2015 XC70 AWD System How Does it Work?

[Hillkidstr]

How does the AWD system work as ive noticed on mine the rear wheels drive and the front dont spin the driveshaft is this correct? and then at a guess does it engage them under loss of traction? is this due to fuel economy ect?

Is it done with locking hubs?

[37 RUBY]

Originally Posted by Hillkidstr

How does the AWD system work

The Haldex system as used on Volvo's drives front wheels only and...

Originally Posted by Hillkidstr

then at a guess does it engage them under loss of traction? is this due to fuel economy ect?

...correct. Why drive rear wheels all the time burning excess fuel on a vehicle that is predominantly aimed at target market who will drive on tarmac, not used like say a Landrover used by a Gamekeeper on moorland.
The front wheel(s) lose traction and vehicle momentum slows, sensors recognise this and send traction via a rear dif that is locked by oil to back wheels.

Originally Posted by Hillkidstr

and is It done with locking hubs?

No just electrickery and oil which locks the power to the rear only with a wet clutch/hydraulics when the front spins up.

So I'm struggling to see how you have this...

Originally Posted by Hillkidstr

as ive noticed on mine the rear wheels drive and the front dont spin the driveshaft is this correct?

How were you testing the system?

...and can another more experienced forum member advise if the front diff could fail as such leaving the rear only driving?

[SwissXC90]

Originally Posted by 37 RUBY

...and can another more experienced forum member advise if the front diff could fail as such leaving the rear only driving?

Only through a major mechanical failure such as broken CV joints, or broken front stub axles

[WARBO]

mainly the car is front wheel drive but has an extra right angle bevel gear box to which the prop shaft is attached, this free wheels all the time and at the rear the prop is connected to the DEM electronic unit. if the cars anti skid systems detect loss of front wheel traction the DEM pressure pump activates ( this is happening all the time on the fly) the wet clutch and provides drive to the rear diff/wheels. Common issue is the bevel gear drive sleeve teeth wear away inside the box and the prop dont spin or the DEM pump or elec box fails and never activates the DEM wet clutch. Front wheel spin indicates either, though a DEM failure would normally include an AWD failure warning on the dash, bevel gear rarely does as being a mechanical issue the car is not "aware" the prop aint spinning as such.

[Hillkidstr]

Feeling like a numb skull now as just realised when I said the rear wheels drive I meant the other way round. Apologises, thank you for all of the explanations I understand now so it isn’t a full time AWD it’s mainly front until they loose traction then the rear kicks in.

[apersson850]

If you just look out through your window on the driver's side, then you may see the rear wheel spin, but the front at standstill.
There's no diff lock side to side. There's the TRACS system, which uses the normal brakes to try to slow down a spinning wheel on one side, in order to get the wheel on the other side to turn. But momentarily, whilst the system tests different brake actions to do this, you may have the front spin on the other side, the side you can't see.

Each time you start the car is AWD, in case it's needed. When the car starts rolling, it will try to release the rear wheel drive. If nothing happens, it will continue with FWD. But if some wheel starts spinning, it returns to AWD.

If you use the Hill Descent Control, you've told the car that the conditions are worse then normal. Although I've never seen Volvo disclose their action here, most brands use such a function to apply the AWD at a greater rate than normal.

If you start with the front wheels pointing in any other direction than straight ahead, rear wheel drive will be weaker. The car is programmed to engage AWD fully when going straight ahead, as a stiff connection between front and rear wheels make it more difficult to turn.

The rear wheel drive is automatic, with no indicators telling you what's going on. However, if the anti-spin/traction control system detects a spinning wheel, it will turn on the yellow spin symbol on the dashboard. It's also lit when the HDC system is breaking in steep downhills.

[SwissXC90]

Originally Posted by apersson850

It's also lit when the HDC system is breaking in steep downhills.

Crikey! I hope that never happens

You want to HDC system to be reliable and not break when going downhills

Brake without breaking has always been my motto.

[apersson850]

Sorry, it was late and English isn't my native language. Although I do know the difference between braking and breaking, loose and lose etc., it slipped my mind yesterday.

It should of course be braking, not breaking.