not have a centre diff lock (unlike my old 964 C4), it may be, that if the 993 C4 loses traction on only one wheel, all drive is lost.

Hi Mark,
Understand all that logic, but the C4 has "3 channel ABD" - ABS is used to brake any wheel that is spinning (except that the rear pair are considered together - hence only 3 channels - but the rears have an LSD so they are only likely to go together).

So that makes me think that the ABD should have actted like a central-diff-lock and stopped the spinning and created some movement in the front wheels. However the green "ABD in action" light was not on while I was stationary with spinning wheels - it only came on once I got some traction and the car was moving.

So it is possible that the ABD simply does not start up if it can't detect some movement in the car.

If we get more snow I'll do some experiments!

Mark

IndischRot · Dec 16, 2008

Assuming it's working, the viscous coupler will "lock" after the fluid heats up (in a few seconds from stone cold?) and if the ABD is working you'll always have at least three driven wheels?

I wonder if there's so little grip that if all the wheels spin then the ABD doesn't kick in as it doesn't sense a difference in wheel speeds so thinks nothing's wrong?

Porker993 · Dec 17, 2008

ORIGINAL: Mark Elder

So it is possible that the ABD simply does not start up if it can't detect some movement in the car.

Sounds very likely. We know there are motion detectors in the car (at least there were in my 964 C4)

Porker993 · Dec 17, 2008

ORIGINAL: phelix

I wonder if there's so little grip that if all the wheels spin then the ABD doesn't kick in as it doesn't sense a difference in wheel speeds so thinks nothing's wrong?

Yes to that, as well. It's a "dynamic" 4WD system, designed to steer you through fast corners, not lug you out of a hole, like a landrover. Interesting different approaches !

The 964 C4 was a bit of both, as it used to go quite well over the rough stuff, with diff lock in.

clyde · Dec 17, 2008

They're the correct size according to the driver's manual.

The car was delivered with 17 inch wheels (with 50 offsets to the rear). My brother-in-law lives in Germany and owned the car for a short while before passing it on to me. He bought the 16 inch wheels second hand with part used snow tyres from his local OPC (honest) a couple of months before I got it.

Would you believe that the same OPC also swapped the left handed headlights over for a pair of second hand right handed ones that they had in their store free of charge? I believe they were from an ex-pat taking his right hand drive to Germany. OK, the left handed set would be more use to them but even so, "second hand" and "free of charge" are two phrases that you don't really hear in an OPC in Britain. Even 8 years ago, at least not in my experience.

I wouldn't hesitate to recommend M&S tyres in this country even though we don't get a lot of snow. They give a better level of grip at this time of year than summer tyres do and are more predicable. The Contisport 2's on my car aren't too bad but the Bridgestones on my son's MX5 are absolutely lethal until they get a good bit of heat in them.

clyde · Dec 17, 2008

ORIGINAL: Mark Elder

If we get more snow I'll do some experiments!

Mark, have you never had it going off when you're pulling away on damp/greasy roads?

The other thing that I've just remembered is when I was down at Shotts a while back Peter showed me a C4 that had a broken UJ at the drive shaft/gearbox. So obviously no drive to the front wheels [8|] but presumably all the electronics still did what they were meant to do. I don't think the owner was of aware of this before he put it in to the workshop if I remember correctly, so the car must have driven OK.

Mark Elder · Dec 17, 2008

Hi,

The ABD definately seems to work well on wet and greasy (even icy) roads when the car is going reasonably straight.

For example I can have the left of the car on a frozen river of ice down the side of the road and the right on good tarmac, accelerate reasonably quickly and the green light comes on and car accelerates well and straight.

But on a wet road and, for example, pulling out of a T junction hard left and giving it some boot, there is no green light and the rear wheels spin and the back of car swings well wide - which is easy enough to correct but you have the feeling you are doing the work and not the electronics.

My favoured explanation so far is that the ABD simply does not work under about 2 mph when there is not enough movement for it to work out what you are trying to do.

Have discovered that the ABD works in reverse gear! That evening after the snow fall I had to reverse over some snow and the green light flickered - car was hardly moving then....

The UJ is well hidden from memory and I don't have access to a ramp. Do you remember how Peter realised that one had failed?

clyde · Dec 17, 2008

Does the ABD not just work on the ABS sensors, only in reverse so to speak. When working as part of the ABS, the sensors detect when the wheels are effectively turning slower than the speedometer is indicating they should be. Whereas when working as part of the ABD they tell when the wheels are turning faster than the speedo? So I would have thought that, as the ABS works at virtually zero, the ABD would have as well. But maybe it's not meant to work below 2mph or so in the same way that it automatically deactivates over 44mph but I can't think why.

I'll need to try it in reverse sometime [

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I think Peter disconnected the drive shaft to replace the clutch and that's when he noticed the failed UJ, rather than taking it apart because it had failed. He did say that he hadn't seen one like it before and didn't know how it could have happened. I would also have thought that the owner would have heard it, even when only working on the normal 5% front drive because the bit of UJ attached to the shaft from the gearbox must have been hitting off the rest of the UJ on the driveshaft. But I suppose if it had been like that since he got it then maybe he wouldn't have known any better.

mwcarrera · Dec 20, 2008

Had a recent go in the snow at Aviemore with my C4 and must say that I did not get stuck at any point. Whether the front wheels did some work was not proven - the feeling was that they did not! or very little if they did! . What did work a treat was the ABS/ABD system that brakes a spinning rear wheel to direct drive to the wheel with more grip - the green ABS light illuminating as it worked. Now looking for a frozen Loch (or big puddle) and video camerman to prove the viscous coupling for real.