Why Tight on Full Lock?

I've been reading a lot of posts about tight lock = vcu prob etc. but there doesnt seem to be any rhyme or reason running throughout. Some posts are saying the lock is tight even though they've a new VCU, some are saying the lock is not even though the VCU is seemingly knackered (or at least hits the high numbers on a torque test).

So can someone explain why Freelanders are tight when on full lock even if there's nothing wrong with the VCU? Are all 4 x 4's tight this way? Are there other issues that could give the same tight lock symptom?

Hi Fritz

Just because its tight on full lock doesn't mean the VCU has gone. Because the FL is permanent 4x4 the VCU is there to allow limited slip between front and back. When driving in a straight line all wheels rotate at the same speed - the vcu does virtually nothing - but on full lock all the wheels are rotating at different speeds. Without the VCU it would virtually not move without skidding the wheels (the same as a traditional 4x4 with the diffs locked). I've always found the best test is to jack up one rear wheel and try to turn it. It should turn smoothly with reasonable force (you may need the wheelbrace!). If you cannot get it to turn at all you may need to get it replaced. If the car is not tight on full lock you may also have a problem! Hope this helps.

Tony

Tony

Thanks for the quick reply. I understand that just because it's tight it doesn't mean the vcu has had it. It's just that a knackered vcu seems to be the diagnosis a lot of people jump to. There are probably lots of people that are changing the vcu when there's probably not a lot wrong with it.

I guess what I'm really wondering is when it's not a vcu problem what other reason is there for the tight lock? Are there are other faults that would give the same tight lock symptom?

Only the obvious ones like binding brakes or you left the handbrake on! You would not know if you had another problem unless the VCU is removed. I have been driving Freelanders for 11 years now and if it wasn't tight i'd be worried. To reduce the chances of prblems make sure you follow the basic rules like new tyres on the rear etc. Search for Torque test on the forum for more info on testing it.

Tony

I posted a similar question last year when I got my Freelander and seemed to get conflicting responses to the same symptoms. Having looked at some detail about how the VCU works and done various tests to try and convince myself that there was nothing wrong I came to the conclusion that the tightness on full lock is normal. Never did work out why it was more noticeable in reverse but after 6,000 miles since I bought the car I have had no problems.

There have been a couple of videos showing testing the VCU, one on the bench and one on the car. The latter shows quite considerable force needed to rotate a wheel but this is in part due to the gear ratios of the diff. When you see how much force is required to make the VCU slip you realise that the tightness make some sense. Provided the vehicle rolls freely in a straight line then there should be no brake binding. Even Land Rover do not seem to give specific measurements to pass or fail a VCU, i.e. if it can slip at all its ok

Thanks for the replies. It is certainly frustrating that LR don't give figures. I did the torque test myself (on hub nut) and got a reading of 90 N-m so I'm guessing that the VCU is ok although I can't deny that I still have a nagging doubt because of stiffness on lock. Thing is when I bought the car I budgeted to replace the VCU (because the lock was tight) but now I think I might just suck it and see, just keep checking the torque regularly.

I also saw the video of the test using a 7 Kg weight on a prop shaft and reckon the max torque (when the prop shaft is horizontal) was about 34 N-m obviously without the Diff etc. to overcome. This is based on a prop length of about 500 mm (7 x 9.81 x 0.5 = 34.3 N-m). The prop length is all important though (i.e. if it were 300 mm then the torque would be only 20.6 N-m and if it were 700 mm it would be 48.1 N-m - it makes a big difference). It would be interesting to see if anyone has done a similar test but with the prop/vcu assembly removed to give us some base data for the torque required to turn the diff. Then I guess we'd all have a clearer picture of what levels are/ aren't acceptable.

Like you I was initially willing to replace the VCU as £400 for one versus £,000 for a diff/ird was no contest. The fact is providing there is good smooth movement in the VCU it will at least act as a shock absorber in the drive train anyway. I think once it locks up you lose that action and everything is under harsh loads leading to failure.

Used to see that with hydraulic tappets, they would feel as solid as a lump of steel even when working but you knew when one had really seized.