Full-time 4wd/AWD Driveshaft Mod Possibilities?

What you're proposing is kind of the opposite of a limited slip. You would start to lose front axle power when you want it the most, irrespective of the rear axle condition. Starting from 0, for example, it would be disengaging.

I think a limited slip device would be more helpful. Allow some relative differntial (if you will) in driveshaft speeds for cornering, but not so much that there is no power delivery.

 

I bought my wife a Limited 4R just for that full-time. It is really nice for mixed road conditions, which is what we usually have in our area during the winter months.

 

I agree with the limited slip idea. If I'm thinking about this correctly, (and there's a solid chance I'm missing something) I'm most interested in having a driveshaft with a viscous coupling. This should transfer some torque whenever the transfer case is engaged, but still slip until the rotational difference between the back and front is enough to make it heat up and send more torque. Essentially it would be the opposite of how the land rover freelander, Volvo xc, or honda AWD work. Rear wheels would do most of the work until they start to slip at which point the front end starts pulling harder. If I could find a company that custom made viscous couplings, I suppose it could be built to transfer whatever percentage of torque was desired (to a certain point). If you want the steering to feel lighter you just switch to 2H. When you want part time 4wd again, or when the viscous coupling eventually needs to be rebuilt, you just switch back to the OEM drive shaft. I believe this is basically how the GM and Jeep Cherokee auto 4wd transfer cases used to work, but they have the ability lock out or bypass the viscous coupling. The biggest challenge I've found with this is space. The couplings used in those transfer cases are about 8 inches in diameter and about 4 inches thick. The ones used in the freelander are about the same. To fit the Tacoma would require something about 5 inches in diameter and probably a lot thicker. I don't know enough about the thermodynamics to begin to guess the required dimensions.

 

Why.
I venture to say tha it would be easier to modify a limited 4Runner into a truck than to develop a full time 4wd that works as you stated above and much cheaper too.
A long weekend, a sawzall and a case of beer and I can give you the pickup truck of your dreams.

 

The only way to do this really would be to find a way to retrofit an existing full time transfer case. On something like the MT FJC or an LC100 or LC200 basically the transfer case has a lockable open diff with some sort of limited slip device(torsen, viscous coupler etc) piggybacked behind it. That way when it's unlocked the LSD gets to do its thing, when locked of course it won't do anything because both sides will always be turning at the same speed.

Key point being that you need to have a different transfer case from the get go. So at that point if you're tackling a transfer case swap anyway may as well just use an MT FJC transfer case. Which it seems like no one can get to work properly, but still more feasible than building your own transfer case from scratch.

Adding a differential on either side of the transfer will just mean that you're giving up four wheel drive for AWD at a lot of trouble and expense. At that point just sell the Taco and buy a Ridgeline(or Maverick I suppose) if you want an AWD truck, or an FJC/Land Cruiser/4Runner if you don't need a bed but want a lockable LSD center.

Note I'm not one of the Ridgeline bashers here, serious suggestion, I think they make a lot of sense for a lot of people and seems like a perfectly viable option if you want a bed and AWD.

 

I love the idea of a full time case like the 4-Runner Limited or the Manual FJ Cruiser. Having a true center differential with a torque sensitive limited slip inside would be the absolute best performing option. I fear all of the computer controlled systems makes this a very difficult swap and beyond my abilities. I'm terrible with trying to trace electrical issues and not smart enough to work out all the circuits involved. Maybe one day when this truck is old enough that I don't care about having christmas tree dash it might happen.

 

How much do you tow usually? Besides the towing you sound like a good candidate for a Ridgeline or whatever Subaru if you really would prefer AWD to part time 4wd.

If you're towing a good amount if I were you I'd probably just flip the Taco for a 100 or 200 series Land Cruiser depending on your budget and call it a day. 100 has the same capacity as a 3rd gen, 200 has more.

 

I think I lot of people are not understanding the disadvantage of part-time 4wd vs awd in the context of daily driving during winter in a climate that has long cold winters and occasional snowfall. Most of the time the hiways and main roads are free of ice and snow but side streets, parking spots, etc can have enough snow/ice to make 2wd less than ideal. It’s a PITA to swap back and forth between 2wd and 4wd in these conditions.

I generally throw some weight in my bed and drive around in 2wd during such conditions. If I get stuck I use AutoLSD. It’s easier and more reliable than getting 4wd to engage and then immediately disengage.

Another disadvantage of part time 4wd is that it doesn’t steer as well in deep snow covered roads.

That said, I do prefer part time 4wd for extreme off road situations. I also have always assumed that part time 4wd is simpler, cheaper, and more durable.

with all time 4wd you have the best of both worlds at the cost of the money and complexity.

 

I want to say thank you for all the responses this has gotten.. Even the guys I don't agree with. It's really helpful with thinking this through.

I don't tow very much, just a small trailer with kayaks, bikes, camping equipment, and what not. The biggest issue for me is the manual transmission. I literally purchased this truck because my only options were this, a jeep gladiator, or a nissan frontier... The Toyota was the only real choice for me from that group. I mention towing because reverse with the manual is either slipping clutch or like 15 mph.. It's a bit too tall. Being able to pop it in 4low and back up with the trailer would be nice.. I'm no hater on Honda's either. I had an Element with their "real time 4wd" for about 12 years and it performed extremely well in bad weather. Spent 15 hours driving home in 'snowpocalpyse' back in 2014 and had great stability with the Copper AT3s on it. It found traction when I needed it without fishing around or sliding. However it struggled some when I loaded it down with camping gear and climbed fire roads in the mtns. I seriously considered a Honda Pilot because of that car, but really wanted that third pedal... I'm just pondering a possible option that would be easily reversible if it didn't work out.

 

You want a Highlander.

 

I would too have concerns with getting the electronics to work right?

With older Jeeps like the TJ (which I have one), it is possible to switch the part-time NP231 with an NP242 (that added full-time), but they were manually engaged TCs (only difference was the light on the dash). The only downfall I heard about the NP242 (having more parts) was it didn't like taking abuse as much. I also had a Jeep Liberty with the NP242, and we left the full-time on most of the time, which is why we bought a Limited 4R.

Maybe the next Tacoma will have an auto setting like a lot of full-size trucks now do?

 

Since the manual transmission is so important to you, have you considered starting with an AWD or full time 4WD vehicle and swapping the transmission? Would that be simpler than changing over most of the drivetrain on your Tacoma?

 

This is a system that Isuzu had, Torque On Demand, TOD.
I had a 2002 Isuzu Axiom with it. Choice of 2Hi, TOD, 4Low. Normally rear wheel drive, but in TOD mode front wheels kicked when needed. No speed restriction while in TOD. Transfer case was made by Borg-Warner.
Power split varied from 0/100 to 50/50 in TOD. 4 low locked in 50/50 front/rear.
It was the best truck for driving in snow on AK highways.

 

Yeah I was in the same boat, although I do a lot of off roading too so the Taco makes sense. Reverse is definitely too tall. You won't hurt it going a few feet in low range to park a trailer, it'll just drive a little funky. Honestly I've forgotten to take it out of 4x4 once or twice coming off the trails and you feel it binding before it does any damage if you're an attentive driver(which I assume you are if you're a manual diehard). Also search around the forums, I know someone was trying to add a 2LO hack which would be much simpler than trying to add a diff to four wheel drive.

I drive in inclement and variable slick conditions a ton as a skier in the Pacific Northwest, 4hi is fine for slick roads. If it's been making you nervous in the snow the stock tires aren't great in it. My KO2s made a big difference, I hear AT3s like you had are even better in the snow and dedicated snow tires would of course be the best. Sometimes you end up twiddling the switch back and forth a lot in patchy snow but it really isn't a big deal. Plus it's kinda fun sliding it around in 2wd. If the pavement's slick or greasy enough you would want AWD you would be totally fine putting it in 4hi. If it's grippy enough to hurt it you'll notice way before you hurt it, it just feels strange.

So with the manual requirement I guess your options really are deal with the part-time 4x4 or find yourself a manual FJ.

 

My 2001 Subaru had a manual transmission and AWD. It might be hard finding one in decent shape these days though. Manuals are a dying breed in the US. There might a few sports cars with AWD and a manual.

 

My pleasure! And...Welcome to TW!!

 

Yes, back in the 70's and 80's, there was an aftermarket company supplying just such a coupling for various 4x4 vehicles. Many of them came w/o locking hubs, so the front drive line was always spinning... adding hubs was obviously much more common.

The issue here, as you aware, is that no one produces such a thing, never mind a unit that will bolt in. So depending on your abilities, it will be up to you to develop and produce it... or pay someone else, which might push the cost up past the easy and affordable point.

If I were to attempt this, a viscous coupling from an FZJ80 might fit the bill.

 

Actually a proper full time 4x4 mode is handy. My last three Jeeps had it and it was great for things like changing road conditions on curvy roads where you want the extra traction for snow/ice but then may hit a sharp bend with dry pavement. Then if needed you could just drop it into part-time 4x4 for the extra traction.

It's a feature I wish I had on my Tacoma and had it been an option I'd have bought it, YMMV.

 

I think I've at least come to the conclusion that a front driveshaft with an integrated viscous coupling would be the better solution for me if such a thing can be acquired. I guess next step will be to reach out to some custom driveshaft shops and see if anyone is willing to entertain this nonsense.. There are plenty of places that do custom truck drive shafts, and I've seen an import tuner that machined a custom viscous coupler, but nothing where the two come together... I guess I'll find out.

I found where Mile Marker used to make a viscous coupler that bolted between the transfer case and the front drive shaft for New Process cases, but they stopped producing it back in the 90s. It would be nice to know why. Was it just lack of demand due to the changes in vehicle technology or is there some issue that developed when using it... Unfortunately that happened back in the dark ages before the internet, so the world may never know...
The 80 series Land Cruiser was one of the trucks started me thinking about this. A friend of mine has had one for a long time and I've always liked it.

 

I dont think there is a single true truck (not your unibody shit) on the market that has an AWD mode with a diff in the tcase.