XC70 always in four wheel drive?

[Tannaton]

Originally Posted by GrantA

Phase 3 or Gen 3 XC70s have a Haldex drive system for AWD and, as 4candles has said the "split" is normally 90/10 until the car starts to lose grip then the Phase 3 cars respond much quicker than previous versions of the system.

Apparently the Haldex coupling on the earlier models of the XC70 were a nightmare and the drive from the gearbox to the rear coupling were forever shearing. At one stage XC drivers were advised to keep the tyre wear between axles at no more than 2mm as excessive tyre wear between front and back tyres put excessive strain on the drive from the gearbox. The problem centred around the number of splines on the gearbox output shaft and it was only when the Phase 3 cars were fitted with much improved splines that the problem went away. I understand that unless the "broken drive" became noisy it was difficult to tell when the splines had failed with the only immediate indication being an improved MPG as the car was only 2 wheel drive.

The Haldex system has been in use for a number of years on Audis and this link explains basically how the system works, if you are bored read through it although it is a bit technical..... http://www.vaglinks.com/Docs/Audi/TT...StudyGuide.pdf pages 34 and 35 explain how the power is distributed between front and back wheels depending on the conditions.

The issue you refer to with the splines failing was the splined sleeve between the gearbox and transfer box on cars with the AW55 5-speed auto gearbox - i.e. 163 BHP D5 and pre 2005 petrol models. The later cars with the 6-speed TF80SC gearbox are much less likely to have this problem. It's a £100 part but quite a bit of labour to sort, it's easy to check you turn the propshaft with at least both front wheels on the ground - if you can keep turning it then you have failed AWD.

If the AWD fails because of the splined sleeve - there is no outward indication until you get stuck, so if you have a AWD with the AW55 you should check it. There is no difference in MPG, the propshaft, rear differential and driveshafts are still rotated by engine power whether through the gearbox or by the rear wheels. You used to see a lot of Freelanders for sale with "propshaft removed for economy reasons" but it makes no measurable difference.

[Simon Jones]

Bear in mind that any 4WD system adds additional weight (transfer box, prop shaft, rear diff, driveshafts etc) so there will be a negative impact on fuel consumption regardless of whether the wheels are being driven or not. The extra drag of the diff etc rotating (again even if no power is being provided to the rear wheels) will also reduce fuel economy.