thanks gents! also thanks to AAdam for the link. i’ve read all the slee intel (and other threads here) but missed that one and obviously didn’t understand the topic.
so (may as well respond to MGoBlue!) -
my 98 4WD tacoma is normally 2WD but shifts into low4 and high4 as noted (thanks). i turn some manual deal at the front wheels (for some reason i don’t understand) and then shift. what is the mechanism called that puts it in low4 or high4? is this a differential...?
and on the 96/97 LC - the center differential “ties” the front and rear axles together but does so in a way that allows for some variability? 50/50 on the highway but it can vary (in turns etc)...?
lastly, the mechanism at the front and rear on some (“lockers”) are specifically not differentials since they lock the left and right side of the axle together is that correct?
oh, what’s a viscous coupling? it’s specifically aftermarket?
In your tacoma, you had 2 differentials, and a part time transfer case. Your differentials are at the center of your front and rear axles, sometimes referred to as pumpkins. The job of a differential is to allow your tires to turn at different speeds when driving on the street. Every time you drive around a corner, the wheels on the outside of the corner have to cover more ground and therefor turn faster than the wheels on the inside of the turn. A differential allows this to happen.
So under normal circumstances in your taco, you'd be in 2wd, and power would only be sent to your rear axle, where your diff is "open" allowing the wheels to spin at different speeds. This is fine for most driving conditions, but if you've ever driven in snow in a pickup in 2wd, you know how things get really interesting, really fast.
So on a snowy day, you'd put your taco in 4wd high, and lock the manual hubs. By shifting your transfer case from 2wd to 4wd, power is now being sent to your front axle, via the front driveshaft. Your front and rear driveshafts are forced to spin at the exact same speed in this mode. The switches in the center of your front wheels are called manual locking hubs, and they are often confused for differential lockers, which I will cover later. Manual hubs in thier "free" position disconnect the wheels from the axle, allowing the wheels to spin freely, but the axle will remain stationary. This will save wear and tear on front axle parts, and reduce the rolling resistance of the vehicle. Most 4x4's these days don't have manual locking hubs, the are just locked all the time, which wears out your front axle and driveshaft components and wastes gas.
So in order to get 4wd in your tacoma, you shift the transfer case into 4wd high, this will transfer power to your front axle, but not to your front tires until you lock your hubs.
Now say that you are on a washed out steep mountain road, this is where you might use 4wd lo. It works the same as high, but through reduction gears in your transfer case you will be able to drive much slower with more control and available torque. In this scenario, your front and rear diffs are still "open" which will allow your tires to spin at different speeds. This isn't an issue for 99% of truck owners, but when you start doing serious offroading, the limitations of open diffs become very apparent. An open diff by design sends all of the power to the wheel with the least amount of traction.
Imagine you are on the flat ground, with one large rock. You want to drive over that rock with your right front tire, but as you climb up the rock, your left from tire and right rear tire start to lose traction and spin, while the tire on the rock, and the opposite rear tire do nothing. This is the problem with open diffs. Being able to lock your front and rear diffs will force all 4 tires to spin at the same speed, and you will easily climb the rock in a controlled fashion.
Now I'll go over the differences between the taco and a full time 4wd 80.
If you try to drive your tacoma on a dry paved street in 4wd high or low with the hubs locked, you'd have a bad day. Every time you go around a corner, you'd feel the drive train binding, your tires would squeel and you'd have a very real chance of breaking something. This is because of another phenomenon that happens when you drive in a circle, your front tires will make a bigger radius than your rears. Since the front has to cover more ground, your front axle needs to spin fast than the rear, and in your tacoma, the transfer case will force the front and rear axles to spin at the same speed. This isn't a issue on dirt roads, or snowy roads, because your tires can slip a little bit to relieve the stress in the drive train, but on a dry street you can break something.
So how does full time 4wd work? It has an additional differential in the transfer case. This "center diff" will allow the front axle to spin faster around the corner, without binding or damaging anything. But if you remember how open diffs are bad for offroading, you can see why it's necessary to be able to lock the center diff, so that you can evenly split the power between the front and rear axles when you are offroad.
The main benefit of full time 4wd is that most people never have to mess with it or even think about it. A dry summer day, or a snowstorm, full time 4wd just works. Now if the conditions are bad enough, you still need to lock the center diff, or put it into 4lo, but for 99% of people, 99% of the time they never have to mess with it.
The drawbacks are numerous however. The front axle is always spinning, so your front driveshaft, U-joints, gears, bearings, axle seals will all wear out faster. All of those extra rotating parts will also cost you extra fuel, and with a full time system, it becomes important that all of your tires are very close to the same size, especially if you have a viscous coupler.
On to the viscous coupler then. The easiest way to explain a viscous coupler, is to explain a limited slip differential. A limited slip is sort of the half measure between a open diff, and a locker. It allows the tires to spin at a different speed, but it does resist and "try" to spilt the power more evenly. A viscous coupler is the same thing, but it's located in your center diff, inside of your transfer case.
OK, that is long enough.
