S90 AWD Information?
 

Hi,
Just bought an S90 D5 AWD and want to know more about the AWD system.
The literature does not give any details of what it is, how it works, etc. Would think it’s front wheel drive and fires up the back wheels when needed. Surprised there is no snow mode. Was looking for some information from the Volvo experts.
Cheers guys

no idea, it just works. Mine is so much better when towing caravan (on wet grass) so something must be happening - but no indication or separate control at all.

My understanding is:-

Car is normally only 2 wheel drive to the front wheels but drive is always being taken via the bevel gear to the front/rear prop shaft which is connected to the bevel gear and to the front of the haldex unit. Inside the Haldex unit are a series of clutch plates which are controlled by a series of hydraulic valves. These valves are controlled by the DEM which is an electronics component attached to the Haldex unit. Associated with the Haldex unit is a hydraulic pump ( which can fail) and a filter ( which can block). Inside the DEM electronics casing are a pressure sensor and solenoid.
Sensors detect when a wheel is rotating at a different speed front to back and depending upon haldex series fitting, the pressure in the haldex locks the clutch plates together, and allows drive to pass through the haldex and into the rear diff and then onto the rear wheels. This locking of the haldex clutch is either within 1/16th of a wheel rotation or if a later series Haldex is fitted "instantant traction"
Issues with the system are usually associated with lack of mantainance as Volvo maintain parts do not require service. But Haldex oil and filters should be changed to preserve the pump. Pressure valves are another fail. However, due to the exposed position of the DEM water/salts etc can penetrate the casing and wreck havoc with the electronics.
Parts from Volvo aint cheap, fortunately 3rd party help is available.

I'm not 100% confident that the above is 100% correct but I believe what I've said is a good laymans guide to how it works.

Cheers
Bob

Thanks guys.
Surprised Volvo don’t make a bigger deal of the AWD system. Found this description on line.

How can Volvo claim they have full time AWD even though primarely only two wheels are powered?


The Volvo automatic AWD system has no traditional transfer case. Primary power goes to the front wheels. The rear drive shaft incorporates a viscous coupling (Haldex on the new XC-90) and as long as front and rear axles rotate at the same speed the coupling unit would remain inactive and the rear tires receive no torque - they are just rolling along. Would indeed slippage occur at the front wheels, the VC (Haldex) will create a power flow to the rear axle.

But if only the front is powered in no slip situations (which is almost always) and the rear does not get any torque - how can Volvo claim it to be an AWD system?

Surprisingly, if only one axle is powered, front and rear axles do not rotate at the same speed. The powered axle always spins a little faster (about 5%) than the axle with the wheels just rolling along. It slips a little on the ground. So, the VC (now Haldex) reacts to the speed difference and compensates with a proportional allocation of torque for the rear. This way the rear axle will get 5% of the torque and thus each rear wheel receives 2.5%.

Voila! All 4 wheels get power all the time. Volvo PR did not lie - it is full time AWD.

However, a power distribution of 95/5 makes for very insufficient traction use. The front axle is very likely to start spinning with the slightest change of traction. A power split of about 50/50 guarantees the safest use of available traction. After all that's the way 4WD was intended to be. Volvo does not offer that.

A serious drawback of the older Volvo system (Viscous Coupling) is, that you will probably have to buy four new tires if only one is bad.

Please note, that newer Volvos with Haldex couplings do not suffer damage when the tire sizes are slightly unequal.

[...

A serious drawback of the older Volvo system (Viscous Coupling) is, that you will probably have to buy four new tires if only one is bad.

Please note, that newer Volvos with Haldex couplings do not suffer damage when the tire sizes are slightly unequal.[/QUOTE]

Panic not - the viscous coupling disappeared in 2003 - so hopefully anyone on here is likely to be buying one nowadays (please)...
It did it's job in normal use and only failed when doing silly things on wet grass or up steep hills in the snow.
The main advantage to folks nowadays is that Haldex distributes the obscene amount of power 'we' get nowadays to all 4 wheels with no penalty - apart from the initial cost...