How to test for VCU/Rear diff... LOSING TRACTION - 3000GT/Stealth International Message Center
3000GT/Stealth International 3000GT/Stealth International
 

Auto Insurance

» Featured Product
Wheel & Tire Center

Go Back   3000GT/Stealth International Message Center > Modifications and Technical Support > All Wheel Drive - DriveTrain

3SI.org is the premier Mitsubishi 3000GT Forum on the internet. Registered Users do not see the above ads.
Reply
 
LinkBack Thread Tools
Old 03-04-2009, 04:55 PM   #1 (permalink)
New User - Please be kind
 
Join Date: Aug 2008
Location: Indianapolis
Drives: 1995 3000gtvr4
scottlesliejones1 Negative 1
Feedback Score: 0 reviews
Default How to test for VCU/Rear diff... LOSING TRACTION

Hey guys, my car has a new shortblock, but now I've noticed when I step on it at high revs (1st and 2nd) it feels like it's losing traction in the rear wheels. I've read a million posts, and most seem to think it's one of the 3 couplings. Excuse me if my car tech talk isn't up to par.

Another board member (venom) also drove it and noticed the obvious slippage at high rev speed in lower gears. Once into 3rd, I can't recall it having slipping problems.

We did ALL the wheel spinning tests, now can someone help diagnose where the problem is. After reading all the posts, it's confusing which could be to blame.

HERE'S WHAT WE DID:

4 TIRES JACKED UP OFF GROUND (in NEUTRAL):

1. We spun a rear tire forward by hand.
Result: Other three tires spun forward. Same thing happened with both rears.

2. We spun front right tire forward. Result: Front left goes opposite direction, rears go forward.

3. We spun the other front tire (left) forward. Result: Front right goes opposite direction, rears wiggle, but don't move as much unless you rock the turning tire back and forth.

4 TIRES JACKED UP OFF GROUND (in GEAR):

1. Spin any tire any direction and other three spin opposite direction.



FRONT TIRES ONLY UP OFF GROUND (in Neutral)

1. Spin either front tire, other goes in opposite direction.


FRONT TIRES ONLY UP OFF GROUND (in GEAR)


1. Spin either front tire, other goes in opposite direction, but spins easier than it did in neutral.

NOTE: You can't spin either front tire while other person holds the opposite tire while it's IN GEAR. But when it's IN NEUTRAL, holding the other tire doesn't stop you from spinning one tire, while holding the other.



REAR TIRES ONLY UP OFF GROUND (in GEAR)

1. Spin either rear tire, and other turns in opposite direction.

REAR TIRES ONLY UP OFF GROUND (in NEUTRAL)

1. Spin either rear tires, and other turns in same direction.



SO ANY DIAGNOSIS?

Thanks,
Scott

Last edited by scottlesliejones1; 03-04-2009 at 05:02 PM. Reason: extra stuff in there
scottlesliejones1 is offline   Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
Old 03-04-2009, 05:31 PM   #2 (permalink)
Forum Member
 
KORMEX's Avatar
 
Join Date: Mar 2000
Location: Ontario,ca,USA
KORMEX Level 1KORMEX Level 1
Feedback Score: 13 reviews
Default Re: How to test for VCU/Rear diff... LOSING TRACTION

The results that you got from the tests are what is to be expected .

Have you thought about the clutch slipping ?
KORMEX is offline   Reply With Quote Quick reply to this message
Old 03-04-2009, 05:43 PM   #3 (permalink)
Forum Member
 
Join Date: Sep 2006
Location: YORK / UK
keithmac Level 1keithmac Level 1
Feedback Score: 4 reviews
Default Re: How to test for VCU/Rear diff... LOSING TRACTION

Normally clutch slip is worse in the higher gears, I generally test a clutch by hitting peak torque in 4th or 5th, if it doesn`t slip then it never will (until it`s worn out).
__________________
AOL`s stolen my picture!.

JDM GTO TT, Emanage Ultimate - 780`s, E85
Hotwired Walbro - 13g`s - K&N - Blitz SSBC
13g`s - 12.4@115
TD04-16g`s - 12.5@117 (work in progress!).
keithmac is offline   Reply With Quote Quick reply to this message
Old 03-04-2009, 06:39 PM   #4 (permalink)
Forum Member
 
HLxDrummer's Avatar
 
Join Date: May 2006
Location: PA
Drives: 1993 VR4
HLxDrummer Level 1HLxDrummer Level 1
Feedback Score: 42 reviews
Default Re: How to test for VCU/Rear diff... LOSING TRACTION

With only the rears up, you should be able to spin one and there should be some resistance (if the VCU in the rear diff is good).

Try having only the rears on the ground and turn one wheel forward and have someone turn the other front wheel the same direction. You shouldn't be able to move them. This tests the center VCU.
HLxDrummer is offline   Reply With Quote Quick reply to this message
Old 03-05-2009, 03:34 PM   #5 (permalink)
New User - Please be kind
 
Join Date: Aug 2008
Location: Indianapolis
Drives: 1995 3000gtvr4
scottlesliejones1 Negative 1
Feedback Score: 0 reviews
Default Re: How to test for VCU/Rear diff... LOSING TRACTION

hlxdrummer: when you say:
"With only the rears up, you should be able to spin one and there should be some resistance (if the VCU in the rear diff is good)."

Does that mean the resistance on the tire that you are trying to spin, or resistance on the opposite tire (say if you're trying to hold it)

Then when you said:
"Try having only the rears on the ground and turn one wheel (I'll call it wheel #1) forward and have someone turn the other front wheel (wheel #2) the same direction. You shouldn't be able to move them."

What is the "them" refer to in the sentence?
Does them mean you shouldn't be able to move wheel #2 because wheel #1 is already moving... or
Does it mean when attempting to move wheel #2, it shouldn't alter the motion of wheel #1, or does it mean something else?

Thanks, Scott

Last edited by scottlesliejones1; 03-05-2009 at 03:38 PM. Reason: clarification
scottlesliejones1 is offline   Reply With Quote Quick reply to this message
Old 03-05-2009, 04:18 PM   #6 (permalink)
Forum Member
 
Join Date: Sep 2006
Location: YORK / UK
keithmac Level 1keithmac Level 1
Feedback Score: 4 reviews
Default Re: How to test for VCU/Rear diff... LOSING TRACTION

It means if you try to spin both raised front wheels at the same time, you shouldn`t be able to (with the rears on the floor or e-brake on).

You should be able to do all tests in neutral.

Other test is to jack one front wheel up only, it should be VERY hard to turn with the other 3 wheels on the ground.
keithmac is offline   Reply With Quote Quick reply to this message
Old 03-05-2009, 05:01 PM   #7 (permalink)
Forum Member
 
HLxDrummer's Avatar
 
Join Date: May 2006
Location: PA
Drives: 1993 VR4
HLxDrummer Level 1HLxDrummer Level 1
Feedback Score: 42 reviews
Default Re: How to test for VCU/Rear diff... LOSING TRACTION

Quote:
Originally Posted by scottlesliejones1 View Post
hlxdrummer: when you say:
"With only the rears up, you should be able to spin one and there should be some resistance (if the VCU in the rear diff is good)."

Does that mean the resistance on the tire that you are trying to spin, or resistance on the opposite tire (say if you're trying to hold it)

Then when you said:
"Try having only the rears on the ground and turn one wheel (I'll call it wheel #1) forward and have someone turn the other front wheel (wheel #2) the same direction. You shouldn't be able to move them."

What is the "them" refer to in the sentence?
Does them mean you shouldn't be able to move wheel #2 because wheel #1 is already moving... or
Does it mean when attempting to move wheel #2, it shouldn't alter the motion of wheel #1, or does it mean something else?

Thanks, Scott
Sorry I didn't really give you a good explanation.

With the rears on the ground, if you spin one of the front wheels and only that one, the other front wheel will spin the opposite direction. This is because the front differential is an open differential. This is normal. However, to test the center differential (VCU) you will need someone else to spin the other front wheel the same way you are (not clockwise or counter, but forward like the car was driving). While you are both applying pressure, you shouldn't be able to move these wheels without some serious force.

For the rear VCU, I'm not 100% but I think you should jack up all four wheels and turn one back wheel. The other back wheel should spin the same direction (forward or backward). You could also try holding one rear wheel still and try to turn the other one. You shouldn't be able to. Sorry for the crappy instructions with this one, but I'm not 100% sure how to do this test.
HLxDrummer is offline   Reply With Quote Quick reply to this message
Old 03-06-2009, 11:47 AM   #8 (permalink)
New User - Please be kind
 
Join Date: Aug 2008
Location: Indianapolis
Drives: 1995 3000gtvr4
scottlesliejones1 Negative 1
Feedback Score: 0 reviews
Default Re: How to test for VCU/Rear diff... LOSING TRACTION

Yea, regarding the rear vcu... my results were the opposite of what you are thinking (although you state you were not 100%).. here are the copied results:

1. We spun a rear tire forward by hand.
Result: Other three tires spun forward. Same thing happened with both rears.

I will try the other resistance checks you both mentioned.
If anyone has any other ideas on the rear vcu, give me a shout out, as I'll be testing these on Monday.

EDIT:

EXTRA STUFF I FOUND ON THIS SITE:

** If your vcu is totally fried, then you will get sever front wheel spin on a launch even with good tires. If you have bald crappy tires, then you will most likely get wheel spin anyways. This usually results in crappy 2.0+ 60' times at the track. You can put the car up in the air and hold both front wheels steady, drop the TC, and turn the output shaft. If it spins really easy, then the VCU is either popped, or on it's way out. There will be some resistance due to the other center parts that are rotating with the shaft, but short of a partial tranny tear down this would be the easiest/fastest way.

** The repair manual test procedure for the rear VCU is to jack up the car with all the wheels in the air and turn one of the rear wheels by hand. If the rear diff VCU is good, then the front wheels and other rear wheel will turn in the same direction. But if the rear VCU is blown, then the front wheels will not, but other rear wheel will turn in the opposite direction, which is exactly what we've seen on most cars.

He doesn't need a new rear diff if the center VCU is toast.

He specifically said that when he turns the rear wheels, the front wheels do not turn with them. That means either the center diff it self blew up, or the VCU is no longer doing it's job.

Essentially, when you're turning one of the rear wheels, the other rear wheel should turn in the same direction. if neither front wheel turns and the other rear wheel turns in the opposite direction, the rear lsd is toast.
a lot of people tend to neglect the rear lsd's when they do a fluid change. after i bought my car, i did a change on it and silver liquid came out. i don't think anyone had done a fluid change on the rear of my car since it was new.

Q: Do you need to have the car in gear to check?
A: no. just in neutral.

Q: Jack the entire car and rotate the tires, if the both rears turn the same direction I am good? What do the fronts do?
A: They should turn the same way.
A: The front wheels will turn opposite each other, they are an open diff.

Last edited by scottlesliejones1; 03-19-2009 at 05:11 PM. Reason: updating
scottlesliejones1 is offline   Reply With Quote Quick reply to this message
Old 03-11-2009, 11:00 AM   #9 (permalink)
Forum Member
 
Join Date: Nov 2007
91GTOtt is an unknown quantity at this point
Feedback Score: 0 reviews
Default Re: How to test for VCU/Rear diff... LOSING TRACTION

That is what my car did, rotated right rear tire and the other rotated the other way but the front right rotated the same direction as I was rotating. So my VCU is good but my rear diff is bad?
91GTOtt is offline   Reply With Quote Quick reply to this message
Old 03-12-2009, 12:55 AM   #10 (permalink)
Rollin, Rollin, Rollin
 
beefy's Avatar
 
Join Date: Feb 2002
Location: 61065
Drives: 98 3000gt VR4
beefy is an unknown quantity at this point
Feedback Score: 14 reviews
Default Re: How to test for VCU/Rear diff... LOSING TRACTION

I would bet that the clutch is slipping.
You probably wouldn't even notice if your VCU went bad.
__________________
98 3000GT VR4
91 Stealth TT retired.
beefy is offline   Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
Reply

  3000GT/Stealth International Message Center > Modifications and Technical Support > All Wheel Drive - DriveTrain

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the 3000GT/Stealth International Message Center forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Free email addresses do not work(yahoo, hotmail, gmail,...)
We are currently being blocked by AOL so any AOL based address may not work(aol.com, aim.com, netscape.net, cs.com)
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On


Powered by vBadvanced CMPS v3.2.2

All times are GMT -5. The time now is 06:20 AM.




Garage Plus vBulletin Plugins by Drive Thru Online, Inc.

Search Engine Optimization by vBSEO 3.3.2