Power Distribution

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I was just curious how the power is distributed between the front and rear wheels when the t-case is in H. Does the power distribution change as wheels slip or grip?

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Eicca

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I know for a fact that the 92s and earlier have a completely open center diff unless locked. Someone hinted that the 93s and up may have some kind of limited slip center diff setup, but I don't know.

Anyone with the desired knowledge care to cash in here?
 
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I believe I've read on here that it has a viscous coupler in the center diff. I don't remember what year it started but it's probably listed in the FAQ or on Slee's website where they have the year to year changes listed and the Cruiser/LX differences listed.
 
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Thanks for the link. I have read it before, and I read it again to see if I missed the information. All I could find was that the 1993-1994, the transfer case changed to the viscous coupling HF2AV. No transfer case changes were listed for the 1995-1997 models. So, I'm assuming my 1997 has the HF2AV.

However, I still don't understand what "viscous coupling HF2AV" means. How is the power distributed between the front and rear axles with the HF2AV transfer case? Is it variable? What's it doing when I'm just driving on dry, flat pavement? What happens when wheels start spinning on snowy or icey roads? I understand what the diffs are doing (left vs right wheels), but I'm trying to understand what's going on between the front and rear axles.

By the way, I'm just really curious as to how the 4WD operates on my truck. It's not critical I know this, but it is bugging me.

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Grench

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Not the expert

I'm not the expert on here, but I'll throw my $0.02 in and maybe one of them will correct me.

The '91 & '92 and maybe the '93 with semi-floats & rear drum (no ABS) had a lockable center diff (CD) without a viscous coupler (VC).

On a '91-'92, with the center diff unlocked, power is supplied more or less equally to all 4 wheels under normal driving. However, if a wheel starts to spin, there is nothing stopping it from taking all of the power and simply spinning 4 times as fast - until the owner of said '91-'92 pushes the center differential lock (CDL) button on the dash. This button engages the locker inside the center differential and forces the front and rear drive shafts (DS) to turn at the same rate - regardless of wheel slip. So, if one wheel is slipping, then there would need to be at least one wheel slipping on the other axle.

On my '96 (and other years) equipped with a viscous coupler, Toyota made a system that somewhat manages itself in very basic situations. If the center differential has one DS spinning faster than the other, it heats up the VC a bit and causes a coupling to engage to balance some of the power back to the non-spinning axle. For gravel roads and snowy street driving by soccer moms this is an ideal no-think set up.

So, what happened to the dash button for the CDL on my '96? Well, the wires are still there in the dash. It is a simple matter of ordering the part from Sleeoffroad, Cruiserdan or similar. Installation is pretty simple too. The button gives the same response as it did on the earlier trucks - to get control over the center differential in high range.

In low range the CDL engages automatically. To wrest control away from the truck in low range, look up 'pin 7 mod'.

Now, if you're going to be driving without a DS as someone above mentioned - you do NOT want to drive on the VC as it is prone to heating up and binding - making it lock all the time and you unhappy with a big transfer case servicing bill. If you want to be able to drive with a DS out, you will need to get that dash CDL button in order to lock the CD in high range - there is an alternate way involving locking in low range and then pulling a fuse if you're in a pinch and don't have time to order parts.

IMHO YMMV - I am still not a mechanic.
 

80t0ylc

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To try and simplify the explanation, if you understand what an axle differential is, think of the transfer case as a differential between front and rear axles. Being as the 80 series is full time 4WD, Toyota refers to the transfer case as the "center differential". The viscous coupler, or VC, which is internal to the center diff., on the '93 - '97 HF2AV, while not locked, makes it a limited slip center diff or transfer case. This allows the rig to perform better in slippery conditions with out locking the center diff. Consequentially, when the center diff is locked, the VC is not a factor any more because in locking the center diff, the front and rear axles are locked together and no slippage is allowed. HTH
 
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On my '96 (and other years) equipped with a viscous coupler, Toyota made a system that somewhat manages itself in very basic situations. If the center differential has one DS spinning faster than the other, it heats up the VC a bit and causes a coupling to engage to balance some of the power back to the non-spinning axle. For gravel roads and snowy street driving by soccer moms this is an ideal no-think set up.

That's not quite how a viscous coupler works.

This is a pretty good description of one (along with other types of differentials):
HowStuffWorks "Viscous Coupling"

For our purposes, the technical way it works (based on the way the fluid works) might as well be magic. You can think of it as a limited slip, that is it will slip to a certain extent, then "lock" and send power to both drive shafts rather than just one. In that aspect it's very similar to a LSD, except it is in the center diff rather than an axle diff.

But the main thing to know is that it's fluid based, not heat based. Heat is actually quite bad for it.


Now, if you're going to be driving without a DS as someone above mentioned - you do NOT want to drive on the VC as it is prone to heating up and binding - making it lock all the time and you unhappy with a big transfer case servicing bill. If you want to be able to drive with a DS out, you will need to get that dash CDL button in order to lock the CD in high range - there is an alternate way involving locking in low range and then pulling a fuse if you're in a pinch and don't have time to order parts.

Bingo.

Because the VC acts as a limited slip, if you're driving with a drive shaft out (or busted) it will be constantly engaging and disengaging. This can cause it to heat up and basically "weld" itself together, meaning it's always engaged. It has basically the same effect as locking the center differential, both front and rear drive shafts have full power, all the time (instead of some of the power, all of the time).

Also a good reason not to drive with mismatched tires.


IMHO YMMV - I am still not a mechanic.

And yet you still know more than some mechanics I've talked to. :cheers:
 
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Cool. Makes sense. So, when driving on dry roads (read, no tire slipping) power is going 50% front and 50% rear. But once wheels start slipping, the vc tries to keep power distribution 50/50. Is that correct?
 
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ezterra,

My version of the short answer:
When you're driving with the center diff open, you're getting equal torque to the front and rear axles. The center differential in the 80-series is a geared differential that splits the torque 50/50. The viscous part is essentially added onto this system to send some additional torque to the axle with more traction when the other one is slipping. This seems to work well in snow and slippery road conditions. It's not really intended (nor does it really work) for low-range crawling.

When you get a wheel spinning on a snow road, the VC starts to kick in, transferring additional torque to the axle with the spinning wheel.

As far as I understand, the viscous coupler is based on a silicon fluid that changes in viscosity due to being sheared (and heated).

It's a pretty cool system, I think. There are a few threads on the forum somewhere that beat this topic to death. But they are interesting reading nonetheless.
 
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This seems to work well in snow and slippery road conditions. It's not really intended (nor does it really work) for low-range crawling.

Oh, it works just fine for low range crawling. I've done plenty of stuff with the center diff open, and the VC will kick in just fine.

It's not ideal though. If you're going to be having a lot of slippage then you run the risk of damaging it. Much safer just to lock the center diff. The only times I've done it is for easy stuff where I know there will be very little slipping.
 
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Cool. Makes sense. So, when driving on dry roads (read, no tire slipping) power is going 50% front and 50% rear. But once wheels start slipping, the vc tries to keep power distribution 50/50. Is that correct?


It's probably more accurate to think in terms of torque (force) rather than power--although your practical understanding is correct.
 
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Oh, it works just fine for low range crawling. I've done plenty of stuff with the center diff open, and the VC will kick in just fine.

It's not ideal though. If you're going to be having a lot of slippage then you run the risk of damaging it. Much safer just to lock the center diff. The only times I've done it is for easy stuff where I know there will be very little slipping.


I've never really tried to power through anything with the CD open. At least to the point where I'm really spinning tires. Have you been able to do this? Tire int he air kind of circumstances?

I often unlock my CD when negotiating easy terrain in low range (makes tight turns on slickrock easier), but I've never had it open under conditions of wildly spinning tires.
 
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I often unlock my CD when negotiating easy terrain in low range (makes tight turns on slickrock easier), but I've never had it open under conditions of wildly spinning tires.

Neither have I. I haven't had it on anything worse than what you'd encounter in ice and snow. I have no doubt, however, that it'll lock up just fine even with a wheel in the air.

Makes it sound more impressive though. "I did that with all three diffs open, and didn't even slip once!" :lol:
 

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