Question for boffins

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The VCU is clever. As MHM said it's all about heat. The slip of the plates causes heat to be generated in the special fluid between the plates. This special fluid then solidifies transferring the power via the plates. As the plates are now rotating at the same rate, the fluid cools off allowing some slip. If the slip is sufficient to cause heat in the fluid the drive transfer repeats. This process will continue until all the wheels are rotating at the same rate. There is a drive ratio but I wouldn't know where to find the information out.
The way the VCU works giving a gentle transition from free to locked is a good way to keep traction on slippery surfaces but allow some slip when negotiating corners, this help keep control of the vehicle without the wheel skip associated with a locked centre diff.
 
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I get all that, honest!

Looking at the test results, they seem to show the resistance offered by the VCU when not locked, rather than what is required to lock one in terms of numbers.
They're going to be all different anyway. Maybe there is no definitive answer.

But, I have to say well done to MHM for sticking with such a valuable thread under strangely hostile circumstances!
 
How much rotation does the VCU need to react then?

I came from a a Haldex gen 4 car to the VCU of the Freelander, and the Haldex reacted in less than an 1/8 of a wheel spin.

So I guess, how much wheel spin is required to generate the heat required for it to start driving the rear wheels?
 
Epicuser said:
How much rotation does the VCU need to react then?

I came from a a Haldex gen 4 car to the VCU of the Freelander, and the Haldex reacted in less than an 1/8 of a wheel spin.

So I guess, how much wheel spin is required to generate the heat required for it to start driving the rear wheels?
Click to expand...

jack a wheel up and try it ,bearing in mind freelander has slight difference in axle speed so kind of pre-tensioning vc
 
Our vcu's "activate" very quickly to give nearly constant 1:1 ratio of prop drive. I filmed it on snow by pulling away quick to spin the front wheels, and film it showing the rear wheels spin almost instantly. I'll put the video up when I get round to it. Working on other stuff at the moment. There's always some resistance across the vcu if it's an original untouched vcu. Ratio front to rear is said to be 0.8% according to rave, with the rear turning 0.8% faster than the fronts.

Nodge is correct in his explanation above.

The one thing I find funny is when so called "expert" vcu reconditionners state the vcu doesn't seize when in use (and say so in their own explanation on the web). They do, but it's only momentarily as nodge ses, when the plates lock if the prop speed difference is high enough for the shearing to heat up the silicon fluid inside. When the plates are 1:1 ratio it cools as there's no shearing and the plates un-seize and rotate at differing speeds. The change form slip to seize to slight slip (repeating) can be quite quick.

The good thing about the vcu is it varies the resistance across itself, depending on the differing prop speed. Resistance across the vcu (ability to drive more power through it) is proportunal to prop speed so the vcu has the ability to find an equiblibriam point of required drive, dependent on prop speed. Difficult to put into words... think of the full on seize/un-seize slightly at high differing prop speeds as opposed to lower differing speeds where the vcu may be shearing and getting hotter then dropping temp slightly then hotter then dropping temp slightly (repeating)... and passing say 80% of full resistance as opposed to the normal value of resistance we measure with the one wheel up test.
 
This VCU question has been going through my head for a few weeks now wondering what to do. My FL has just reached 65000 on the clock and reading a lot of posts about changing it has got me a bit concerned. I've owned the car from new everything seems to be fine with the VCU so do I leave well alone or change it for a recon VCU ????.

Ps Sorry for butting in.
 
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Minescoming up to about 58k on the clock too, not sure what to do about the VCU coz Im skint innit!? :D Theres some tests you can do on the VCU thread that tell if youre VCU is knackered/on its last legs, personally I reckon weekly checks and as long as its ok no need to replace it, if it aint broke dont fix it right? Then on the first signs of it going, get it changed. I gotta do the cambelt too around those kinda miles too right? And Ive got the 1.8 k series petrol, so probably worth checking the head gasket too, eurgh...

I dont trust my mechanical skills enough to change the cam belt or VCU myself, been quoted about £400 for a cambelt change... that seems bloody extortionate, my last car only cost £180 for a cam belt & water pump change! God knows how much a VCU change would cost from a garage!
 
Tiercel said:
Minescoming up to about 58k on the clock too, not sure what to do about the VCU coz Im skint innit!? :D Theres some tests you can do on the VCU thread that tell if youre VCU is knackered/on its last legs, personally I reckon weekly checks and as long as its ok no need to replace it, if it aint broke dont fix it right? Then on the first signs of it going, get it changed. I gotta do the cambelt too around those kinda miles too right? And Ive got the 1.8 k series petrol, so probably worth checking the head gasket too, eurgh...

I dont trust my mechanical skills enough to change the cam belt or VCU myself, been quoted about £400 for a cambelt change... that seems bloody extortionate, my last car only cost £180 for a cam belt & water pump change! God knows how much a VCU change would cost from a garage!
Click to expand...

I changed my belts but mines a TD4 not to bad a job on mine. The VCU is not to bad either just a case of dropping the prop and VCU in one then just changing the unit over and replacing. I changed my prop bearings at the beginning of the year after I had a garage change then about 8 months before that, they didn't last so when I changed them myself I used the GKN bearings and so far no problems.

Ps I think a recon VCU is around the £300 mark.
 
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Hippo said:
The one thing I find funny is when so called "expert" vcu reconditionners state the vcu doesn't seize when in use (and say so in their own explanation on the web).
Click to expand...

I haven't seen that, although I have seen them say that they haven't yet found a VCU fail by being fully seized as opposed to VERY stiff.

It makes sense that they would stiffen over time given their nature.
Apart from any heat damage to the fluid, they will collect impurities through use.

I'd love to know what the chemical make up is of that fluid. Usually materials become more fluid when heat is applied. It's probably captured alien technology :eek:
 
bukko said:
Has anyone ever tried replacing a VCU with a Torsen centre diff?

(I feel like the annoying kid who always says "Whyyyy?")
Click to expand...

It couldn't be done as the rear output from the IRD is constantly driven.
The VCU works better than a Torsen diff would in this application.
 
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