National Park Service cited AWD owner on 4WD trail!

[Kira]

...and now a word on soil erosion.

 

[Bailes1992]

Something here doesn't make sense - you may need to go farther than dealer brochures to figure out what is happening here.

It can't be a lockable center differential if it's a FWD car in normal mode of driving. A center differential means that power is essentially sent to the "middle of the vehicle" and then "splits out" front and rear. A limited slip center differential would then add in that it would enforce some sort of torque transfer to the other axle from one with wheel slip. If the normal mode of operation is FWD, then the thing in the middle is clutch, open/closed, limited slip, and/or lockable. How much torque these clutches can handle varies in reality, and most designs require a constant slippage on the front axle to keep the rear engaged, unless/until a full lock might be commanded from a switch.

There won't be a need for a haldex-like unit if there is a center diff. ??

It's a transverse layout so the centre differential is bolted straight to the gearbox and sits in the engine bay. However the 'centre diff' is permanently locked and then the clutch to 'activate the back wheels' sits on the input of the rear diff.

 

[meep]

It's a transverse layout so the centre differential is bolted straight to the gearbox and sits in the engine bay. However the 'centre diff' is permanently locked and then the clutch to 'activate the back wheels' sits on the input of the rear diff.

I was curious so I did some reading. The Nissan marketeers have confused things by talking about the center differential. Your Dacia uses the Nissan driveline, in its AWD configuration. So here’s the clarification - there doesn’t seem to be a center “differential” in this design. If there was a center diff, there could never be a time when both axles were not powered. In addition, you could not have a variable clutch system such as a haldex downstream from the center diff, because an unlocked haldex would leave one side of the differential unloaded, and either (a) no torque would make it to the other axle, or (b) the center diff would have to lock up to send power forwards, while the haldex freewheeled until a slipping event. If the center diff had to always be locked, then there’s no need to install a center differential. They have minced their terms. See PS.

What you most likely have is a “PTO”, or Power Take Off on the transaxle which is a “fully locked,” if you will, output shaft, sending power rearward. Then that goes to the haldex (or similar) clutch system which engages on demand. So - your observations of what you’ve seen on the vehicle are more accurate than the Dacia (and Nissan) marketing, which is calling it a center differential, but I don’t think it truly is.

The performance in this system is all determined by the controls demanding lockup. Purely mechanical systems require some slip before they engage. Drive a gen 1 CRV hard in a tight circle and you’ll feel it buck. They can’t make it any tighter than that, but the tighter you get it, the more work the rear axle can perform.

When they add electronics, however, they can read the steering wheel position and provide more aggressive lockup when they know it’s going straight, and ease up when the vehicle is steering. I believe that’s how the earlier Subaru automatics were designed, and I’ve never heard someone complain about those. If anything, they received complaints from being a little too aggressive and binding some in turns.

The robustness of such a system then comes down to the torque capacity of the rear clutch, and general strength of the PTO. Similar systems from aisin, used in Volvo and ford, bathe a bevel gear and short PTO drive in a heavy gear oil, but a small qty.. people don’t know it’s there and it never gets changed. But if you know it’s there, and change it every 30k, they do well. There should be a drain bolt and a fill bolt, see if you can find them, and use a good hearty gear oil in it.

I have zero experience with the torque capacity of the haldex clutches, and certainly yours which is probably more advanced with the electronic drive modes. We had a gen 1 mdx which had a “full lock” mode but their implementation was a shop spec of 145 ft pounds per rear axle, which was *quite* effective in the snow and rain but barely useful in more than light mud off road. “Locking” therefore didn’t mean something like “locked gears,” but more like, “an electromagnet is applying its full force to 2 clutch plates. All 6 amps. Use them wisely.”

Now - the haldex will be before the final drive, so before that makes anyone panic, if the gear ratio is 3:1, a 150 ft pound clutch effectively provides 3x that torque at the axle, and that is useful power on a trail.

I’ve typed to the edge of my coverage. I hope this helps.

PS. Here’s how you test for a center diff. Jack up the full side of the vehicle. Engine is either in gear (manual) or in park (auto). Grab one wheel and turn it. The other will turn the opposite direction if there is a center diff. IF the center diff has a limited slip element to it, it will be very hard to turn, but if you can get enough force on it, the other wheel will turn the opposite direction.