Viscous coupling/clutch

[dragonvan]

The newer NV136 is a definite upgrade in that regard. It has a fiber/steel plate clutch like in an automatic transmission. When it feels like the front diff is engaged when making sharp turns at low speed (jerk, binding, or shudder feeling). It is time to change the fluid if you have an NV136 so that you don't wear out the clutches. If you have the BW-4472 it is time to pull the t-case and replace the viscous clutch assembly. If you don't do this on the BW you will probably melt down or break the front diff. It just wasn't made for the constant load of having no slip point (differential) between it and the rear diff. It's the same sort of thing as driving a 4WD that has no clutch or differential in the t-case in 4High down a dry paved road (good traction = no wheel slip). The NV136 won't do that if you ignore the symptom, you will just wear out the clutch plates early. The jerking or binding on tight turns at slow speed feels just like making a tight turn on pavement with a 4WD that is in 4High maybe just a bit less severe in the case of the NV136 when it first starts doing it.

[mdmead]

The newer NV136 is a definite upgrade in that regard. It has a fiber/steel plate clutch like in an automatic transmission. When it feels like the front diff is engaged when making sharp turns at low speed (jerk, binding, or shudder feeling). It is time to change the fluid if you have an NV136 so that you don't wear out the clutches. If you have the BW-4472 it is time to pull the t-case and replace the viscous clutch assembly. If you don't do this on the BW you will probably melt down or break the front diff. It just wasn't made for the constant load of having no slip point (differential) between it and the rear diff. It's the same sort of thing as driving a 4WD that has no clutch or differential in the t-case in 4High down a dry paved road (good traction = no wheel slip). The NV136 won't do that if you ignore the symptom, you will just wear out the clutch plates early. The jerking or binding on tight turns at slow speed feels just like making a tight turn on pavement with a 4WD that is in 4High maybe just a bit less severe in the case of the NV136 when it first starts doing it.

Well, I disagree the new one is an upgrade.....

The old style is pretty bulletproof and failures are pretty rare. And I'd hazzzard a guess that most failures are operator error, i.e., mismatched tire sizes. It seems the bulk of problems brought up on AS.com over the years on t-cases has been on the new electronic units and it seems "electronic" is usually the problem with high repair bills to follow.

Some have also questioned the "reaction time" of the new vs old regarding the frontend kicking in to pull. I have no experience with the new units and would love to test one out on icy roads. I know my viscous case is quick and seamless and I've had it in some really deep snow and it behaved just like a truly locked t-case.... (minus the much-needed low-range though!).

What's funny is that when I bought my van, my main concern was the t-case and I thought I would be lucky if it lasted 100K miles. Then after finding a bunch of fellow Astro/Safaris enthusiasts at MiniVanReview.com way back when, I learned that 200K was just breaking in the van and t-case.

But hey, I'm opposed to change just on principle!

[peter]

I believe that the silicone-based oil in there actually becomes less viscous as it heats up. Sorta the opposite of what usually happens with any oil. It's that decreased viscosity (thickening of the oil) which helps the front drive-shaft hook up.
No?

[dragonvan]

Peter, heat usually thins oil. 10W-30 doesn't get thicker as it gets hotter it just only gets as thin as a 30 weight at temperature instead of being as thin as a 10 weight at the upper temperature measuring point.

I have replaced several of the old viscous clutch t-cases. I think one of them grenaded the front diff too if I remember right. Though I admit not many, they are pretty bulletproof. I have seen many viscous clutches fail in cars... many! It was a common problem on GM's early AWD cars and also on the Subaru's that had them. It does work very seamless though... but hey it's analog it should.

I have only replaced one motor on the new t-cases. Granted these aren't as old yet. I have replaced a couple of motors on the 2 speed versions. I have replaced quite a few t-case motors on the fords... especially the rangers. This is just what I have seen though and only reflects my corner of the world.

You guys may be right about the reaction time of the system. The PCM in these vans is no speed demon and fuel and ignition get the bulk of clock timing I would bet. I bought my van right at the end of winter and didn't have to go anywhere the one little snow we had.

[rlsllc]

I think there are variations between the NV 136 cases, just like there are variances and tolerances in everything. I've read of some members experiencing rear wheel spin before their front wheels engaged. I can't get my '99 to spin the rears on snow, wet grass, anything without the fronts pulling. It will even bark a front tire when you pull out at a 90-degree approach to a road under throttle. You can hear it with the window down. And no, the case isn't stuck, I've done the AutoTrac II flush and it drives figure eights smooth. When the case dies, it's getting a NP233 and a front disconnect. Then I have to blow the 4.3 up so I can put in a 350. Tangent! Sorry!

[dragonvan]

rlsllc, I would bet the different reaction times of the system has more to do with the programming that's flashed into the PCM than machining variances. My father's '02 K1500 Silverado has the heavier 2-speed version of this case, the NV246. Neither he nor I have ever noticed a delay in "Auto" mode. I did recommend to him however to just put it in 4High if he is driving in a snow storm though, to make sure it doesn't decide, for whatever reason, to go into 2High and stay there.

When I test drove mine, I played around in an ice covered parking lot. When I gave it some gas the rear stepped out some. The van had bald tires on it though, and I could continue to do this even after I was sure the front was engaging. That was the point of the fun was to test the t-case. It was most likely occuring as the system is designed to never give more than something like 40% torque to the front IIRC, to protect the front diff I believe. I think the viscous clutches were designed to never completely lock up either, when working correctly, also to save the front diff.

Neither of these t-cases are a substitute for a real 2-speed case, but they are light years better than 2WD in my neck of the woods, even with a limited slip. If there is 3 feet of snow and I really need to get somewhere I'll just jump in the ZukTracker, that's what I built it for. The van is to pull the camper and haul the troops in every day.

The majority of the problems that I have seen with the electronically controlled t-cases in GM's have been the switch modules in the dash going bad on the 2 speed cases, in the t-series and full size. I have replaced a lot of them.

I am sure mine will get a work out in about 6 months here in the "Frozen Tundra." (You might not get that last sentence if you aren't a football fan. Hint, hint.)

[smokinsiggy]

I wouldn't even bother taking it apart. I've seen several given away. They can be had for very little at a salvage yard, or if you want a real cool van, stick a 2 speed case in it like mine and several others around here.

This idear really strikes me as good to go but what happens with no front locking hubs ? Does this mod make it work the same as the AWD concept ? Or an actual all the time 4x4 ?

[LiftedAWDAstro]

I believe that the silicone-based oil in there actually becomes less viscous as it heats up. Sorta the opposite of what usually happens with any oil. It's that decreased viscosity (thickening of the oil) which helps the front drive-shaft hook up.
No?

You are 100% correct in how a viscous coupling works!

smokinsiggy...with a 2 speed t-case, in 2wd hi mode, it will act exactly as a regular 2wd van. You will not get any engagement of the front wheels at all. When in 4wd hi or lo, you get a mechanical connection between the front and rear diff's. There is NO slip between them and they will bind up on real tight turns in dirt or wet/dry roads. With no locking hubs, the front cv half shafts, front diff and front prop (drive) shaft will turn anytime the front tires are turning.

In my 03 awd, there is basically no noticeable delay between when the front engages. On Lynne's 2005 TrailBlazer, there is a definite delay between rear wheel slip and front engagement when in AWD mode.

[smokinsiggy]

"There is NO slip between them and they will bind up on real tight turns in dirt or wet/dry roads". Thanks for this comment "liftedAWD", now here is another question on this matter . With the binding on tight turns as mentioned is it proper to say that one can expect problems down the road as experienced in my AWD earlier before it came down . That was a common problem from my understanding with AWD and it only makes sense to think that you are to going to experience a mechanical failure of some kind with this flaw . Now what can be done to rectify this "flaw in design" as per say? Swap out everything to actual 4x4 design , with transfer case, front diff., locking hubs as with the s15 or jimmy/blazer. ??? May not be understanding this issue properly and am willing to ask all kinds of dumbass questions to figure it out , Cuz if I ever grow up I'm gonna be a GMC driveline guru . LOL

[LiftedAWDAstro]

I don't mind all the questions.

When I say it will bind up, I don't mean that it will cause problems as much as you will feel it binding slightly. With a true 4x4 t-case, you CAN NOT run in 4WD on wet or dry roads as the added traction of the pavement WILL cause problems. Really, in dirt, you will feel a slight bind but the tires will slip slightly rather than break anything.

Lockdoc and I have many miles of 4WD Low range driving of our vans with true 4x4 and we never had any troubles with breakage while making sharp turns. The breakage came from spinning while climbing hills and getting the front ends off the ground whether it was 1 wheel or both and having the tires slam the ground while spinning. THIS causes major breakage!!!!

The s-10's never had locking hubs so that isn't an option. What they had was a disconnecting axle shaft on the right side. This is done to eliminate the very slight vibrations caused from the high speed running of the front drive shaft while cruising down the road. There is a definite advantage to swapping out the stock diff with one from an S-10. There is also a very slight fuel economy savings. Maybe Peter can say if there is any savings there...

[ScottS]

Are yous sayng the S10 front diff can be used on a Safari ?


There is a guy selling an astro diff on ebay but it is for a 99 + , anyone know why it won't ft a 97 ? If I have to change the front prop shaft to make it work I have no problems with that

[LiftedAWDAstro]

I have a 98 front diff and I will have to compare it to my 03's but it looks the same from memory.

[ScottS]

For some reason the Hollander interchange lists 97-98 as the same and 99+ as different- if they changed the input flange to mate with the new transfer case, I have no problems with changing either the prop shaft or the output flange . Can't believe they would change the axle tube and he way it mounts to the truck, but I could see a shorter/longer prop shaft due to the electric transfer case being in a different position.

[ScottS]

Oh and if you have a 98 Diff can I by it ? What ratio ?

[LiftedAWDAstro]

Oh and if you have a 98 Diff can I by it ? What ratio ?

4.11 ratio and no you can't buy it. LOL I will be hanging onto it for installation into my 03 at some point down the road...maybe.