1981 blazer, rules for transfer case and 4wd?

[derherr65]

I have a 1981 blazer that appears to be stock. As best I can tell it has 305 OR 350, aluminum np208, dana 44 front, gm 12 bolt rear, th350 automatic, automatic locking hubs.
1) Is this right?
2) How exactly does the 4wd system work, what's the difference between 4 lo, 4 hi, n, 2 hi?
3) What are the rules for when you should and shouldnt use each, and how to shift from one to another?

 

[Silver84k5]

2 hi is what you run around town in,

4 hi is four wheel drive, with the same gearing as 2 hi.

4 lo is four wheel drive, with lower gearing

n is neutral

Use 2 hi around town,

Typically i would use 4 hi in town, when there was snow on the road or something.

Use 4 low, when you want more control, slower speed, technical driving.

Im pretty sure on the 208 you can shift into 4hi from 2 hi, while moving, AND your rear tires arent spinning. Im not sure how fast you can be driving while you do this. Ive never had to shift into 4 hi while cruising at highway speeds, i doubt you will either

shifting into 4 low, you want to be stopped, and in neutral or park.

Ive never had a rig with auto hubs, but ive heard sometimes you have to drive in reverse a few yards, to get them to unlock after going from 4wd to 2wd.

 

[derherr65]

OK. How does the 4wd system work? Are front and rear driveshafts locked together, or is there a clutch? Are front and rear diffs open, limited slip, locker? Can any wheel overrun or must all tires move at the same speed?

 

[hunter29078]

The 4wd drive works because of the 208 transfer case. It is mounted to the back of the transmission. When you shift it into 4wd a fork moves internally in the case to transfer power to the front axle. In 2wd it is only giving power to the rear. Unless modified you probably have open diffs in the front and rear.

 

[derherr65]

And the 208 has no clutches or other provision for slippage? In other words the front and rear outputs must both turn at the same rate or something breaks, right?

 

[hunter29078]

And the 208 has no clutches or other provision for slippage? In other words the front and rear outputs must both turn at the same rate or something breaks, right?

The rear and front will turn as one, however the 208 has a chain which will slip when it needs replacing.

 

[shovelbill80]

hey....i'm new here and been away from K's for a while, but i think you should have a 28 spline 10 bolt up front if it wasn't swapped out. i recall '81 was the last year for the 12 bolt in 1/2 tons. i know i reused my 44/12 setup for a few trucks that i got from my '75....that 208 has a chain in it and sometimes they stretch and make things a bit touchy to operate as i remember, popping out of 4WD as i recall.

 

[Fordum]

And the 208 has no clutches or other provision for slippage? In other words the front and rear outputs must both turn at the same rate or something breaks, right?

Pretty much, yes. That is why in general, you do not use 4hi on dry pavement.

If the tires are exactly the same size, you might get away with it for a short distance, for instance pulling up a boat ramp, but things get really tight if you try to make a turn on dry pavement, since each wheel is going a different distance.

If you are on a surface that will allow slip, such as dirt, mud or snow, 4hi is fine.

Running different sized tires front and rear is not recommended.

With auto hubs, you usually can Not shift on the fly. Transfer cases that allow that, have what amounts to synchronizers that spin up the front drive-shaft before it goes into gear.

With manual hubs, if they are engaged, and the wheels are not slipping, then the front and rear driveshafts are turning at the same or almost the same speed.

On my truck, which has a 205 transfer case with no clutches or synchronizers, I can drop into 4hi at 60 with the hubs engaged.
But I don't recommend driving that fast in 4hi except for special circumstances.

Now, if you have a truck like a lot of folks here have, that has been overbuilt to extreme, you can run on dry pavement in 4hi and the system is so strong that it forces the tires to slip on the pavement.
But, even with the build these folks have, its still a strain.

 

[derherr65]

That makes sense. Explains the lurching on tight corners and that mine seems to like shifting into and out of 4wd while stopped.
On further investigation I'm going to say it's a GM 10 bolt. It looks just like the dana, acorn shape, drain plug and all, except the 'ears' on the bottom.