89' 4x4 operation

[Straylox]

89' GMC Sierra 5.7 V8

I've done alot of searching but couldn't find any answers specific for my truck.

I have always thought that the truck must be staionary before shifting into 4-hi, but i was told recently by a friend that it was safe to shift from 2 wheel to 4x4 while going down the road.

My question is what speeds are safe to do this?

Basicly i will be traveling on slick, snow covered residential roads, to the clear interstate, then back onto slick, snow covered side roads when i get off the interstate. if i can shift in and out of 4-hi and 2-hi while in motion, it would make life easier for my commute. I need to know if this is safe to do or not, heh. Thanks.

[leolkfrm]

Sure do it all the time. The thing to remember is don't run 4wd on wet or dry pavement

[bradrhale]

didnt know that a 89 GMC had shift on the fly.

[75K30]

Shift in the fly is nothing more than a great marketing campaign. This ability to shift into 4wd high range is shared by all 4wd systems.
If the front hubs are locked, then the front driveshaft will be spinning at the same rate as the rear driveshaft. This means that when you call 4wd by shifting the lever, it will easilly go into 4wd high range. This can be accomplished at almost any speed.
This is a true statement provided that the outputs are matched. This means that you cant do this if the rear wheels are spinning, or have lost traction. The driveshafts have to be turning at about the same speed in order to accomplish this.
My 70's vehicles, and even a couple from the 60's can be shifted on the fly.

Just a little bit of info that may never ever be usefull, but,,,,there it is.

[leolkfrm]

See Brad ya'll in the warm weather don't use 4wd, like we do up in snow country
for us it's a good day when we don't need it

[bradrhale]

wasnt sure. but you are right, 4wd is never needed unless a friend calls and needs a pull. but alot of times, only 2 wd is needed for that for someone that knows how to work the truck.

[leolkfrm]

You are right, but I stopped buying heavy lug tires in favor of tire wear(and rotation on 2wd). but it still gets 500#s in the back. It's more of a convenience then need

[bradrhale]

i have never been a fan of huge knobs. they make alot more unnecessary noise. i dont have any extra weight in mine besides the box in the back. always figured since i drive an anchor, i really dont need to add any. that, with the oversized tires and non updated gears (35" with 3.42s) i really try to keep mine out of places that might call for more weight.

[Straylox]

ok, so can i shift in and out of 4-hi at any speed or only slower speeds? how fast can i go in 4-hi? about 40 mph is the fastest ive done.

So there is no need to back-up after shifting into 4x4 to lock in the hubs? (this is what i was told to do when i first bought the truck by the previous owner)

[leolkfrm]

If you are going faster5 than 40mph you should not need 4wd. if I remember correctly you need to backup to aid in unlocking the hubs