Centre diff and to rear air locker or not?!

Hi you great SC owners!

I've got a 96' SC full-time 4wd and am wondering exactly how the centre diff / drive shaft works?

I've read some posts about a viscous centre shaft where as some other people have said its an open centre diff.. Help!

I've called ARB in Hobart, TAS and they said I could look into a rear air locker but my question is: If I went to the expense of air locking the rear - Then if one of the front wheels lifts will it still just sit there and spin the front wheel even with a rear locker engaged or would there still be 50/50 power to front & rear so the rear wheels would have traction (with or without locker)



I'm now going to try and post a picture of my chassis build plate so you guys can see my axle codes etc - Unfortunately I don't have a LSD as I've read the G292 code means open diff??

Cheers, J

I think the way the viscous coupling works in concert with the planetary gear set on the Hiace 1KZ full-time 4WD models is that the planetary gear set splits the torque between the front and rear output shafts in the transfer case (post referenced below states the split is 35%/65% Front/Rear). Then, if one of the output shafts starts spinning faster than the other (over a certain threshold) the viscous coupling will begin to lock up to try and maintain that torque split between the output shafts.

If my assumptions above are correct and you get a front wheel in the air, the front shaft will start spinning faster and the viscous coupling should start locking up (does it ever lock 100% and how much slippage is required? see forum post linked below) and redirect some of the torque to the rear wheels. It may benefit you to put in a rear locker, but maybe not as much as you'd hope. Sounds like front and rear lockers would be the way to go if you can afford it.

I do wonder what the service life is of the fluid used in the viscous coupling, it sounds like as the fluid ages, it locks up less readily. Seeing that these full-time 4WD vans are now up to 28 years old, it could be time to replace the coupler. Since they're sealed units, it could be more dependant on kms driven and other factors like driving style and tires being of similar circumference.

Take a look here for more detail on the 4WD system:

https://toyota-club.net/files/faq/04-02-20_faq_4wd-2_eng.htm

And this SC forum posts details some real-world experience:

https://hiace-super-custom.forumotion.com/t1102p15-not-a-real-4wd

I found a coupling unit for sale on Amayama. Still you'd have to partially disassemble the xfer case to R&R it, fun!

https://www.amayama.com/en/part/toyota/4133026012

Lastly, this Toyota video is a good watch as well, even though it only pertains to the transaxle-based All-Trac system, it still goes over the principles of the viscous coupling (17:53 is where the viscous coupling section begins):

I kind of went down a rabbit hole on this one, but it's been fascinating trying to understand the engineering.

Here's my best description of how the planetary diff and viscous coupling interact (please correct me if I'm wrong!).:

1. The input shaft from the transmission drives the ring gear in the planetary set. The ring gear also meshes with one side of the viscous coupler.

2. The planet carrier turns a sprocket that meshes with the other side of the viscous coupler. This sprocket also drives the chain connected to the front output shaft.

3. The planet gears in the carrier mesh with the rear output shaft, the sun gear.

Here are some good photos of all these gears:

https://1gfe.ru/toyota/vse-modeli/val-razdatki/val-razdatki-toyota-36437/

https://1gfe.ru/toyota/vse-modeli/val-razdatki/val-razdatki-toyota-36435/

Here is an exploded parts diagram of the unit:

https://jp-carparts.com/toyota/partlist.php?maker=toyota&type=711140&cartype=144&fig=3610

With the van at rest on dry pavement, the planet carrier and sun gear are stationary since they ultimately are connected to the front and rear axles and will resist turning freely. The ring gear turned by the engine rotates the entire gear set as one and the van moves forward.

Let's say the van is parked with a rear wheel on ice. When the engine turns the ring gear the planet carrier will remain stationary due to traction on the front tires, but the planet gears will rotate in place and drive the sun gear. On the viscous coupler the ring gear is spinning one half of the unit, but since the planet carrier is stationary, the other half of the coupler doesn't turn. The shear forces will cause the coupler to begin to lock up and transfer torque to the planet carrier and the front wheels will start to turn and move the van off the ice.

Now, let's put the ice patch under a front wheel. The engine turns the ring gear, the sun gear is now stationary due to traction at the rear wheels causing both the planet gears and planet carrier to turn. The ring gear is turning one side of the coupler and the planet carrier (which is now rotating) is turning the other side of the coupler, but at a slower speed than the ring gear due to gear reduction. The coupler will start to lock up, attempting to speed up the planet carrier's rotation to match that of the ring gear. The carrier can't rotate at the same speed as the ring while the sun gear is stationary so the coupling force gets applied to the sun gear and turns the rear wheels.

Based on the above, the point at which the coupler is engaged for the same speed wheel slippage at the front vs rear should be different, as the speed differential between the two sides of the coupler is greater when the rear wheel slips (ring is turning and the carrier is stationary),  vs the front slippage, where the ring and the carrier are turning, just at different speeds. That would mean the front wheels will have to spin faster than the rears in order to cause the coupler to start locking up.

Obligatory YouTube video:

Hey DJathens :-)

Thanks for your long educational reply! I find it all pretty interesting too but I'm still learning the basics of it. I would say that having a rear air locker would definitely make it more capable but given the lack of suspension articulation I wonder if it's worth spending the money?

I noticed on your profile you have a 1991 SC? Is yours the full-time or part-time version? Do you do much off-roading / tracks over there in and around Oregon? I'm in Tasmania, AUS and we have LOTS of 4x4 tracks that probably are beyond the capabilities of my van but there's also plenty of forest roads and tracks that it could drive on.

Have you seen that Youtube channel Vanlife Northwest? I'm guessing they are somewhere close to Oregon or Washington? Or BC? They wheel part-time 4x4 Hiaces though!

I would love a part-time one - I'd nearly go as far as importing one from Japan but that's probably not a great idea during this pandemic haha!

Look forward to hearing back from you!

Hello there groovy ozy guy, yes my 91 SC is part-time 4WD. Toyota switched the SC to full-time in August 1993. I imported mine back in 2018, it's a lot of work, but saves you a lot vs going through a dealer.

Vanlife Northwest is located here in Portland, and Danny is my go-to mechanic when a job is too big for me. He does excellent work, and has a great YouTube channel. If you keep an eye out, you can see me in the video from this past January when we did a run in the snow. That was a blast and showed me some of what the van is capable of handling. I was impressed!

There are some 4x4 tracks out here, but I've largely kept to Forest Service roads as I'm not big into advanced off roading, and like you said, the van doesn't have the best clearance/articulation/suspension travel. I'm using it mainly as a rig to take my friends skiing, car camping at established campgrounds, and hopefully soon some dispersed camping in the national forest.

Best of luck in your adventures! There's a great community around this forum and a lot of helpful information.

Thank you - I'll have a look at the Youtube channel again :-)

That's great that you imported yours! Did you do all of it yourself? I know you said you didn't use a dealer so do you mean you bought it from a Japanese auction online and then had it shipped over? I know it'd be a bit different importing to the U.S. than it would to Australia but I know both of our countries have pretty tight regulations for vehicle imports.

There is an Australian broker I've emailed who charge a flat $1100 fee for finding, inspecting and I think bidding for the customer. If I were to ever go down the import route It'd be great to know how you did it? At least the part up to getting it to the U.S. (From then the process would be a bit different)

I did import it from Japan myself. I worked with a company in Japan called Pacific Coast Auto. They get you access to the auction system, translate the auction sheets and then bid on your behalf up to your max bid. Once you win an auction, they handle all the paperwork and logistics to get the vehicle deregistered and onto the cargo ship, plus send you any documents necessary for import customs and registering in your locale. I believe they charged $1000 USD for their services. The main restriction for the US is the manufacture date of the vehicle you are importing must be at least 25 years ago on the date it arrives in the US.

Howdy - Ahh yes the US's 25 year rule, I've heard a lot about it as I watch quite a lot of vehicle channels from the States. I quite like Doug Demuro's Youtube channel! I don't think we have a year rule here in Aus but we have very specific requirements for a few safety features and indicator lights must be on the side of the vehicle etc (Except old cars sold in Aus I guess)

Thanks for the info re Pacific Coast Autos - I have seen them in my searches :-) I am pretty sure the Aus broker I found does the same thing as they're actually based in Japan but I assume they have a few people locally working with them too. I reckon that's the way to go for importing as you pay the $1000-1500-ish (AUD) fee and you are guaranteed a great example of what you're after opposed to paying 5k more from a local (in the same country) dealer who obviously need to make their money.

DJAthens how do you find the clearance with your van? I notice that the transmission hangs down quite a lot in these vans... It is the transmission right? haha! I can see if would be an issue if you were trying to go over a large bump/rock where both front & rear axles are either side of it...

I have between a 2 and 2.5" lift on my van after installing taller springs and cranking up the torsion bars, and so far I haven't scraped anything underneath. I took off the bash plates under the engine and transmission to get a little more clearance, as they're heavy, get in the way of doing service, keeping an eye out for leaks, and can drag in deeper snow.

I think the point that hangs down the farthest is the transfer case, but it has its own shield that I've left on. Or, it could be the front cross member. I can't recall exactly what my max clearance is. It's basically in a line from front to back that passes between the rear diff and rear lower control arm. I think it is around 11 inches, or 28cm.

Ahh yes that's a decent clearance. I'd say yours would look a little different to mine underneath due to being part-time 4x4 vs the full-time drive system in mine. I see you quite enjoy snow driving! Do you camp out much in the snow too? We only have a few areas in Aus that get snow and I have never been 4WD'ing in them but it looks fun from a few Youtube channels I watch.

This is the start of the camping area and then there's a road that winds up the mountain and somewhere on top there's a small ski field (I haven't been to that area yet)

There's a snow field in Tasmania close to where I live but I don't think it's consistently covered - I'd like to fill up with diesel and take a drive out there one weekend!

djathens - Brilliant description of the function & operation of the center diff gears & coupling! It’s like I’m there in the casing!

Hi Pete_nz - How's your SC going? How do you find it off-road? Does yours also have an open rear diff? I still haven't seriously looked into a rear locker yet - money constraints however I did get my van over a bit of a boggy ditch on the weekend and up onto a bumpy grassy area for our camp just this weekend gone. They are decently capable as they are!

Hi, It’s good. Don’t really off road as such but am often on dirt roads/driveways and my city has lots of steep and awkward driveways where normal 2wd vans will too often spin a wheel and get stuck even on concrete or other sealed surfaces.

Last month I towed 4 separate loads of paving stones each at about 2.2 tons incl the trailer. At the end had to get them back down a grass slope, then get the empty trailer back up. Started raining by the last trip. Discovered the van will spin all 4 wheels at once to get moving then grip and just pull it’s way up. All on standard road tyres. I don’t think a locking or even lsd will give any advantage with the full-time AWD system. As described so superbly above by djathens, if a rear wheel spins, the drive gets sent forwards. The only scenario it might help is if one rear is the only one with grip out of all 4. That my van had all 4 wheels spinning then gripping on uphill wet grass/mud surprised the hell outta me. On flat slippery surfaces there’s no slip at all.

I can’t remember if the lsd from the Surf is a direct fit. The ratios may be off given their tyre size difference. Seems the way to go if extreme situations are required.

Btw, My van has ~420,000kms and I guess the fluid coupling (as described in the 2nd post at the top) is still functioning fine.

I would imagine the AWD system was better than any LSD.
In my experience LSDs have limited use, I have one on a BMW. In snow it just means both back wheels spin at the same time, so the car drifts into the ditch with road camber

The fluid works so well - I suspect - because if things are fluid coupled and one exit loses drive, the fluid pressure lowers for that item and more power is available for the things that don't.
That's my assumption, but surely what any fluid AWD system would aim for.

Hey guys :-) Pete_nz that's awesome that you were able to haul that trailer up the grassy hill and that the AWD system kicked in and you got grip! I am starting to understand a bit better now that these AWD super customs are actually pretty capable! I desperately need new tyres on mine but hopefully can afford them next week. I've got reasonably beefy all-terrains on mine..

I was pretty impressed my van got me over this ditch last weekend on slippery grass in a pine plantation - And my current tyres are pretty bald!