Engaging 4WD in older Chevy Pickup

I just bought a 1991 Chevrolet K1500 and I have a question about the 4WD. Can I switch to 4-high “on the fly”–as in while the vehicle is moving–or do I have to be parked/stopped/or in neutral? The guy at the used car lot didn’t seem to know for sure, but my neighbor said “no problem,” to shift it to 4-high from 2WD as long as I’m not going too fast. Is this true? I don’t want to trash my car! I don’t know much about 4WD on older cars. I’m used to “push button” 4WD on newer models. Also, what’s the highest safe speed for 4WD? High and low…Thanks!

Would you like to be safe or sorry? Check the owner’s manual if there is one. I have driven a Ford at 40 MPH on really icy roads and hills with ordinary tires and had no problem. 4L would be kind of risky when moving. 4H you could probably do when moving.

Maybe the manual is in the dash, we have a few gmc vehicles of that era and have always engaged 4wd before moving, 4wd should be used cautiously and sparingly imho. 4 lo you will not need unless pulling a boat out of a sandy launch etc. In fact I would skip using 4wd except when absolutely necessary and never over 40 mph. But then again that is my experience with early 90’s vintage vehicles still working fine.

You should be able to shift between 2H and 4H on the fly as long as you are going straight ahead and all the wheels have equal traction. Now if your vehicle has some sort of “automatic locking hubs” you must stop first as the front axle is not “live” until SOMETHING engages the front hubs. Your truck may have a live axle which is engaged all the time, 2-wheel or 4 wheel drive…On older trucks, many were equipped with manually locking hubs where you had to stop and get out and lock or unlock the front hubs…SERIOUS off-roaders still favor this fool-proof set-up…

You can ALWAYS shift from 4H to 2H, that’s no problem. But be gentle when first trying to go from 2wd to 4wd on the fly. If the transfer case lever will snap in with no gear clashing, then you have a live front axle and can shift on the fly. If you get gear grinding, forget it, stop before you engage 4wd…

Now if your vehicle has some sort of “automatic locking hubs” you must stop first as the front axle is not “live” until SOMETHING engages the front hubs

My 84 GMC pickup with auto-locking hubs could be switched into 4wd-hi at speeds under 50. I did NOT have to stop to engage 4wd. Did they change something in 91???

Exploding transfer cases are just part of the Four Wheel Drive experience…Grab that lever and jam it in there and see what happens! After that thrill, you might like to try driving into a big ol’ mud-hole and lock 'er in Four-Low then crank the steering wheel to full lock, right or left, makes no difference, and pour the coals to her! $2000 worth of axle shafts, hubs and steering knuckles destroyed in seconds! What fun!

We do some serious rock crawling out here in Tucson. Most of mine was with a 71 Wagoneer (yes they did very well). If the situation presented itself that I needed 4 low I would always come to a complete stop when the really rough stuff stopped and then shift to 4h (Dana 20 transfer case)Never felt the need to experement with my equipment when I was in 4low country, you always take the conservative route when dealing with country that requires 4 low, just to far back in to get stuck.

I leave the snow stuff to thoses “other” 4 wheelers.

I appreciate replies to my question, but the sarcasm isn’t very helpful. I’m new to this style of 4WD truck and I’m asking an honest question and hoping for useful advice. I have no idea how to take this response.

I don’t anticipate using 4-low very often–or ever. I simply want to know when it is safe to engage the 4WD-high when conditions go from dry pavement to snow and ice. Do I have to pull over, stop the car, put it in neutral before I shift? Or can I just shift into 4WD while driving when the conditions change? The manual offers no advice in this regard. I experimented this weekend and found that it seems to shift into 4-hi no problem. It usually takes a few second for the light on the floor to indicate that the front axle is engaged, but it seems to shift in and out no problem. I haven’t tried it at speeds over 30 mph. I do need the 4WD for driving up the mountain on snow and ice.

You can engage while moving. Just make sure wheels are nearly dead straight and you are not slipping. If it seems to engage or disengage hard using a slight application of throttle will help.

Usually the safe high speed is around 50MPH or so. Basically if you need 4wd and need to go faster than 50-60MPH then something is wrong.

In my 4wd vehicles I have used (Toyota 4runner, Pickup, Suburban, Subaru) all the 4wd info was usually printed on the backside of the sun visors.

I used to own a similar vintage K2500 4x4. You can shift between 2HI and 4HI on the fly. To go into 4LO, you should be stopped.

The advice to have the wheels straight ahead is especially true with this vintage setup. If you crank the wheels to the side and engage 4WD, you can damage the actuator.

There are sources online for OEM Owner’s Manuals if you are interested. I’ve seen them for around $15 and worth every penny IMO.

You should be able to go from 2H to 4H when moving. I’d try a slower speed engagement at first, say 15 to 20 mph and do this on a dirt or gravel road. You have an older truck and the condition of the transfer case, front diff, and hubs could be questionable. If everything seems to work OK, then you can try it at higher speeds. Above 50 it can be too big a jolt to the system if something isn’t right. I believe you are aware that these older 4WD set ups don’t do well if you leave them in 4WD and drive on dry pavement, just in case I thought I’d mention it.

It would be a good idea to get all the fluids in the trans, transfer case, and both differentials changed. Fresh fluids will keep things from going snap, crackle, and pop on you.

Exploding transfer cases are just part of the Four Wheel Drive experience.

So you’re saying that GM (actually chryco) purposely designed a system that is suppose to explode. I don’t think so.

I have seen vehicles with the freshest fluids known to man go snap,crackle and pop. Fresh fluids are not protection from overstresing your vehicle.

The incidents would be 10 miles from the start line in an off-road race like the Parker 400,really rough stuff, Mint 400 was rather exciting also, 300 cars behind you all hauling a** and you can’t see 10 ft in front of yourself.

I have a 2004 GMC automatic with a hump mounted 4x floor shift. In no way would i shift-on-the -fly at any speed. Electrically operated shift is O.K.

I forgot to mention this: Don’t worry about the sarcasm. It’s half the fun of giving advice and expressing feelings, or the lack of them. My own sympathy meter goes from zero to maximum at any time. Depends which way the wind is blowing.

this link will show you how you can use it

That should help the OP, who posted his question 11 years ago, and whose truck is now 29 years old…
if it is still running.