99 GMC Sierra Z71 4WD Problems

[Briant_k]

Well, my "Service 4WD" light came on while driving home today. When I got home I started messing around with it and switching from all 4 modes worked fine. I drove a little in 4WD HI and restarted. The light didnt come on again, so I figure the parts just needed a little movement. Aside from some noises that made me grit my teeth while I was switching modes, everything seems to be working fine, except....while I was in 4WD Low, and in Reverse under 5mph, it felt like it was going in and out of gear. Also, I went to a gravel parking lot and messed with the 4WD Low some more. I noticed that it wouldnt shift up while in 4WD Low. Even at about 2300rpm's. Also, just after hitting the gas in 4WD Low, it kinda feels like its double shifting. Or like, I can take off, but after a second or two, it shifts again...kinda hard to explain. I know about pretty much everything EXCEPT 4WD systems. I dont know if this is normal or not. Any info would be greatly appreciated. Otherwise, if im bein retarded, feel free to let me know. Thanks
Brian

[92GMCstepside]

On newer 4wd's when you back up, that is how the hubs are disengaged, instead of having a manual lock on the hub it is all automatic, when u shift into 4wd low it locks the front hubs solid with each other, when you back up that is how you unlock them. So the feeling of it shiftin in and out is from the hubs unlockin, lockin and unlockin again because u are still in 4 lo. The newer 4wd's also need to be run once in awhile to keep things lubed up. I generally try to drive at least 10-15 miles every other week in 4 hi just to keep the oil flowing. The noise you heard if it was a grinding sudden jerk or just metal on metal that is fairly typical. That is just the gears and shift collars engageing or disengaging. You might go to your local parts store and see if they can pull any codes with the 4wd light. This I think is a good place to start. Also when you are in 4lo the transimission is tryin to put maximum power to the ground they generally don't shift like normal, low range is designed to be very torquey hence the feelin of taking off, this is normal. Low range is designed for slow driving, puttin the most power to all 4 wheels possible using the engines torque range.

[Briant_k]

So I really don't have to worry about the shifting up in 4 low considering I was on a flat surface and not towing anything. The whole "shifting in and out" thing just freaked me I guess. Since this was the only time I drove in 4 low with it. So, according to your reply, when I'm in reverse while in 4 low, the hubs automatically disengage? I drove it again tonight and the light has remained off, so I think I'm good as far as that goes.

[92GMCstepside]

thats correct, the hubs automatically disengage. And even though you were on a flat surface and not towing anything they still wont shift like normal, transfer cases are geared for lots of torque and the computers are set to help maximize that torque. So it sounds as if all is well.

[Briant_k]

Good news. Thanks a lot.