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switching from 2H to 4H

G

gibson

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Hi everybody.
Maybe a stupid question, but my K5 is my first non-permanent 4x4...
At the garage in Italy where I found my '83 Blazer, they told me: "If weather conditions are getting bad, and it starts raining while you drive on the highway, don't hesitate to put on 4H to increase the traction."
This sounded a bit strange to me cause I had in mind that only Quadra-track transmission could do that. Or permanent 4x4 systems like the one I had on my Landrover Defender.
For me, Blazer K5 rustic transmission could not do that. I thought that you could put on 4H only on muddy tracks or if the street is covered with snow.
So I was wondering...who is right ? me or the garage ?:dunno:
Is it really possible to switch on 4H on streets ? Isn't there a risk of damages to the transmission especially if the road is full of sharp curves?
Thanks by advance for your answers.
Oh by the way, I'm looking for a 83 Blazer instruction manual. Where could I find this document ? Is there a site where I could buy one.
 
For winter, I used to lock my front hubs and run in 2HI. if things got snowy I'd pull it into 4HI "on the fly", then back to 2HI when the roads were more clear. Wet pavement...never. Wasn't necessary. Typically it'd be 4HI on side streets, and 2HI on main roads.
 
OK, here is the deal. Don't listen to the guys with massive drivetrains that are overbuilt beyond belief.
They know what they are talking about, better than me, but for us poor folk that have to live with normal factory drivetrains...........

No matter how carefully you might try to match tire size, and other variables, the front and rear axles will always turn at different rates even going straight forward.
Curves, even more so.

Since the transfer case in your vehicle locks the front and rear driveshafts together when you put it in 4wd, this will cause strain to build up quickly going down a dry road with good traction.
At some point, something has to give. If you have great traction, and an automatic tranny, it may just slow to a stop.
But, usually the tires will start to slip. If you are lucky.

Being a Ford man, I'm a little fuzzy on which transfer case you have, but if its one of the heavy cast iron ones, then it might handle the load. But you are putting an abnormal strain on U-joints, slip yokes, basically the whole system.

If you had one of the lighter duty newer aluminum boxes, well, I have seen two of them explode hauling a boat up a boat ramp on dry concrete.

In your case, I would do like TRusty said. Lock the hubs in, and just drop it in when things get slick.
Remember, as long as the front and rear driveshafts are turning at about the same speed, you have "shift-on-the-fly" since the gears are turning at the same speed and you can just slide it in.
But, if you are spinning your rear tires, then it will grind the gears.

I do this in my Ford all the time. When I am driving in the woods, going from bad spot to bad spot, I leave the hubs engaged. When I hit a slick spot, I back off the gas to let the rears stop spinning, drop it into 4hi and get back on the gas.

As soon as I hit dry dirt, I back off the gas, kick the automatic tranny into neutral to take any load off, and knock it into 2hi.
I can do it so fast the truck never hesitates.

Remember this though. You have been running on the rear drivetrain at speed. Not so the front.
The front U-joints may have wear, the spline might be floppy, and the shaft may even be out of balance.
So driving at speed with the hubs engaged might cause lots of vibration.
It can be fixed, of course, just be prepared to encounter it.

Also, the truck is going to steer and handle differently in 4hi than 2hi. How much so depends on steering geometry and tires. So you might want to practice on an empty snowy road before dropping into gear in heavy traffic.......

Bottom line is, if the road is slick enough for the tires to slip without too much strain, its not going to hurt anything to put it in 4hi.
 
Slight tangent, but I could swear my old Chilton or Haynes book advised to drive around on dry pavement in 4HI/hubs locked for a while before doing a fluid change on the transfer case. I did it about 20 years ago when I was just starting to learn about trucks and nothing bad happened. Given that the 208 takes ATF, I don't really see the point though. Does this ring a bell with anyone else?

Anyway, I agree that standard procedure would be to keep the hubs locked if you expect to possibly need 4WD to make life more convenient. I wouldn't drive much over 45 this way myself, but I can't think of why it would be a problem to go faster. The Jeep Wranglers all have un-unlockable hubs, so turn everything up to the tcase all the time. When I lived in Iowa the hubs on my CUCV would be locked all winter.
 
I wouldn't suggest running 4 hi on wet pavement unless it was an absolute downpour with standing water on the road. With that said if everything is in good working order it shouldn't hurt anything to run the truck in 4wd on pavement. Yes, I said it should not hurt anything. This comes from spending many years performing durability testing at an automotive proving grounds. Every 4wd vehicle from diesel duallies to SUV's would run locked in 4wd on dry roads half of the time, which was usually 15,000 to 30,000 miles in 4wd. They would do everything from 100 mph on a dry concrete track to lock to lock turns on pavement. I only saw one specific vehicle setup on a certain type of test that had issues. This SUV had a CV style front driveshaft (true CV, not a double cardan) that ran consistent high speeds. If you had new tires on one end and very worn on the other it would burn up the CV, but if you kept the tires at least half way matched it was okay. We tested hundreds of vehicles over many years doing this and besides for the above issue (again, one particular vehicle configuration on a specific test) never saw a failure that could be directly attributed to running in 4wd.

Think about it, people run in 4wd on high traction rock trails all of the time and do full throttle assaults climbing hills. If the extra torque of 4 low and the tires bouncing off the ground while climbing rocks and hills is okay how can running on the pavement but that big of a deal?

Still, I wouldn't recommend running locked in 4wd down a dry freeway for no reason. Simply no reason to.
 
Can't quite grasp why people want to run 4wd in the rain? If the water is deep enough that you can't cross in 2wd then 4wd isn't going to help you. 4wd won't stop hydroplaning.

Unless its slick enough that you can't keep traction in 2wd (mud,snow,sand) then the 4wd is probably gonna bind on you.
 
I remember reading in a Chiltons book to lock in the front hubs and drive at least 10 miles monthly "to ensure adequate front end differential lubrication"...didn't say to shift into 4wd though..

I see no reason to do that however,as most all front driveshafts will spin all by themselves even in 2wd,just from the fluid in the transfer case being thick enough after a first cold run is enough "fluid drive" to make it rotate,especially with the t-cases that use gear or motor oil...and the diff will spin along with it and splash enough lube around to keep it lubed up and prevent rust from forming..the things that would suffer most would be the axle u-joints probably,they can seize up quickly if they dont get spun around for a long time..(ones in full time 4x4's often outlive part time ones,even though you would expect just the opposite to be true!)..

I could see the front shaft in my 72 K5 spinning in 2wd thru the shifter boot hole for the first 5 miles or so with it in 2wd and the hubs unlocked..then like magic,it would come to a stop ,after the gear oil thinned out enough..

In my pickup with a NP-208 I usually lock in my front hubs if its snowy and leave it in 2wd until I need 4wd...did the same in my old NP-205 equipped trucks too,that way when your on a highway thats salty and dry,you can just use 2wd,then if you turn off onto an unplowed or snow covered road ,you can just pop it in 4 hi...with my plow on,if I gun it to cross an intersection in 2wd and its wet or icy,I'll sit there spinning the rear tires!--almost got myself t-boned that way--now I put it in 4hi every time I need to cross a busy intersection..
 
I have switched from 2-hi to 4-hi on the fly several times, needs to be a swift movement and nothing over 45mph.

I did forget to unlock my hubs one day after a wheelin trip in Moab, transfer case was in 2wd, but since the hubs were still locked, the front drive shaft was spinning. I could NOT figure out why my truck was vibrating so badly, until I gave it more thought and remembered the hubs.

Turns out, like most of these older trucks now, unless you have had your front drive shaft rebuilt and tightened up, the slip part gets really loose over time and use. mine was wobbling at higher speeds.
 
Since the O.P. says he has an '83, there's a chance that the Blazer could still have the factory auto hubs. Let's not forget that the NP208 came out at the same time as the auto hubs and I believe it was designed to work with them. In the glove box there's a sticker that says you can shift from 2Hi to 4Hi at speeds of up to 50 mph (80-85 kph?). That was on my '85 with the NP208 and auto locking hubs. I tried it and it worked. I even drove it on the pavement in 4Hi when there was ice and snow on the road and my 2wd truck couldn't safely make it. It was sliding all around. The only time I think I could have done some damage was when I was turning into a parking space and the wheels were close to full lock in either direction, I felt the front wheels hopping and binding a lot since they were both trying to drive at once. I can see where it would be possible to blow a u-joint on an axle shaft or even break a shaft in that instance, but only with big, heavy tires, lots of throttle and rough conditions.

So I say don't be afraid to drop it in 4Hi if you feel you need to, just don't do it all the time, especially if you're going to be doing tight turns on pavement. This would be for a chain drive t-case with a planetary drive setup like the NP208, of course. If it were an all-gear case such as the NP205, you will have to stop and put the transmission in park or neutral to go to 4wd. Totally different setup on those older style cases.

Welcome to Colorado K5 by the way. :D
 
'83 should be an NP208, but this should be confirmed. If the transfer case is aluminum, it's almost certainly NP208. With the front hubs locked, it's safe to shift 2H<-->4H at pretty much any reasonable speed, as everything in the case is already going the same speed. I suppose high speed shifting will put a little more wear on the shift fork pads.

With auto hubs, you need to be sure they are locked to shift at higher speeds. So you would shift to 4H (low speed) and drive, then go between 2H and 4H as you wish. If you are in 2H and back up, the hubs will unlock (at least most of the time!). When the hubs are not locked, I would only shift at lower speeds, like 15MPH and below. Auto hubs are finicky, so it's best not to push your luck. Overall, I would be more comfortable with "shift-on-the-fly" with manual hubs.

As said above, this isn't really for rain, but useful for snowy roads.
 
GMCSuburban2500 said:
Can't quite grasp why people want to run 4wd in the rain? If the water is deep enough that you can't cross in 2wd then 4wd isn't going to help you. 4wd won't stop hydroplaning.

Unless its slick enough that you can't keep traction in 2wd (mud,snow,sand) then the 4wd is probably gonna bind on you.
Click to expand...

4wd or AWD does help handling in the rain, but I agree for normal driving and just wet roads it is not really needed. The hydroplaning example is way over simplyfing what is going on.
 
It won't hurt anything to shift on the fly with an NP208.

Martin
 
I remember reading in a Chiltons book to lock in the front hubs and drive at least 10 miles monthly "to ensure adequate front end differential lubrication"...didn't say to shift into 4wd though..
Click to expand...

HA! Got you on that one. Sometimes age is good for something!
You are correct until you go back far enough.
The early Jeeps and other trucks with the sealed oil bath steering knuckles had steep angle tapered roller bearings for the top and bottom ball joints/kingpins.

The bottom bearing sat in gear lube and would last forever. But the top bearing was constantly splash lubed by the axle joints running in the lube.
Since there were no locking hubs when they were designed, the system worked great.
Afterwards, not so much.
I went through several top bearings before I learned to drop it into 4hi between red lights about every week.
 
Thx everybody for your answers and your warm welcome.
6.2 BLAZER, I share your point of view. It makes sense. If people could drive for miles in 4H on rocky tracks without any damage, I don't really see how temporarily switching in 4H on pavement under the rain could be dangerous for the transmission.
I understand also the point of GMCSUBURBAN2500. 4H doesn't prevent from hydroplanning. That's true. But I was wondering about this possibility to drive in 4H on pavements, because the last 5 minutes to my house are quite tricky under the rain: A few sections are steep slopes (about 30%) and i already saw some people in trouble with rear-wheel drive car. On the contrary, I never had a problem with my daily car (2011 Kia Sorento).
I don't want to drive all the time in 4H under the rain. I was just wondering if it might be painful for the transmission to switch in 4H during the last 5 minutes to my house if it's raining. In order to avoid excessive slippage of rear wheels...
Thx guys. This forum is very useful for newbies like me.
 
I would think that driving in 4H in the rain would be okay. Perhaps you should avoid using 4H on dry pavement.

Welcome to the forum. Lots of great info and people on here. I've learned a lot and I'm still learning.
 
I have a couple of time accidentally driven in 4 hi on dry roads for several miles. Realized it and put it back in 2 wheel drive and no problems.

On rainy roads perfectly fine traction is decreased.
 
It's certainly reasonable to use 4WD if tires are slipping in 2WD. You just get more tire wear and maybe driveline wear if you use 4WD a lot when traction is good. On dry pavement, the truck really fights tight turns in 4WD as well.
 
Going straight down the highway it won't affect anything, as the wheels are all turning at the same speed. It's when you try to take a corner, and the wheels want to turn at different speeds that you'll run into issues (tire scrub) on surfaces with decent traction. Keep good tires on it, and save the 4x4 for the truly slick stuff.
 
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