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Haldex system function question -

Help, Advice, Owners' Discussion and DIY Tutorials on Volvo's stylish, distinctive "P2" platform cars.

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Haldex system function question -

Post by BlackBart » Tue Feb 04, 2020 9:32 am

I'd like to understand the Haldex functioning a little better. I know there are different generations - my '04 is slightly different from the early P2's I think.

My question is about the continuing operation of the rear diff and axles after actuation.

Normally you're in Front Wheel Drive (FWD). When sensors perceive front wheel slip relative to back wheels (and this happens nearly instantaneously), the clutches in the Haldex unit at the rear diff engage the diff and rear axles to provide drive. It doubt it's 50/50 front to rear, but at some ratio between the two.

So then what happens? It can't be permanently engaged or it would be full time AWD. It can't disengage right away or there would be wild cycling between FWD and AWD and probably jerking or instability. How does the system know that you're on a sheet of ice road and it should remain engaged? If the sensors determine no difference in wheel speed between front and back (which makes sense if all wheels are driving now), does it let go of the rear axles?....and then the fronts will slip again. Is there a clock timer function in the Haldex? That would seem arbitrary.

What keeps it engaged?
What makes it disengage?

Thanks for your experience and input.
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Re: Haldex system function question -

Post by jimmy57 » Tue Feb 04, 2020 11:35 am

Yours is still first generation but with improved pressure and temp sensors. The electric pump on the first few generations was a low suds pump, its job was to effectively push out air and make the clutch apply as quick as possible via as little as one mechanical pump stroke. The solenoid is the control of leakage from the clutch apply circuit. The mechanical pump has two round concentric cylinders. Really more like two groves with a sealed washer in each. Under the washer is a set of rollers and under rollers is a wavy surface where rollers will bob up and down if the rollers and wavy surface are moving different speeds. The driveshaft turns one part and the other part is splined and carried by the diff in rear. Once engine runs the electric pump is started and the solenoid is pulsed to keep a low 35 psi +/- pressure that is enough to close clutch but not so much to transmit much torque. If car is on a lift one person can hold front tires and another can turn the rears and the clutch will slip without much effort applied. If there is wheel spin it will be the fronts slipping and the driveshaft to rear will spin faster than the diff at rear. Actually by a multiplied factor since the angle is 1:2.62 (the 97-2000 P80 are 1:3.31). So the parts carried by driveshaft will spin and the rollers and wavy plate deal will stroke those washer pistons and pump oil but since the system is already at a low pressure the output of pump will increase pressure immediately and clutch will lock and torque will go to rear and away you go. The solenoid control signal will change to seal the clutch apply piston circuit as the DEM gets all 4 wheel speed signals from network and knows to do this. If you were moving at speed the engagement is slower to prevent sudden jerks and such and the DEM would change the solenoid signal to closed more slowly.
Now that it is locked the system is sealed well enough that it will remain sealed and locked for a short bit and in most cases that would have been enough anyway. If the traction needs are much greater then there will be a cycling of the system where it loses apply pressure, slips, builds apply pressure via the different rotation speeds of driveshaft and diff again and locks again. If you ever see some of the VW, Audi, or other Haldex or similar system vehicles on slippery surfaces you will see the cycling occur, some of that also owing to ABS applying brakes to shift torque left-right as needed.

The upgrade made next (2005 v8, 2006 others) was to increase pump capacity a little and increase the static pressure. This gives a low amount of torque to rear and is enough to reduce wheelspin chance up front during take off. Volvo called this "instant traction". V8 torque and a heavier vehicle.

The newer systems, I think 2012 intro, use a high capacity pump and accumulator and no longer have the annular piston pump stuff. These can have continual apply and are lighter and smaller.

Most AWD systems on car based platforms do not have a locked, full time capability. Old Subarus did but the target here is to get you going and keep you going on improved roads and not bother you with knobs, switches, levers or with any bumping or knocking when turning at full lock.
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Re: Haldex system function question -

Post by BlackBart » Tue Feb 04, 2020 1:54 pm

Very good explanation, thanks!

So it does cycle in and out of traction / driving the rear wheels. Not intended to be in AWD for very long, just get you out of traction problems? Can you see or feel the time lag? How does it act when climbing a steep icy slope? Are the rear wheels kicking in and out?

I have actually climbed roads like that, say a frozen pass, where you needed all the traction you could get or you were going sideways. A fwd Scirocco where the fronts, without a limited slip diff, were constantly letting go and jerking the steering wheel, and the front would push sideways. A non-limited slip rwd will swing the tail all over the place trying to grip.

The old Subarus were good, yes. Old Audis with quattro were fantastic, always pulling with four wheels, but with limited slip built in. Ours is a 2001 and may be about the last of them before traction controlled individual braking. It's a blast in the snow, and the best ice car I've ever driven.

The BMW xdrive (I've only driven a 2013 3-series) has a clutch pack at the rear of the transmission and a transfer case with a driveshaft forward from there. It's a rwd car and intended to handle that way, so the max the front can pull is 50%. So the rear is pulling from between 100% and 50%.

I grew up climbing out of my dad's Bronco, manually turning in the front hubs, climbing back in, shifting big levers clank clank, and motoring off locked in 4wd. You can't drive it on pavement like that or you'll bind it up or break u-joints.

What was the system in the older P80 cars? That was pre-Haldex?
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Re: Haldex system function question -

Post by matthew1 » Tue Feb 04, 2020 2:36 pm

BB I have an overview here https://www.matthewsvolvosite.com/volvo ... guide.html

I'll add Jimmy's explanation to it, or link to this from there.
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Re: Haldex system function question -

Post by - Pete - » Tue Feb 04, 2020 3:00 pm

IIRC, Haldex 3 allows for a maximum of 15 degrees front wheel spin before sending power through the rear final drive. On video, even in slow motion, it is truly almost imperceptible.
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Re: Haldex system function question -

Post by BlackBart » Tue Feb 04, 2020 3:49 pm

Nice information, Matt. I thought there was some change in '03, there it is.

I've had little experience so far with mine, but what I've seen seems impressive. And you can get the tail end to kick out on a slow corner like rwd, which is fun. That info says 5 to 65% at the rear wheels.

It seems good for low traction and not getting stuck etc, but I have yet to see how it acts at speed in low traction - hills, passes, slippery back roads.

And good dedicated snow tires would make a world of difference no doubt.

I saw these posts on an XC70.com forum from 2003.........

"I have never had any problems with traction on the XC. And I tried to get it stuck. We had plenty of snow this winter in New York, and I live around Riverdale which is very rocky and has a lot of hills. The area is also private, so snow removal takes place last there.
Moral of the story: The XC had no problems even stopping on an uphill and then driving off from a complete stop. I actually have a post here somewhere when I pulled out a Ford Bronco from a ditch with my XC that day, much to the surprise of the Bronco driver.
Anyway, love the traction on the XC, never got stuck in snow yet, but can't say I never tried."

___________
"Remember to compare apples and apples. Prior to 2003 the AWD system was not electronic and no DSTC available. For 2003 it is the Haldex system and DSTC is available."
__________

What about the bits I read where there has to be some forward motion for the system to kick in? Example was front wheel spin at rest on a slippery boat ramp.
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Re: ICE

Post by BlackBart » Tue Feb 04, 2020 4:40 pm

Here's an interesting video on glare ice. Looks like a newer P2, 2006 or so? Eastern Europe, maybe, doesn't look to have ice tires. You can see various discrepancies in front and rear wheel speed with varying levels of traction.

Go to 1:17 where he's at a stop and in a rut actually, and moves forward from rest.



Also note he nearly crashes an innocent passer-by because he's a fool.
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Re: Haldex system function question -

Post by jimmy57 » Tue Feb 04, 2020 10:52 pm

Haldex started with the AWD S60 in 2002 and the XC70 got it in 2003. The 2001-2002 XC70 had the viscous clutch unit which was a longer term apply system but still had to have some slip to work. The gel in the viscous unit has a behavior like wetted corn starch (antithixotropic) and will stiffen and transmit torque when there is slip but it will yield and then stiffen again when it gets stirred (wheel slip). If it gets hot it can get more solid and stay that way so a long icy uphill might get you a near lock in one of those. If you keep it long enough and the oil in the gel seeps out then it is locked in many cases as the remaining dirt (cut one open and it falls out as a dust or dirt) stops the rippled plates from passing by each other.
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Re: Haldex system function question -

Post by matthew1 » Wed Feb 05, 2020 10:17 am

I'm making edits to the Volvo AWD Systems page and want to make sure I have it right. So Jimmy...
jimmy57 wrote:
Tue Feb 04, 2020 11:35 am
Yours is still first generation but with improved pressure and temp sensors.
A 2004 is first gen Haldex? Are we talking generations of Haldex tech in absolute terms, or generations of Volvo using Haldex?

jimmy57 wrote:
Tue Feb 04, 2020 10:52 pm
Haldex started with the AWD S60 in 2002 and the XC70 got it in 2003...
Same question here.
jimmy57 wrote:
Tue Feb 04, 2020 10:52 pm
If you keep it long enough and the oil in the gel seeps out then it is locked in many cases as the remaining dirt (cut one open and it falls out as a dust or dirt) stops the rippled plates from passing by each other.
So it fails -- in this contaminate case -- in the ON state essentially, causing binding at low speed/tight turns?

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Re: Haldex system function question -

Post by BlackBart » Thu Feb 06, 2020 3:47 pm

A design question -

Why did they not put the clutch system at the front, ahead of the driveshaft, so that the shaft isn't rotating all the time? Is it because of all the inertia in getting the shaft plus the diff and the wheels moving in a very short time?
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