Alltrac Previa Transfer Case observations (long)

[Valdanilov]

Howdy,

I'm new to the forum, and fairly new to Previas.
Here's my story:
I got my first Previa last year. I lucked out (or did I?), stumbled across a 5-speed Alltrack right around the block from my house. The price was good, so I jumped on it.

Cleaned it up a bit, and took the van up to the mountains to try it in the snow. Well, what do you know, I got stuck the first time I drove into the snow. One of the rear wheels kept spinning, and that was it.

After getting back home, I fugured out what was wrong. The front driveshaft was missing! OK, I find a local guy via Craigslist, who's parting out a '93 Auto Alltrac. I buy and install the driveshaft, and go for a test drive. Bad noise from the front diff!

OK, put the van on the jackstands again, crawl under, sure enough, the front R&P is missing a few teeth. Aha, I think, now I know why the front driveshat was off!

So, go back to the parts car, buy the front axle, come back and install it, go for a test drive. On no, every time I turn the steering wheel, the tires start scrabbing on the pavement! Great, so we have a bad transfer case.

(Lesson 1: when your viscous coupling freezes up in the transfer case, you better take off your front driveshaft right away, or risk breaking your front axle!)

OK, go back to the parts car, only to find out the the transfer case has already been sold, argghh! Go back, start Googling..., educate myself on the subject.

Things I learned from the Net:
- viscous couplings fail, and they are expensive to replace.
-5-speed Alltrac transfer cases are unique, and only came on '91-92 models.
No, you cannot swap in a transfer case from an automatic! Nobody knows why (more on that later).
- you can drive with your front driveshaft off w/o doing any damage.

OK, so I proceed to drive w/o the front driveshaft for the next year.

Fast forward 6 months: we go to Europe for an extended vacation, buy a '96 Toyota TownAce 4x4 Turbodiesel, camp for a month in the van all over Turkey and Cyprus.
Love that Van! Easy 30mpg, Hi-Low transfer case, go anywhere!

Fast forward to January 2011: we are back in US, and I keep day dreaming about a 4x4 van.... do more research on the Net, perhaps shop for an early Toyota Van 4x4.... Come across Tim's write up on the subject of lifting a Previa, and possibility of retrofitting a TV 2-speed transfer case to a Previa. This sounds like a great project!
So, I do more research about Previa trannies, transfer cases, and interchangeability. It's amazing how little information is out there on our vans!! Even Toyota dealer couldn't tell me exactly what type of transmission and transfer case my van came with. WTF?!

This is when I decide to drop the transfer case, and try to figure things out. Perhaps I can substitute a 2-speed transfer case from another model?

The T-case came out easy enough, and I got it all apart.
Looking at the mating surfaces, I don't believe I can use any other T-cases from other Toyota models, this T-case is too different.
My T-case accepts a tranny tailshaft with 18 splines, while the spare T-case I'm getting from an automatic Previa has 21 splines.
(now we know why auto and manual T-cases do not interchange!)

Meanwhile, I'm going to take the good viscous coupling from the spare auto T-case, and install it into mine. While I won't have the High-Low, at least I'll be back to AWD, until I come by needed parts.

I learned a few things after taking this sucker apart. It's a chain driven T-case of rather simple constructon. And the way it's configured, it's giving all the torque to the FRONT wheels, until the viscous comes into play. So, essentially, Previa is a FWD van most of the time.

Another interesting thing is that my T-case has a locking feature, which is used "for testing only". However, it appears that this T-case was originally designed to be 2-speed, and manually lockable. Therefore, you can rig it up to have a manual diff-lock pretty easily, after disabling the viscous.
This way you can have your Previa as a FWD only, until you get stuck, at which point you can engage the diff-lock, and get out of trouble.

Oh yeah, the T-case ratio is 1:1.

Question still remains:
-can a 2-speed TC from a 4x4 TV be adapted to a Previa AWD? Before I can answer that, I need to know the spline count on the tranny tailshaft of the said 4x4 TV.
Does anyone know?
I'm also in the market for such a transfer case, so feel free to email me if you have one for sale at valdanilov at msn.com

Val

[timsrv]

Great write-up Val! Your observations are of great interest to me. One of my next projects is to install a 2 speed transfer case on my 91 Automatic LE lifted All-Trac Previa.

I've done a bit of research and up to this point I have been hopeful the transfer case from an 87-89 5 speed 4wd Toyota van will work here. I have not yet checked and compared the internals of these parts, but externally these t-cases are identical. If you look up the gasket (the one between the transmission and transfer case) in Toyota's EPC (Electronic Parts Catalog), you will see they use the same part number for both the Previa and the earlier vans. I also measured and compared both drive-line output areas and found these to be identical too. Yes, aside from exact dimensional placement, even the spline count and diameters are the same.

That totally blows me away that Toyota would use a different shaft configuration between the tranny and t-case for manual vs auto.......WTF??? This stifles my hopes a bit when it comes to mounting the earlier version 2 speed t-case on my automatic Previa.

One thing I do know is it's a popular thing to retrofit 2 speed transfer cases from the 5 speed manual transmissions vans over to the automatic transmission vans (87-89 4wd vans). But, from what I understand, the input shafts/hubs must be swapped over from the auto transfer case to the 2 speed case. On the earlier vans these t-case parts are interchangeable but I'm not so sure about the Previa case.

FWIW, I had the t-case off of a 95 Automatic SC All-Trac Previa about a year ago and I made some notes regarding the t-case shaft and splines. Here's what I wrote: OD of transfer case shaft is 1.595". Major diameter of interior splines is 1.210". Minor diameter of splines is 1.120". If counting the recessed splines, there are 17 small and 3 Larger ones. When you counted yours, I'm assuming you counted the raised splines (on the inside of the t-case shaft). If that's the case, then your comment of 21 splines would be consistant with what I observed.

At the time I made my notes, I also took some close-up pictures with my dial calipers and machinist scale on key parts, but due to a recent computer crash those (and a lot of other things) are lost forever......(don't ask, it's a very sore subject).

I will follow this post with great interest as I had also discovered a surprising lack of available information (I don't know of anybody else messing around with Previa t-cases and/or possible swaps). If you get a chance to take pics please post and keep us informed of your progress. It sounds like you're beating me to the punch here but I don't mind because I can learn from your experiences. Thanks for posting. Tim

[Valdanilov]

The spare T-case I'm getting is from a '95 Alltrac, and I only got description over the phone. I suspect it's going to be as you said, 20 count of recessed grooves in the shaft, 3 large and 17 small.

What we know for sure so far:

-'86-89 TV 4x4 T-cases DO interchange between auto and manual.
-'86-89 TV 4x4 have the same mating surface to '91-97 Alltrac Previa,
-'91 manual Alltrac has 18 grooves on the T-scase shaft,
-'95 automatic Alltrac has 20 grooves on the T-case shaft,

While I assumed that the difference in spline count is between manual and automatic models, it is entirely possible that the difference is in production years, i.e. '91-93 = 18 grooves, and '94-97 = 20 spline.

The only way to tell is to inspect the T-case from an early automatic.

And once we know what the groove count is on the '86-89 TV 4x4, we'll have our answers. I believe, Tim, you've got the right parts to investigate so the ball is in your court at this point.

I would really like to know the answer, since converting to a 2-speed TC is still in my books. Once I get my spare T-case, I'll swap the viscous unit, and will take pics of the process, and post them here for the next poor soul that has T-case troubles.

Val

[timsrv]

Yep, you are correct. I have everything I need here except time . I have several priority jobs waiting before I can investigate this........plus it's cold & wet out there and I don't currently have an indoor place to work . I am ordering a JDM engine and automatic transmission for my 91 All-Trac Previa on Monday. I already have a 5 speed transmission and transfer case out of a 87 van sitting here, so after the JDM stuff arrives it won't take too much time or effort to pull things apart for inspection. I do have other obligations/priorities though and this is not pressing (as much as I'd like to jump right on it). I will post my findings here as they become available. Tim

[NEC]

I have been looking for a dual range transfer for my 87 auto. I can't remember where I read it but I'm pretty sure the a/t and m/t transfers have different input splines even on the old 87-90 models. The swap can still be done, but requires dis-assembly of both transfer gears to swap out the input spline. By that thought process, It could be argued that the same could be done to retrofit a later model a/t-m/t swap. Somebody please do this and let me know if it works. I currently have a spare a/t with no xfer gear on it if anyone needs specific measurements of the output shaft.

[Valdanilov]

Hi NEC,

yes, if you'd be so kind to count how many splines your spare auto tranny has on it's tailshaft (where it mates to the transfer case), that would help.

You are probably correct in that if you are trying to put a 2-speed transfer case into an automatic '87-89 (90?), you have enough parts to make the swap.

This is also true for interchanging the transfer cases between auto and manual Previas (well, at least in the case of a '91 manual and '95 automatic). Even though the input shafts are different, you can just swap those parts around, and still use either T-case in either model (pending to be fully confirmed later this week).

However, if you are trying to fit an automatic Previa with a 2-speed T-case, you'll likely need another case from an '87-89 automatic to get the correct input shaft from.

A lot of variables here, but we can get to the bottom of it, if we collect enough data.

BTW, how does an early T-case work in an automatic '87-89 Van? Is this a full-time open diff, or can it be manually locked?

Val

[timsrv]

I tried ordering the All-Trac automatic transmission today but found they are out of stock. I'm now on a waiting list. I'll probably look around to see if another semi-local importer has these. Tim

[Valdanilov]

FWIW, there's a '95 Alltrac being parted out in the Lynnwood, WA Pick-a -Part.
I got a transfer case from it yesterday. The van is otherwise mostly complete, but the gauge cluster is gone, so cannot verify the mileage.

I used the viscous coupling from it last night to install into my '91 Alltrac 5-speed T-case. As expected, it was a direct swap. The 2 T-cases only differ in the input shaft spline count, but besides that part, everything else interchanges (with exception of the manual locking feature).
I just have to bolt the T-case back to the car, and take it for a test drive, but won't have any time to do it until the weekend :(

Val

[Valdanilov]

OK, I am happy to report the viscous coupling swap went well, and I'm back on the road!

Val

[timsrv]

That's awesome!

[Valdanilov]

I have a little bit more info to add:

I finally have a 4x4 5-speed TV in my garage, that needs a bit of love. I have removed the transmission/T-case combo from it last night, and sure enough, the manual transmission tailshaft has 18 spline count. I have also confirmed that the mating surfaces between said T-case and a Previa T-case are the same.
This means that the 2-speed transfer case from 5-speed 4X4 TV is a direct bolt on to a 5-speed Alltrac Previa!
It can also be said, that the 2-speed T-case WILL NOT bolt on to an Automatic Previa, w.o. swapping the input shaft in the T-case (sorry, Tim!). Since the input shaft in the Alltrac T-case also houses the viscous coupling, I seriously doubt that you can swap this input shaft into the 2-speed T-case during the conversion. But, perhaps it is possible? I cannot tell for sure, w/o taking the 2-speed T-case apart.
So, to sum it up:

-'86-89 TV 4x4 T-cases DO interchange between auto and manual.
-'86-89 TV 4x4 have the same mating surface to '91-97 Alltrac Previa,
-'91 manual Alltrac has 18 grooves on the T-scase shaft,
-'95 automatic Alltrac has 20 grooves on the T-case shaft,
-'86-89 TV 4X4 2-speed T-case WILL bolt on to '91-93 Alltrac Previa MANUAL tranny

[timsrv]

It seems odd that Toyota uses different splines on the t-cases from auto to manual.............but Toyota does a lot of odd things.

That's good news for the few of you with manual transmission Previas, but kind of sucks for the rest of us. I checked the EPC for transfer case input shafts (87 manual 4wd van VS the 91 manual Previa) and that pretty much sealed the deal. Due to the differences you mention (viscous) and the planetary for the high/low, these shafts are completely different (so no dice on swapping input shafts from the Vanwagon into the Previa). Sounds like the only thread of hope left would be swapping transmission output shafts from a Vanwagon automatic into a Previa automatic. Unfortunately, due to different spline counts the part numbers would be different, so there's no way I could verify that would work unless I tore down both transmissions (and that's a big job). I'll need to rethink my strategy here . Thanks for the info. Tim

[Valdanilov]

Yes, it sounds like more trouble than it's worth! You'd be better off starting with a 5-speed Previa, and transfering your lift bits onto it. I know where you just might be able to find such a car!
My plan is to build an ultimate 4x4 TV, then sell the Previa. Want to trade?

As a side note, it appears easy to bolt up an Alltrac T-case onto a 4X4 TV, and end up with an Alltrac TV! Not that it would be a worthwhile conversion, but hey, it would be different, and kind of cool.

[timsrv]

Ha ha. If I go the route of a 5 speed Previa then I'll need ot figure out a way to unlock the front hubs. I think for now I'll just settle for my lifted All-Trac. Tim

[NEC]

Ha ha. If I go the route of a 5 speed Previa then I'll need ot figure out a way to unlock the front hubs. I think for now I'll just settle for my lifted All-Trac. Tim

You should be ok with leaving the front hubs locked in as these hubs/axles are designed to be always locked anyway. The xfer case would be what un-coulpes the front and rear. The only difference is that you would now have 100% rear wheel drive unless you have the xfer case locked in 4wd mode. The transfer gear should still get plenty of lubrication as the oil pump will be turning whenever the input shaft is turning.

[timsrv]

Yeah, I was thinking more about fuel economy and reducing wear on my front drive components. I'm sure the benefit would be small though and probably not worth the trouble.

[timsrv]

It just occurred to me that I could take the input shaft from an automatic transfer case (87-89 van), swap it into a 2 speed transfer case (87 - 89), then install it on my automatic All-Trac Previa. Assuming of course that the splines on the on the automatic Previa transmission match up with the splines on the automatic vanwagon t-case. I'll look at this a little closer when I do my drive-train swap..........not sure when that will be now. They had told me about 3 months to get the stuff from Japan, but not sure how the tsunami may have affected that. Tim

[Valdanilov]

So, there are still a couple of unanswered questions:

-can the input shaft be swapped between the auto and manual T-cases?
-will the front/rear driveshafts need modifications?

In my mind, converting an automatic Previa to an earlier T-case would be viable for the following reasons:

-increase fuel economy by using 2wd most of the time, and 4wd - on demand,
-convert, because the original viscous coupling in the Alltrac T-case went bad.

Installing a 2-speed T-case is a bonus, but for the automatic Previa really isn't necessary, as the auto tranny would take up the slack in most situations where a low range would be desired. Combine that fact with the ease of conversion, and a push-button control for the 4wd with the auto T-case (single speed), and I'd be tempted to install a single speed T-case instead of the 2-speed T-case.

Converting a manual Previa to a 2-speed T-case would make more sence though.
But it's not like Previas are really off-road capable, with the long-wheel-base chassis. More like snow and sand kind of off-roading, and a Hi 4wd will do just fine.

Just my 0.2c worth...

Val