VCU, IRD, what the...?!

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The Mad Hat Man said:
how do you suggest that yu check for a VCU which has failed in a "no drive" mode rather than a "locked" mode?
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Jack a rear wheel up and try and turn it, if it easily turns then VCU does not transmit drive, or go in a muddy field :) Oh and you get better MPG ;). To be honest i have only ever seen two VCU's that didn't transmit any drive and they were actually both so called recon units sold by another company. These units are a complete waste of time as they have been rebuilt incorrectly and will never transmit any usable drive.
 
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austen said:
Jack a rear wheel up and try and turn it, if it easily turns then VCU does not transmit drive, or go in a muddy field :) Oh and you get better MPG ;). To be honest i have only ever seen two VCU's that didn't transmit any drive and they were actually both so called recon units sold by another company. These units are a complete waste of time as they have been rebuilt incorrectly and will never transmit any usable drive.
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there is a thread on here somewhere, trying to ascertain a typical "good" torque figure for testing VCU's - can you give any advice on what should be acceptable? preferable when tested at a rear wheel.

Apologies for being a cynical Hector - but, again, you are bound to say " To be honest i have only ever seen two VCU's that didn't transmit any drive and they were actually both so called recon units sold by another company."
 
The Mad Hat Man said:
there is a thread on here somewhere, trying to ascertain a typical "good" torque figure for testing VCU's - can you give any advice on what should be acceptable? preferable when tested at a rear wheel.

Apologies for being a cynical Hector - but, again, you are bound to say " To be honest i have only ever seen two VCU's that didn't transmit any drive and they were actually both so called recon units sold by another company."
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mmmm, we test the VCU's on a bench on a rig that we built ourselves that uses weights and is timed. Trouble with measuring torque is the way it is applied, you get a high torque figure the faster the load is applied. If i stand on my breaker on a rear wheel it take about 5 or 6 seconds for the wheel to rotate enough for me to fall of it when i test s good VCU. Sorry not very scientific, but thats not the way i test them normally!

Don't worry Hector, i understand that. I'm not going to name them or say anything else, its just blindly obvious when you test them back to back. I'm not saying its not possible, but I would like to see a non modified one that had failed as i can't see how it happens, unless the VCU casing has cracked or a spline gone??? I've seen plenty with only rear wheel drive that drive like they have a rubber prop shaft!!!
 
austen said:
mmmm, we test the VCU's on a bench on a rig that we built ourselves that uses weights and is timed. Trouble with measuring torque is the way it is applied, you get a high torque figure the faster the load is applied. If i stand on my breaker on a rear wheel it take about 5 or 6 seconds for the wheel to rotate enough for me to fall of it when i test s good VCU. Sorry not very scientific, but thats not the way i test them normally!

Don't worry Hector, i understand that. I'm not going to name them or say anything else, its just blindly obvious when you test them back to back. I'm not saying its not possible, but I would like to see a non modified one that had failed as i can't see how it happens, unless the VCU casing has cracked or a spline gone??? I've seen plenty with only rear wheel drive that drive like they have a rubber prop shaft!!!
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Care to tell us what weight at what distance from centre and time taken?? then we'll have a benchmark for testing....
 
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The old rig was knocked up from a front prop shaft with the uj welded up at 90 degs and lump of steel on the end in place of the front cv joint for weighting. Not sure of the weight, I'll see what I can do
 
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If you apply enough force to anything you could move it.
So even though a VCU won't actually be seized, if it is too stiff for the drive-train to turn then technically it is siezed, isn't it?
Has anyone ever had a VCU fail the tippex-test?
 
howardo said:
If you apply enough force to anything you could move it.
So even though a VCU won't actually be seized, if it is too stiff for the drive-train to turn then technically it is siezed, isn't it?
Has anyone ever had a VCU fail the tippex-test?
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We apply the same force to everyone we test and never had one not move.

I also see a lot of VCU's with "tippex test" lines drawn on them in totally the wrong place where they will never move. If the unit is truly seized the diff or IRD would not last 5 minutes (see my earlier post in this thread about the unit that was seizing due to incorrectly matched tyres)
 
Exactly as i tried to say here...http://www.landyzone.co.uk/lz/f9/tyres-86002-3.html

The VC is only capable of slipping at a certain speed before it locks and causes serious damage.
The key is knowing whether your VC will slip easily/fast enough for driving normally on road.
The older the VC is reduces it's ability to slip fast enough. Early signs are judder on full lock.
 

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Chaser said:
Exactly as i tried to say here...http://www.landyzone.co.uk/lz/f9/tyres-86002-3.html

The VC is only capable of slipping at a certain speed before it locks and causes serious damage.
The key is knowing whether your VC will slip easily/fast enough for driving normally on road.
The older the VC is reduces it's ability to slip fast enough. Early signs are judder on full lock.
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And i bet the diff and vcu were red hot! the VCU should be cold to the touch after driving on a straight road, the warmer it is, the bigger difference in tyre diameters and the more damage caused to the drive train.
 
Due to load diffierence between the front and rear axles the tyres have a different rolling radius. On mine, new tyres all round, this is approximately 3% (31cm front/32cm rear), causing the front wheels to rotate faster than the rear. Can anyone quote the true gear ratio between the front and rear axles, ratios aren't given in an literature that I have seen, so it is not possible to determine which way the VCU is slipping in straight-ahead driving.
It would however suggest that it is prefferable to put new tyres on the front to match rolling radii, rather than the other way around as has been advocated in another thread.
 
copy & paste from the handbook in rave.....

"Ideally, tyres should be replaced as sets of four,
but if this is not possible, replace the tyres as
axle sets. When replacing tyres in axle sets,
always fit the new tyres to the rear axle."


Fitting newer tyres on the rear axle negates some of the ratio difference between front and rear drive and so helps relieve some of the pressure on the VC as it is not required to slip as many revolutions/mile.

It states somewhere in rave (can't find it just now) that the rear is underdriven 0.08%. I don't know if this is the early or revised ratio.

If you think about it the rear diff is in overrun until there is any front wheel slip. The rear wheels are driving the rear section of the prop faster than the IRD drives the front section hence why you put the newer tyres on the rear.
 
Chaser said:
copy & paste from the handbook in rave.....

"Ideally, tyres should be replaced as sets of four,
but if this is not possible, replace the tyres as
axle sets. When replacing tyres in axle sets,
always fit the new tyres to the rear axle."


Fitting newer tyres on the rear axle negates some of the ratio difference between front and rear drive and so helps relieve some of the pressure on the VC as it is not required to slip as many revolutions/mile.

It states somewhere in rave (can't find it just now) that the rear is underdriven 0.08%. I don't know if this is the early or revised ratio.

If you think about it the rear diff is in overrun until there is any front wheel slip. The rear wheels are driving the rear section of the prop faster than the IRD drives the front section hence why you put the newer tyres on the rear.
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Freelanders aren't the only car that have suffered from this, the cavalier/calibra 4x4's had an apparently "weak" transfer box, infact it was a very similar system to the freelander, and unmatched tyres caused the failures.
 
My td4_2003 is now about 126 000 km...and VCU never been changed. All the tests are good...recently i went to the garage for a check up and as i talked about changing it, the mechanic answered " no panic...:)"
I just want to say that they is no intangible rule...anyway, i have planed to change it after 130 000 km , just to be quiet
 
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This site although aimed at The VW Syncro van gives a good description of what a VCU is, how it works and why it goes wrong. It also goes in to some details of the implications of unevenly worn tyres, a matter of interest to me as mine had 3 worn and one brand new tyre. I am not too concerned as the tyre was absolutely new so had not run far in that state and I insisted on new tyres before I bought the car.

The Syncro AWD System

A lot of it is obviously vehicle specific and does not apply in our case but it interesting to see that many of our issues are to do with the concept of a Viscous Coupling and not specifically Freelander based. One of the linked pages refer to a typical life of their unit as being about 60-90 K miles.

What is important is that it does support the idea that a VCU can get permanently locked. I am a little confused about the maximum free speed as being at 6% as if the unit is in itself speed sensitive and locks at higher differential speeds when travelling faster. I would absolutely not recommend carrying out the test suggested in this document as it is extremely dangerous.

The document does however support my concern mentioned in another post that slow speed manouvres feel like I have the handbrake on.

Incidentally I did own many years ago (don't laugh) a FIAT Panda 4x4. That had a dog clutch between the front and rear propshafts operated by a handle in the car, only to be used on loose surfaces. Dead simple!!! Now how can I mod a VCU to do the same??
 
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