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Thread: Let's talk VCU...

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    Forum User Not Verified Feedback Score 2 (100%) RealMcCoy's Avatar
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    Let's talk VCU...

    Of all the parts on these cars, the viscous coupler stands out as one of the most misunderstood... Is it good, or is it bad? which one is it? how do you test it?

    First lets work on how to identify the VCU:

    There are two basic design housings; The first design has no thrust surface on either side of the outer housing. The inner sleeve protrudes from both sides approx. 1/4". The second design has a thrust surface built into the outer housing on the left side, so that the sleeve is flush with the housing. In the vast majority of cases, the first design will be an 18 spline unit, and the second design will be a 25 spline unit. (but don't bet the farm on that, because there are some second design 18 spline units floating around)



    The primary difference between the two, is that the 18 spline unit has the splines biased toward the left side, with splines all the way to the end of the sleeve. The 25 spline unit has the splines centered in the sleeve. You cannot physically put an 18 spline unit in a 25 spline trans, but care must be taken not to mix them up and put a 25 spline unit in an 18 spline trans.(It will fit in place, but only give you about 1/4" spline engagement to the front diff.)

    The following three pics, combined with the previous one, show the differences in the output, sun gear and VCU inner sleeve:






    So now we know how to identify it, how do we tell if it's any good..?

    The first thing to check is the splines... Excess wear is bad:



    Then if you're working on an 18 spline unit, you want to check for excess end clearance. they have a habit of walking the inner sleeve right through the housing. (this is where we find out WHY they re-designed it)



    I have a preventative fix for that particular problem:




    The next thing you want to check is find out if it's obviously blown. (we'll do a more comprehensive test shortly if it's not obviously junk) Just grab the sun gear, and try to spin the center by hand. If you can easily spin it, we're done, it's now a core to be sent back to your favorite vendor when you buy your rebuilt one...

    If you can't turn it by hand, or can barely turn it slowly, we can move on: The next thing we want to check is the seals. The only thing you can check here, is visually look for signs of leakage, and check the grip of the rubber on the inner sleeve. The seals are a large O-ring that when in good condition have significant preload on the seal surface. You check that grip by working the endplay of the inner sleeve back and forth. If it easily rattles around, the seals are most likely hard as rock, and you run the risk of puking the viscous fluid the first time you get it hot. If they show a good resistance to movement then you have a good chance of them continuing to do their job...

    So now we get to the meat of the issue.... How do we test one to find out if it's any good? what is good, what is bad? I may disappoint some of you here, I have no intention of telling you where that line is. What I am going to do, is show you how a new unit behaves, and hopefully give you the information you need to make that decision for yourself.

    A while back I bought a brand new 18 spline unit, it's never been dipped in gear oil... So I figured it would be the perfect test subject. I stuck it in a vise, and broke out the torque wrenches.

    The VCU's job is not to stop movement, but to stop RAPID movement... So the first thing I checked is the actual breakaway torque. I found that at a very slow movement, it took a pretty low 15 ft/lbs to maintain movement. At that pace, it would take well over a full minute to make one revolution. Next I checked the rapid movement torque limiting. This is a bit subjective to how much of a gorilla the guy pulling the torque wrench is, but I found that I could pop 85 ft/lbs with a hard fast pull.

    I wanted to get this on video, but it didn't work out as well as I wanted. I only had my phone, and nobody to help me, so they aren't much... The low torque one I pushed the button to turn it off at the end and it kept recording for a couple seconds until I caught it, so you get to look at the floor for a bit.. The high torque one just suffers from me trying to hold the phone with one hand, and pop 85 ft/lbs with the other...

    VCU :: VCU1.mp4 video by lnmccoy - Photobucket
    VCU :: VCU2-1.mp4 video by lnmccoy - Photobucket


    Real Performance Automotive (541)816-4500 www.FB.com/RealPerformanceAuto

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    LW fears my posts Not Verified Feedback Score 1 (100%)
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    Good post. I'm surprised you were able to get 85ft lbs, that seems like a lot compared to the force needed to turn a brand new TC assembly.

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    Member verified Feedback Score 0 DocWalt's Avatar
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    Good to see someone finally put VCU knowledge in one place.

    You could theoretically have near complete lockup from the VCU, but by that point I think it would blow a seal or overheat and destroy the fluid.
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    Forum User Not Verified Feedback Score 2 (100%) RealMcCoy's Avatar
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    Quote Originally Posted by UTRacerX9 View Post
    Good post. I'm surprised you were able to get 85ft lbs, that seems like a lot compared to the force needed to turn a brand new TC assembly.
    You completely lost me... maybe I haven't had enough coffee yet. How does the turning resistance of a T/case factor?

    Quote Originally Posted by DocWalt View Post
    Good to see someone finally put VCU knowledge in one place.

    You could theoretically have near complete lockup from the VCU, but by that point I think it would blow a seal or overheat and destroy the fluid.
    Yes, they will destroy themselves if abused. You'll never get one to fully lock up unless it welds itself. This is one of the worst I've seen:



    Someone tried to drive the car with a spit output shaft....

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    Member verified Feedback Score 0 DocWalt's Avatar
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    Good old VCUs.... lol. Just so everyone knows, TTs also have a VCU in the rear diff for LSD action. In my experience, the rear diff goes way before the center diff VCU. My rear diff was toasted, no LSD action at all. Center VCU was practically perfect.

    When a VCU blows and you're not 100% sure it's shot, you can drain your tranny or diff and check the fluid. If there's a lot of gray looking thick metallic stuff floating around in the normal drab gear oil, your VCU has popped like that one ^^

    Murcielagos and Gallardos both use the same style of VCU. I have no idea if theirs are as susceptible to damage as 3S VCUs.

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    Administrator Feedback Score 3 (100%) Alan92RTTT's Avatar
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    NIce post. I copied it to the front page tech section.

    3000GT / Stealth / GTO Forum - Let's talk VCU......

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    Forum User Not Verified Feedback Score 2 (100%) RealMcCoy's Avatar
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    Quote Originally Posted by Alan92RTTT View Post
    NIce post. I copied it to the front page tech section.

    3000GT / Stealth / GTO Forum - Let's talk VCU......
    Thanks Alan, I'm honored, and happy I could help build the knowledge base here...

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  10. #8
    LW fears my posts Not Verified Feedback Score 1 (100%)
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    Quote Originally Posted by RealMcCoy View Post
    You completely lost me... maybe I haven't had enough coffee yet. How does the turning resistance of a T/case factor?
    It doesn't directly. Just an observation I noticed when putting the output shaft into a brand new TC and the amount of force needed to turn it, vs when the OS is installed.

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