Viscous Coupler Going Bad??

[Bear80]

I drove my sister's '95 and it almost seems like the center diff is locked, however it is not. It is hard to turn at low speeds, tires chirp, and the drive line has noticeable bind. When I lock the center diff it gets really hard to turn, etc, etc which tells me it is locking and unlocking.

I've heard that if a VC is bad it will seize up and put full torque to front and rear output shafts. So could the VC be on the way out and not properly splitting torque?? And why would it go bad, since it has always had a matched set of michelins on it?? It has over 220k and the last 80k been 90% highway.

Someone please school me on VCs, I don't know too much about them.

 

[landtank]

When My truck warms up I get a groan when shifting from reverse to drive. I thought it was either in the tranny or the VC. The tranny was swapped out last spring so that leaves the VC. Stuff does wear out and it looks like the VC in that truck is on it's way out.

 

[RavenTai]

Someone please school me on VCs, I don't know too much about them.

http://webpages.charter.net/raventai/HF2AV TRANSFER.pdf

the VC rarely has any problems, there was a recent thread where one was destroyed by mismatched diameter tires, this thread also has soem diagnostice info and a resolution:

https://forum.ih8mud.com/showthread.php?t=116398

 

[Tool Pusher]

Bear80,
I just went through this exact thing. When turning sharply the driveline would bind as if the center diff. were locked.

To test if the VC is bad, make sure the center diff is not locked, block the rear wheels, jack up one front tire and put the t-case in N. Try and turn the front tire that is in the air. If the VC is working correctly you should be able to turn the front tire with some resistance. Mine would not turn at all, except for some driveline backlash.

I decided to replace the VC so I bought a used t-case that had only 88,000 miles on it and verified that the VC worked by testing if the front and rear output flanges were not locked. They turned oppositely, as you would expect with a diff., but had resistance between the two.

The VC is pretty easy to replace as you only need to remove the rear t-case housing. Now my t-case works as it should.

David Sword

 

[Bear80]

Thanks! Funny it was so recent of a thread. Good read on the pdf too, I didn't even think of it as a LSD device and it makes alot of sense now.

 

[Cruiserdrew]

Bear80,


The VC is pretty easy to replace as you only need to remove the rear t-case housing. Now my t-case works as it should.

David Sword

True-The same work as replacing the rear seal. I wonder how much a new one is, but why is it necessary when there is a center diff?? With a center diff unlocked, it sould act like any AWD car even if the VC is "locked". Am I missing something?

 

[landtoy80]

I need more info.
Does the wheels bind up when its vehicle is cold or hot?
Does it do it after driving at hwy speed or right after you start it?
Feel the t-case to see how hot it is.
Have you greased the yokes on the drive shaft? Is the yoke plugged, grease flinging out from the yoke after greasing?
When you remove the grease zirk, does the grease fly out the hole?

To test:
remove either the rear of front drive shaft and drive, if you have a center dif lock, lock it, drive it at hwy speed then stop, unlock the center dif and see if you can turn the output shaft of the t-case.
If you can turn the output (it will turn very hard) its ok.
If its locked, then you have a problem.

When my t-case went bad, I thought it was the t-case.
It would get VERY hot after driving at hwy speed. When it got hot, the VC locks up.
There was no info out there when my t-case VC locked up and I only removed the front drive shaft and drove.
Had I removed the rear, I might of found out it was the drive shaft yoke that was the problem...NOT! I would of thought it was the rear output shaft bearing and replaced the rear bearings.
Anyhow, the yoke on the rear drive shaft that was plugged on my 80 that caused my t-case to overheat.

 

[landtoy80]

True-The same work as replacing the rear seal. I wonder how much a new one is, but why is it necessary when there is a center diff?? With a center diff unlocked, it should act like any AWD car even if the VC is "locked". Am I missing something?

The use of the VC is to lock the t-case when you are in hi range and dif unlocked.
When you remove one drive shaft (or lift up one axle) and try to drive:
If you don't have a VC and try to drive, you will be stuck/ wont move.
Add the VC and try to drive, the VC will lock up after several turns or it gets hot (not sure how it works but I did test my VC and when it is heated, the VC locks up) and you will move.

The VC is a way for you to drive in 2wd and instantly have 4wd when you need it and back to 2wd just as fast.

 

[RavenTai]

why is it necessary when there is a center diff?? With a center diff unlocked, it sould act like any AWD car even if the VC is "locked". Am I missing something?

It sounds like you may be under the impression that the VC in an 80 works like an AWD vehicle where the VC IS the center differential? Where in many VC AWD systems the VC is what transmits the power to one of the axles, without the VC one axle receives no power. I think many people familiar with those other systems automatically hear viscous coupler and they that is what they think the FZJ80 has.

The HF2AV in the 93-97 FZJ80is technically full time 4 wheel T-case not AWD, difference being the front and rear drive shafts are directly geared to each other via the center differential. the CD and VC are in parallel instead of series. The VC instead of being a method of transmitting drive just becomes a limited slip device, dampening the action of the center differential similar to a positraction but instead of clutches it uses a viscous silicone fluid, Even if you removed the viscous coupler both drive shafts would still be driven,


In a turn the front drive axle follows a wider radius, When the VC locks up the center differential can no longer differentiate the front and rear drive shafts. And the driveline binds until a tire slips, acting just like having the CDL locked,


See the PDF linked above and also the unfortunately named but very informative “Diffs for Dummies” page linked in the FAQ they explain it better than I can.

 

[Cruiserdrew]

Thanks RT-I have had the mechanism apart and sort of know how it works. It just seems to me that if there were no VC, and it was replaced by a splined shaft, the truck would still drive normally. My understanding (which may be flawed) is that the VC introduces some resistance and so forces the transfer of some torque to the rear wheels, in the setting where the front wheels are spinning.

It does interest me, though, why the 100 series trucks with the same basic tranny and t-case left the VC out.

There is lots of paranoia about running slightly different sized tires, that you wouldn't need to worry about if there was a center diff and no VC.

 

[RavenTai]

Thanks RT-I have had the mechanism apart and sort of know how it works. It just seems to me that if there were no VC, and it was replaced by a splined shaft, the truck would still drive normally. My understanding (which may be flawed) is that the VC introduces some resistance and so forces the transfer of some torque to the rear wheels, in the setting where the front wheels are spinning.

It does interest me, though, why the 100 series trucks with the same basic tranny and t-case left the VC out.

There is lots of paranoia about running slightly different sized tires, that you wouldn't need to worry about if there was a center diff and no VC.

read my post again, if you were to look at the parts you may get the idea that the VC transmits drive to the rear axle but this is not the case, the output shaft in the T-case from the center differential to the rear drive shaft travels through the center of the VC uninterrupted, were you to replace the VC with a splined collar the rear drive shaft would be locked the the center dif carrier, just like the front and rear lockers when they are engaged therefore locking the center diff,

If you wanted to remove the VC and have it drive normally you woudl remove the VC and replace it with nothing, you have just turned the HF2AV into an HF2A of the FJ80 and UZJ100, the center differential would still differentiate in a turn but would allow either axle to spin should it encounter low traction until you locked the CDL.

 

[landtoy80]

<<<Even if you removed the viscous coupler both drive shafts would still be driven,>>>

Without the VC only one drive shaft will turn. The one with no resistance will turn.
So you would be stuck until you lock the center dif if one wheel is off the ground.

 

[RavenTai]

<<<Even if you removed the viscous coupler both drive shafts would still be driven,>>>

Without the VC only one drive shaft will turn. The one with no resistance will turn.
So you would be stuck until you lock the center dif if one wheel is off the ground.

right, if one axle were on a slippery surface that axle would get all the revolutions, does not matter front or rear.

I was trying to point out that in an 80 under normal driving on pavement all wheels are still driven even without the VC, as opposed to an AWD vehicle if you remove the VC the axle down stream would not be driven, for instance the rear axle on a transverse mount AWD car.

 

[Tool Pusher]

There was some mention in earlier VC threads that the reason Toyota used the VC in the HF2AV in the FZJ80 and the HF2A in the FJ80 and 100 series was due to the anti-lock braking system. The following is what I gathered from these earlier threads and not my direct knowledge. The FJ80 has no anti-lock system and the 100 series has a four-wheel sensing system so does not need any coupling between the front and rear drives. The FZJ80 has a two-wheel anti-lock system that needs feedback between the front and rear drives and hence the need for the VC. I guess we would need someone in Toyota's driveline engineering section to answer this. I did briefly consider leaving out the VC when I found it was locked, but decided to replace it since I figured it had been put in there for a reason.

David Sword

 

[dropthattaco]

I just read through the PDF, and even though it's off topic, I just noticed that this T-case has an oil pump?!?!??! That's freaking awesome!!

Ok, back to the topic at hand: has anyone ever heard of the VC going out in the form of not transfering torque? They fail by locking up, but has anyone ever failed with no lockup? Also, is this the type of thing that needs "exercising" like the lockers?

 

[dropthattaco]

There was some mention in earlier VC threads that the reason Toyota used the VC in the HF2AV in the FZJ80 and the HF2A in the FJ80 and 100 series was due to the anti-lock braking system. The following is what I gathered from these earlier threads and not my direct knowledge. The FJ80 has no anti-lock system and the 100 series has a four-wheel sensing system so does not need any coupling between the front and rear drives. The FZJ80 has a two-wheel anti-lock system that needs feedback between the front and rear drives and hence the need for the VC. I guess we would need someone in Toyota's driveline engineering section to answer this. I did briefly consider leaving out the VC when I found it was locked, but decided to replace it since I figured it had been put in there for a reason.

David Sword

Actually, I can field this one . I used to work at lexus, and as we all know, the LX's are Land Cruisers. They sent me to training for this. The limited slip action in the center diff is accomplished using the traction control system. The center diff is just a regular open diff, and as such, when one output is held, and the housing is turned, it forces the other output to spin instead. What happens when a wheel starts to spin is the TRAC ecu identifies the slippage using the wheel speed sensors, and uses the ABS pump to pulse the brake at that wheel. This stops spin and forces the spin to go to the other axle. This happens for all corners all the time, regardless if it's already pulsing another wheel. So, if both rear tires are spinning, the ABS is applied on those wheels, and the power is transferred up front. It's actually a pretty ingenius design, but it's noisy and vibrates the truck like crazy. Works great, though!

 

[CJF]

Ok, back to the topic at hand: has anyone ever heard of the VC going out in the form of not transfering torque? They fail by locking up, but has anyone ever failed with no lockup? Also, is this the type of thing that needs "exercising" like the lockers?

Never heard of one failing "open". Doubt you'd notice anyway; 91-92's don't even have one, and plenty of later year folks have removed theirs.

It gets "exercised" every time you drive it.

 

[lndcruz]

When My truck warms up I get a groan when shifting from reverse to drive. I thought it was either in the tranny or the VC. The tranny was swapped out last spring so that leaves the VC. Stuff does wear out and it looks like the VC in that truck is on it's way out.

Don't tell me thats what that groan is

 

[7schulz]

Take it out or get a new one
https://forum.ih8mud.com/80-series-tech/124794-tutorial-removing-viscous-coupler-photos.html
https://forum.ih8mud.com/80-series-tech/403229-vc-viscous-coupler-caused-damage.html
It is made of a number of circular plates with tabs or perforations, fitted very close to each other in a sealed drum. Alternate plates are connected to a driving shaft at one end of the assembly and a driven shaft at the other end. The drum is filled with a dilatant fluid, often silicone-based. When the two sets of plates are rotating in unison, the fluid stays cool and remains liquid. When the plates start rotating at different speeds, the shear effect of the tabs or perforations on the fluid will cause it to heat and become nearly solid because the viscosity of dilatant fluids rapidly increases with shear. The fluid in this state will effectively glue the plates together and transmit power from one set of plates to the other. The size of the tabs or perforations, the number of plates, and the fluid used will determine the strength and onset of this mechanical transfer.